uut1,21 12 otmoayPyisHaghighipour-M Physics Contemporary 11:21 2018 10, August arXiv:1108.0031v1 [astro-ph.EP] 29 Jul 2011 o.0,N.0,0 0 1–42 00, 00 00, No. 00, Vol. Physics Contemporary iiyo h xsec fpaesaon te tr n pre and stars Gio other philosopher, around earths” planets Italian countless of the existence when the ago of years bility 500 almost was It Introduction 1 http://www.informaworld.com 0000 DOI: online 0000 print/ISSN 0000 ISSN: les no and remai worse planets no are ideas. their seve universe his but the the of as luminous, in way worlds are countless same and These the bodies exactly largest in are suns their around rotating 1 o oeta 0 lnt nldn h rteolntr bo exoplanetary first the including planets 500 than more now ailvlct hti asdb h rvttoa attract nearb of around gravitational bodies spectrum the planetary by the many identify caused in to is astronomers shifts that the velocity of radial Measurements changed. trend perturbatio sensitivi its of through level or directly, the effo either reached their body not ago, planetary had decades techniques two trie t until detection astronomers and However, unique, centuries systems. not For are planetary stars. system other solar around our and revolve Sun the that evident t Technique ity ∗ c rmteqoeb h tla hlspe,Godn rn (1 Bruno Giordano philosopher, Italian the by quote the From orsodn uhr mi:[email protected] Email: author. Corresponding hnst dacsi bevto n eeto technologi detection and observation in advances to Thanks 0Tyo Francis & Taylor 00 lntDetection. Planet Keywords: presented. revi is a stars Giv and low-mass discussed, of objects. also zones these is and super-Earths of super-Earths, i of on habitability detectability zone research possible habitable of and state the evolution, current where the stars of low-mass int review dynamic around possibly common exopl and more atmospheres the relativ moderate are physica in maintain they have can planets, place terrestrial may unlike special whereas super-Earths a Earth planet, found terrestrial recently typical have Uranus, than ue-ats ls fpaeaybde ihmse rangi masses with bodies planetary of class a Super-Earths, lokonas known also , xrslrPaes lntr neir lntFormation Planet Interior, Planetary Planets, Extrasolar ue-ats e ls fPaeayBodies Planetary of Class New A Super-Earths: 1 ic hn stesineo srnm rgesd tbeca it progressed, astronomy of science the as then, Since . a nttt o srnm n AAAtoilg Institute, Astrobiology NASA and Astronomy for Institute nvriyo aai oouu I982 USA 96822, HI Honolulu, Hawaii, of University ope Velocimetry Doppler ae Haghighipour Nader REVIEW anuscript ( 0000 niil ou eas hyaesalradnon-luminous. and smaller are they because us to invisible n l airt eet eas fterszs super-Earths sizes, their of Because detect. to easier ely naie hnorErh”Buowsbre lv because alive burned was Bruno Earth.” our than inhabited s 4-60:”hr r onls usadcuteserh a earths countless and suns countless are ”There 548-1600): wo h rset fterdtcini h habitable the in detection their of prospects the of ew lnt forsse.W e nytesn eas they because suns the only see We system. our of planets n ) ntercn dacsi eeto ehius the techniques, detection in advances recent the en n yaia hrceitc iia otoeof those to similar characteristics dynamical and l Fgrs1 a enscesu nidentifying in successful been has 1) (Figures icse h oeso h omto,dynamical formation, the of models the discusses roswt lt etnc.Te lose obe to seem also They tectonics. plate with eriors gfo e at-asst lgtysmaller slightly to Earth-masses few a from ng ntr cec.Bigsihl agrta a than larger slightly Being science. anetary ncoe itne.Ti ril rsnsa presents article This distances. closer in s a ∗ nishs star. host its on n etdteie f“onls usand suns “countless of idea the sented o famsiecmainenabled companion massive a of ion ieesyt eetsuch detect to tirelessly d lntr yais Habitability, Dynamics, Planetary , yta a eesr oietf a identify to necessary was that ty tr.The stars. y dn rn,dsusdtepossi- the discussed Bruno, rdano t eernee fruitless–their rendered were rts s nteps w eae this decades two past the in es, y . uie-asojc in object Jupiter-mass 4.7 a dy, eems emn lnt that planets many be must here h ih fasa u oits to due star a of light the rcso ailVeloc- Radial Precision emr n more and more me extrasolar ll August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

2 Nader Haghighipour

Figure 1. Schematic illustration of Doppler shift of stellar light. As shown by the second panel from top, the light is red-shifted when the star is moving away from the observer. When the star moves towards the observer (second panel from bottom), light is shifted towards blue. By measuring this Doppler shift, astronomers can determine the semimajor axis, eccentricity, and minimum mass of the unseen planet. Figure from spheroid.wordpress.com .

a 4-day orbit around the Sun-like star 51 Pegasi [1], and possibly an Earth-sized planet in the habitable zone of the near-by star Gliese 581 [2]2. Technological advances also enabled astronomers to detect planetary bodies by measuring the dimming of the light of a star due to a passing planetary companion. This technique, known as Transit Photometry, has been successful in detecting now more than 130 planets. Figure 2 shows the schematics of this technique. As an example, the actual light curve of the star HD 209458, the first star for which a planetary transit was detected, is also shown. The transiting planet in this system is a 0.64 Jupiter-mass object in a 3.5-day orbit [4]. In addition to the detection of planets, transit photometry has also enabled astronomers to determine the size, density [5, 6], and in some cases, the chemical elements in the atmospheres of transiting planets [7]. Other detection techniques such as microlensing, where the gravitational fields of a star and its planetary companion create magnifying effects of the light of a background source (Figure 3) [8, 9], transit timing variations method, where the gravitational perturbation of an object creates variations in the time and duration of the transits of a close-in planet [10, 11], and direct imaging (Figures 4 and 5) [12, 13, 14] have also been successful in detecting extrasolar planets. We refer the reader to the extrasolar planets encyclopedia at http://exoplanet.eu, and exoplanet data explorer at http://exoplanets.org/ for more information. To-date the number of detected extrasolar planets exceeds 550. Almost all these planets depict

2It is important to note that the first planets outside of our solar system were discovered around pulsar PSR B 1257+12 by Wolszczan & Frail in 1992 [3]. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 3

Figure 2. Top: Schematic illustration of a planetary transit. Bottom: Actual light curve of HD 209458 caused by the transiting of its 0.64 Jupiter-mass planet [4]. Figure from [4] with the permission of AAS.

physical and dynamical characteristics that are unlike those of the planets in our solar system. While in the solar system, giant planets such as Jupiter and Saturn are in large orbits, and smaller planets such as Earth and Venus are closer in, many extrasolar planetary systems are host to Jupiter-like or larger bodies in orbits smaller than the orbit of Mercury to the Sun. Also, unlike in our solar system where planetary orbits are almost circular, the orbits of many extrasolar planets are considerably elliptical. These unexpected dynamical characteristics of exoplanets have had profound effects on our views of the formation and dynamical evolution of planetary systems. The theories of planet formation, which have been primarily developed to explain the formation of the planets of our solar system, are now constantly revisited and their applicability to exoplanetary bodies are continuously challenged. The complexities of extrasolar planetary systems are not limited to their orbital dynamics. The physical characteristics of many of these objects are also different from the planets of our solar system. While in our solar system, terrestrial and (gas- and ice-) giant planets form two distinct classes of objects with two distinct ranges of masses (giant planets are one to two August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

4 Nader Haghighipour

Figure 3. Detection of a planet using gravitational microlensing. The time sequence can be taken from either left or right. The second panel from left shows the lensing of a background source by a star and its planet. As the planet moves away from the lensing state, the image of the background source is only lensed by the planet-hosting star. Figure courtesy of D. Bennett and Lockheed Martin Space Systems.

orders of magnitude more massive than terrestrial planets), several extrasolar planets have been discovered with masses in an intermediate range from a few Earth-masses to slightly smaller than Uranus. Dubbed as Super-Earths, these objects form a new class of planetary bodies with physical and dynamical characteristics that may be different from those of the terrestrial planets and yet significant for habitability and planet formation theories. This paper presents a review of the physical and dynamical characteristics of these objects. The first super-Earth was discovered by Beaulieu et al. (2006; [9]) using the microlensing technique. To-date, the number of these objects has passed 30. Table 1 shows the masses and orbital elements of these bodies1. Two of the more prominent super-Earths are CoRoT-7b, the 7th planet discovered by CoRoT (COnvection, ROtation and planetary Transits) space telescope with a mass of 2.3-8 Earth-masses [5, 15, 16, 17], and GJ 1214b, the first super-Earth discovered by transit photometry around an M star with a mass of 5.69 Earth-masses [6]. These two objects are the first super-Earths for which the values of mass and radius have been measured (CoRoT-7b: 1.65 Earth-radii, GJ 1214b: 2.7 Earth-radii). This is a major achievement and a great milestone in the field of exoplanetary science which for the first time allows for estimating the density of an extrasolar planet and developing models for its interior dynamics. The semimajor axes of the majority of super-Earths are smaller than 0.2 AU and their ec- centricities range from 0 to 0.4. This orbital diversity, combined with the values of the masses of these objects, has made super-Earths a particularly important class of extrasolar planetary bodies. The larger-than-terrestrial sizes and masses of super-Earths point to the less challenging detection of these objects compared to the detection of Earth-sized planets. They also suggest that super-Earths may have dynamic interiors and be able to develop and maintain moderate

1Note that Table 1 does not include the three terrestrial-class planets around the pulsar PSR 1257+12. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 5

Table 1. Currently known extrasolar planets with masses up to 10 Earth-masses. The quantities M,P,a and e represent the mass (in terms of Earth’s mass M⊕), orbital period, semimajor axis, and orbital eccentricity of the planet. The mass of the central star is shown by M∗ and is given in the units of solar-masses (M⊙).

Planet M(M⊕) P (day) a (AU) e Stellar Type M∗(M⊙) GL 581 e 1.70 3.15 0.03 0.0 M3 0.31 Kepler-11f 2.30 46.69 0.25 0.0 G 0.95 GL 581 g 3.10 36.56 0.15 0.0 M3 0.31 MOA-2007-BLG-192-L b 3.18 0.62 0.06 HD 156668 b 4.16 4.65 0.05 0.0 K2 HD 40307 b 4.19 4.31 0.05 0.0 K2.5 V Kepler-11b 4.30 10.3 0.09 0.0 G 0.95 Kepler-10b 4.56 0.84 0.02 0.0 G 0.89 CoRoT-7b 4.80 0.85 0.02 0.0 K0 V 0.93 61 Vir b 5.08 4.21 0.05 0.12 G5 V 0.95 GL 581 c 5.60 12.92 0.07 0.0 M3 0.31 HD 215497 b 5.40 3.93 K3 V 0.87 OGLE-05-390L b 5.40 3500 2.10 M 0.22 GJ 1214 b 5.69 1.58 0.01 0.27 0.16 GJ 667C b 5.72 7.00 M1.5 GJ 433 b 6.04 7.00 M1.5 Kepler-11d 6.1 22.68 0.159 0 G 0.95 GJ 876 d 6.36 1.94 0.02 0.14 M4 V 0.33 HD 40307 c 6.86 9.62 0.08 0.0 K2.5 V GL 581 d 5.60 66.87 0.22 0.0 M3 0.31 GL 581 f 7.00 433 0.76 0.0 M3 0.31 HD 181433 b 7.56 9.37 0.08 0.39 K3I V 0.78 HD 1461 b 7.59 5.77 0.06 0.14 G0 V 1.08 55 Cnc e 7.63 2.82 0.04 0.07 G8 V 1.03 CoRoT-7c 8.39 3.69 0.05 0.0 K0 V 0.93 Kepler-11e 8.40 31.99 0.19 0.0 G 0.95 HD 285968 b 8.42 8.78 0.07 0.0 M2.5 V 0.49 HD 40307 d 9.15 20.46 0.13 0.0 K2.5 V HD 7924 b 9.22 5.39 0.06 0.17 K0 V 0.83 HD 69830 b 10.49 8.67 0.08 0.1 K0 V 0.86 HD 160691 c 10.56 9.64 0.09 0.17 G3 IV-V 1.08

atmospheres–two conditions that would render super-Earths potentially habitable if their orbits are in the habitable zones of their host stars. Although the close-in orbits of super-Earths pose a challenge to the planet formation theo- ries (many efforts have been made to explain the formation of these objects in close-in orbits, and several models have been developed. However, this issue is still unresolved.), the physical characteristics of these objects, namely their densities, when considered within the context of different formation scenarios, present a potential pathway for differentiating between different planet formation models. In that respect, the study of super-Earths plays an important role in identifying the most viable planet formation mechanism. In this paper, we discuss these issues and review the current state of research on the formation, interior dynamics, and atmospheric evolution of super-Earths. We also review the prospects of the detection of these objects using ground- and space-based telescopes as potential targets for searching for extrasolar habitable planets.

2 Formation of Super-Earths

Planet formation is one of the most outstanding problems in astronomy. Despite centuries of theoretical efforts in explaining the formation of the planets of our solar system, this problem is still unresolved and the formation of planets is still an open question. Although it is widely accepted that planet formation begins by the coagulation of dust particles to larger objects in a circumstellar disk of gas and dust known as nebula, the details of this process are unknown and the formation of giant and terrestrial planets is not fully understood. The issue is even more complicated in extrasolar planetary systems. The current models of planet formation, which have been developed primarily for explaining the formation of the planets August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

6 Nader Haghighipour

Figure 4. Image of the giant planet of Fomalhault by Kalas et al (2008; [12]). The central star is a 2 solar-masses A star at 25 light-years from the Sun. The planet has a mass of approximately 4 Jupiter-masses. It orbits the central star at 115 AU with an eccentricity of 0.11. Figure from [12] with the permission of AAAS.

Figure 5. Planetary system of HR 8799 imaged by Marois et al (2010; [14]). The central star is of spectral type A with a mass of 1.5 solar-masses at a distance of 128 light-years from the Sun. The planets have the masses of Mb = 7MJ , Mc = Md = 10MJ , and Me = (7−10)MJ , with semimajor axes of 68, 38, 24, and 14.5 AU, respectively. Figure from [14] with the permission of NPG.

of our solar system, cannot explain the formation and dynamical diversity of many of extrasolar planets. The unexpected properties of these bodies have raised many questions about the validity of the current theories of planet formation and their applicability to other planetary systems. For instance, many of the currently known extrasolar giant planets have orbits smaller than the orbit of Mercury around the Sun. This is an anomaly that cannot be explained by the current theories of planet formation (as explained below, giant planets are expected to form at large distances). The discovery of these hot Jupiters prompted theoreticians to revisit models of giant planet formation, and attribute the close-in orbits of these objects to their interactions with their surrounding nebulae and their subsequent radial migrations to closer orbits (Figure 6)1.

1The resultant of the gravitational forces that a planet receives from the portions of the nebula interior and exterior to its orbit causes the planet to radially migrate. The migration is classified as type I when the planet is small and does not accrete nebular material (i.e. it does not create a gap in the nebula while migrating). When the planet is large and accretes August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 7

Figure 6. Graphs of the type I (left) and type II (right) planetary migration. In the left panel, the mass of the planet is small and no gap-opening occurs. As the planet grows, it clears its surrounding and a gap gradually appears. Figures courtesy of F. Masset.

We refer the reader to Chambers (2009; [18]) and Armitage (2010; [19]) for a comprehensive review of planet migration and its implications for the formation of planets. The existence of super-Earths is another of such unexpected findings. While in our solar system, planets belong to two distinct categories of terrestrial (i.e. Earth-sized or smaller) and giant (approximately 12 times more massive than Earth and larger), super-Earths, with an intermediate mass-range, introduce a new class of objects. The planet formation theories not only have to now explain the formation of these bodies; in some case, they also have to explain their unusual dynamical properties. This section focuses on the formation of super-Earths. It begins by explaining different mod- els of the formation of giant and terrestrial planets in our solar system, and discusses their applicability to the formation of super-Earth objects.

2.1 Models of Planet Formation Planets are formed in a circumstellar disk of gas and dust by the coagulation of micron-sized dust grains to larger objects. In general, this process proceeds in four stages;

• growth of dust particles to centimeter- and decimeter-sized bodies through gentle hitting and sticking,

• growth of centimeter- and decimeter-sized objects to km-sized planetesimals,

• collisional growth of km-sized planetesimals to the cores of giant planets in the outer regions of the nebula, and to moon- and Mars-sized bodies (known as protoplanets or planetary embryos) in the inner regions, and

• the accretion of gas and formation of giant planets followed by the collisional growth of plan- etary embryos to terrestrial bodies.

material during its migration, a gap will appear and the migration is classified as type II. See figure 6. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

8 Nader Haghighipour

Figure 7. Top: Hit-and-stick collisions lead to fractal growth and subsequently to a narrow size distribution [26]. The collision among dust grains occurs for several reasons including their Brownian motion in the gas. Bottom: Graph of the fractal growth of aggregates during collisions in Brownian motion. The data points are taken from experiments by Krause & Blum (2004; [25]). The solid curve is the analytical model. The time on the horizontal axis is normalized to the collisional timescale for grains. The inset on the upper left corner shows examples of fractal dust aggregates found in the space shuttle experiments by Blum et al (2000; [28]). Figure from [29] with the permission of ARAA.

The first stage of this process is well understood. At this stage, dust grains are strongly coupled to the gas and their dynamics is driven by non-gravitational forces such as radiation pressure, and also by gas drag. Because dust particles follow the motion of the gas, their relative velocities are small. As a result, they slowly approach one another and gently collide. Laboratory experiments and computational simulations have shown that such gentle collisions result in the fractal growth of dust grains to larger aggregates (Figures 7 and 8) [20, 21, 22, 23, 24, 25, 26, 27]. The second stage (i.e. the growth of centimeter-sized objects to kilometer-sized planetesimals) is still a big mystery. The collisions of centimeter- or decimeter-sized bodies with one another do not seem to facilitate the growth of these objects to larger sizes [29, 32]. As dust grains grow, their coupling to the gas weakens (i.e., they move faster in the gas) [31] and they show more of their independent dynamics. When two objects reach several centimeters or decimeters in size, their relative velocities become so large that their collisions may result in breakage and fragmentation August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 9

Figure 8. Molecular-dynamics simulations by Paszun & Dominik (2006; [30]) showing compaction of fractal dust aggregates. The upper left panel shows two aggregates before collision. The merging and compaction of the aggregates are shown in subsequent panels. Figure from [29] with the permission of ARAA.

(e.g., [32]). Known as the centimeter-sized barrier, such disruptive collisions prevent the growth of small bodies to larger sizes. The difficulties do not end here. In the event that some centimeter-sized bodies manage to grow, the subsequent increase in their velocities causes many of them, in particular those with sizes of 1 to several meters, to either collide and shatter each other, or rapidly spiral towards the central star. Known as the meter-sized barrier, these effects deprive the nebula of enough materials to form planets. The puzzling fact is that despite these difficulties, planets do exist and the above-mentioned issues were somehow overcome. Many theoretical models have been developed to solve this puzzle [33, 34, 35]. However, they all have limitations and none has been able to present a complete and comprehensive scenario for the formation of km-sized planetesimals. We refer the reader to articles by Blum & Wurm (2008; [29]) and Chiang & Youdin (2010, [36]) for reviews of the current state of research in this area. At the third stage of planet formation, the situation is different. Here, the interactions among planetesimals are primarily gravitational. Since the protoplanetary disk at this stage is populated by km-sized and larger objects, collisions among these bodies are frequent. In general, frequent collisions in a crowded environment result in low eccentricities and low inclinations which facil- itate the merging and accretion of the colliding bodies. As a planetesimal grows, the influence zone of its gravitational field expands and it attracts more material from its surroundings. In other words, more material will be available for the planetesimal to accrete, and as a result the rate of its growth is enhanced. Known as runaway growth, this process results in the growth of km-sized planetesimals to larger bodies in a short time (Figure 9) [37, 38, 39, 40, 41, 42, 43, 44]. At large distances from the central star (e.g. > 5 AU from the Sun) where the rotational velocities are small, the collisional growth of planetesimals is more efficient. At such distances, planetesimals approach each other with small relative velocities and their impacts are likely to result in accretion. Also, because far from the star, the temperature is low, the bulk material of such planetesimals is primarily ice which increases the efficiency of their sticking at the time of their collision. As a result, in a short time, planetesimals grow to large objects with masses equal to a few masses of Earth. As this process occurs while the nebular gas is still around, growing planetesimals gradually attract gas from their surroundings forming a large body with a thick August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

10 Nader Haghighipour

Figure 9. Snapshots of the evolution of a disk of planetesimals during their collisional growth to larger objects. The left panel shows the formation of a protoplanet after 2 × 105 years. The right panel shows the interaction of protoplanets with planetesimals and the formation of several moon-sized embryos. Figure from [44] with permission.

gaseous envelope and a mass equal to a few hundred Earth-masses. At this state, a gas-giant planet is formed. This mechanism that is known as the core-accretion model is widely accepted as the model of the formation of gas-giant planets in our solar system (Figure 10) [45, 46, 47, 48]. At distances close to the central star, the accretion of planetesimals follows a slightly different path. Similar to the process of the formation of the cores of gas-giant planets, the collisions of planetesimals at this stage may result in their growth to larger bodies. However, the efficiency of planetesimal accretion will not be as high and as a result, instead of forming objects as big as the cores of giant planets, accretion of planetesimals in this region results in the formation of several hundred moon-sized bodies known as planetary embryos. Computational simulations [49] and analytical analysis [50] have shown that when the masses of these embryos reach the lunar-mass, planetesimals can no longer damp their orbits through dynamical friction, and the runaway growth ends. The gravitational perturbation of the resulted planetary embryos affect the dynamics of smaller planetesimals and cause them to collide with one another and/or be scattered to large distances where they may leave the gravitational field of the system. This growth and clearing process continues until terrestrial planets are formed and the smaller remaining bodies (asteroids) are in stable orbits [51, 52, 53, 54, 55, 56, 57, 58]. Figure 11 shows the time evolution of a sample simulation of terrestrial planet formation [56]. The above-mentioned processes, although seemingly straightforward, are extremely complex. The immensity of the nebula, the enormous number of interacting objects, and the complicated August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 11

Figure 10. Graphs of the mass of the giant planet (left panels) and its radius (right panels) for simulations by Lissauer et al (2009; [47]). Each color corresponds to a different value of protoplanet surface density (see [47] for details). The solid line represents the mass of the core, the dotted line shows that of the gaseous envelope, and the dashes-dotted line corresponds to the total mass of the planet. The upper left graph shows the mass-growth only up to 40 Earth-masses in order to show the details of mass-accretion at early stages. The top right panel represents the radius of the planet during this stage. The panel on the bottom left shows the total mass of the planet. As shown here, the giant planet accretes more than 300 Earth-masses in approximately 3 Myr. The bottom right panel shows the total radius of the planet. Figure from [47] with permission.

physics that is involved in their interactions make it impossible for any simulation of planet formation to include all necessary components and to be fully comprehensive. These simulations are also constantly challenged by observations that reveal more characteristics of planet-forming environments. For instance, during the formation of giant planets, the core-accretion model requires the nebular gas to be available as the core of Jupiter grows and accrete gas from its surrounding. The computational simulations presented in the original paper by Pollack et al (1996; [45]) suggest that this time is approximately 10 Myr. In other words, in order for gas- giant planets to form by the core-accretion model, the lifetime of the nebular gas has to be comparable with this time. However, the observational estimates of the lifetimes of disks around young stars suggest a lifetime of 0.1-10 Myr, with 3 Myr being the age for which half of stars show evidence of disks [59, 60, 61, 62]. Any model of gas-giant planet formation has to be able to form these objects in less than approximately 3 Myr. Additionally, the simulations of the core-accretion model suggest that the core of Jupiter grows to ∼ 10 Earth-masses. However, computational modeling of the interior of Jupiter and Saturn point to values ranging from 0 to as large as 14 Earth-masses [63, 64]. It is unclear what the actual masses of the cores of our gas-giant planets are, and if smaller than 10 Earth-masses, how they accumulated their thick envelopes in a short time. We refer the reader to a review article by Guillot (2005; [63]) for more details. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

12 Nader Haghighipour

Figure 11. Radial mixing of planetesimals and planetary embryos, and the formation of terrestrial planets. A Jupiter-sized planet, not shown in the figure, is at 5.5 AU. The size of each object has been scaled with its mass assuming that it is a perfect sphere. The color of each object corresponds to its water content (water/mass fraction). Red represents dry, light green represents 1% water, and blue corresponds to 5% water content. The black circle in the middle of each object shows its solid core. Figure from [56] with permission.

Several efforts have been made to overcome these difficulties. As shown by Hubickyj et al (2005; [46]) and Lissauer et al (2009; [47]), increasing the surface density of the nebula to higher than that suggested by Pollack et al (1996; [45]) significantly reduces the time of the giant planet formation (Figure 10). Furthermore, an improved treatment of the grain physics as given by Podolak (2003; [65]), Movshovitz & Podolak (2008; [66]), and Movshovitz et al (2010; [48]) indicates that the value of the grain opacity in the envelope of the growing Jupiter in the original core-accretion model [45] is too high, and a lower value has to be adopted. This lower opacity has led to a revised version of the core-accretion model in which the time of giant planet formation is considerably smaller [46, 48]. Most recently, Bromley & Kenyon [67] have developed a new hybrid N-body-coagulation code which enables this authors to form Saturn- and Jupiter-sized planets in approximately 1 Myr. An alternative mechanism, known as the disk instability model, addresses this issue by propos- ing rapid formation of giant planets in a gravitationally unstable nebula [68, 69, 70, 71, 72, 73, 74, 75, 76, 77]. In this model, local gravitational instabilities in the solar nebula may result in the fragmentation of the disk to massive clumps which subsequently contract and form gas- giant planets in a short time (Figure 12). Calculations by Boss (2000; [68, 69]) and Mayer et al (2002-4; [70, 72]) show that an unstable disk can break up into giant gaseous protoplanets in approximately 1000 years. Although this mechanism presents a fast track to the formation of a gas-giant planet, it suffers from the lack of an efficient cooling process necessary to take energy away from a planet-forming clump in a sufficiently short time before it disperses.

2.2 Application to the Formation of Super-Earths As explained above, the current models of giant and terrestrial planet formation have been devel- oped to explain the formation of the planets in our solar system. Since there are no super-Earths August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 13

Figure 12. Snapshots of the evolution of a protoplanetary disk in the disk instability model. As shown here, while the disk evolves, spiral arms appear where the density of the gas is locally enhanced (bright colors correspond to high densities), and clumps are formed. Figure from [70] with the permission of AAAS.

around the Sun, it may not be obvious whether these models can also explain the formation of these objects. However, a deeper look at the ranges of the masses and sizes of these bodies suggests that super-Earths might have formed in the same way as gas-giant planets. The key is in the intermediate range of the masses of these objects. With masses ranging from a few Earth-masses to slightly smaller than Uranus, super-Earths are basically as massive as the cores of gas-giant planets. In fact some researchers consider super-Earths as giant planets’ “failed cores”. We recall that according to the core-accretion model and the simulations of the interior of Jupiter, this planet may have a core with a mass between zero and 14 Earth-masses [63, 64]. The extent to which the current models of giant planet formation can be used to explain the formation of super-Earths varies from one system to another. The diversity of the currently known super-Earth planetary systems, both in spectral types of their host stars and the orbital dynamics of their planets (see Table 1), suggests that while in some systems (e.g., around M stars) super-Earths might have formed in-place [78, 79, 80, 81, 82, 83], in other systems (e.g., around G stars) the formation of these objects might have occurred while their orbital elements were evolving [81, 82, 84]. In such systems, the larger than terrestrial masses of super-Earths, combined with the fact that many of these objects are in short-period orbits, points to a formation scenario in which super-Earths were formed at large distances (where more material was available for their growth) and either were scattered to their current orbits as a result of interactions with other cores and/or planets [84], or migrated to their current locations as they interacted with the protoplanetary disk [82]. This mechanism naturally favors the core-accretion model of gas- August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

14 Nader Haghighipour

Figure 13. Graph of the core and envelope of a planet at different distances in a disk around a 0.4 solar-mass star. Black circles represent cores and the large circles are their envelopes. The mass of the core (Mc) is given in Earth-masses and its radius (Rc) is in centimeters. The total mass and radius of the planet are shown by Mt and Rt, respectively. As shown here, in about 100 Myr, the planet’s envelope contracts and the planet reaches its final size. Figure from [78] with the permission of AAS.

giant planet formation, although attempts have also been made to explain the formation of super-Earths via the disk instability scenario [85].

2.2.1 Formation of Super-Earths Around Low-Mass Stars: Core-Accretion To determine whether super-Earths can form in-place around low-mass stars, Laughlin et al (2004; [78]) simulated giant planet formation through the core-accretion model in disks around stars with masses smaller than 0.5 solar-masses. These authors showed that around M stars, this mechanism produces planets ranging from terrestrial-class to Neptune-sized (Figure 13). The results of their simulations also indicated that the lower-than-solar masses of M stars (typ- ically smaller than 0.4 solar-masses), which implies low masses and surface densities for their circumstellar disks as well, results in less frequent collisions among planetesimals and planetary embryos, and prolongs the growth of these objects to larger sizes. Consequently, the time of the core growth around M stars will be several times longer than the time of the formation of Jupiter around the Sun. During this time, the gaseous component of the circumstellar disk is dispersed, leaving the slowly growing core with much less gas to accrete. The short lifetime of the gas in circumstellar disks around M stars can be attributed to two factors: 1) the high internal radiation of young M stars (these stars are almost as bright as solar- type stars), and 2) external perturbations from other close-by stars. Since most stars are formed in clusters [86], their circumstellar disks are strongly affected by the gravitational perturbations and the radiations of other stars [87]. For M stars, this causes the circumstellar disk to receive high amount of radiation from both the central star and external sources. The high amounts of radiation combined with the low masses of M stars, which points to their small gravitational fields, increases the effectiveness of the photo-evaporation of the gaseous component of the circumstellar disk by up to two orders of magnitude. As a result, the majority of the gas leaves the disk at the early stage of giant planet formation. The slow growth of planetary embryos around M stars and the rapid dispersal of the nebular gas suggest that no giant planet should exist in these systems and the planets around M stars August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 15

Figure 14. Top: The graph of the time evolution of the surface density of a disk around a 0.25 solar-mass star. The initial mass of the disk is 0.065 times the mass of the central star. Each curve on this graph corresponds to the disk evolution at that fixed radius. Bottom: The graph of the mass of a planetary embryo at the indicated distance during the evolution of the disk. As shown by the two panels, the inward migration of snow line increases ice condensation which in turn results in an increase in the disk surface density, and formation of larger objects. The latter is more pronounced in the region between 2 and 8 AU. Figure from [79] with the permission of AAS.

have to be mainly super-Earths or smaller. While the observational evidence is in agreement with the latter (e.g., the M star GL 581 is host to 4-6 planets with masses similar to that of Neptune and smaller), it does not support the first suggestion. Several giant planets have in fact been discovered around M stars among which one can name GJ 876 with two planets with masses of 0.6 and 1.9 Jupiter-masses on 30-day and 60-day orbits [88], and HIP 57050 with a Saturn-mass planet in its habitable zone [89].

Effect of Stellar Evolution The above-mentioned simulations do not take the effect of stellar evolution into account. As opposed to young solar-type stars whose luminosities stay almost constant during the formation August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

16 Nader Haghighipour

of a planet (e.g. 10 to 100 Myr), the luminosity of a pre-main sequence low-mass star (e.g., 0.5 solar-masses) fades by a factor of 10 to 100 during this time [90]. As a result, the internal temperature of the circumstellar disk will decrease which causes the region known as snow line (or ice condensation limit, the region beyond which water is in the permanent state of ice) to move to close distances. The inward migration of the snow line results in an increase in the population of icy materials (km-sized and larger planetesimals), which in turn increases the efficiency of the collisional growth of these objects to protoplanetary bodies (we recall that as mentioned in section 2.1, sticking is more efficient among icy bodies). As shown by Kennedy et al (2006; [79]), around a 0.25 solar-mass star, the moving snow line causes rapid formation of planetary embryos within a few million years. Subsequent collisions and interactions among these objects result in the formation of super-Earths in approximately 50–500 million years (Figure 14).

Effect of Planet Migration A common feature among the formation scenarios mentioned above is the implicit assumption that planets are formed in-place. Although the post-formation migration has been presented as a mechanism for explaining the close-in orbits of super-Earths, these scenarios do not include the effect of the possible migration of still-forming planets (for instance, at the stage when cores of giant planets are forming) on the collisional growth of protoplanetary bodies. They also do not consider the possibility of the migration of planetary embryos during the accretion of these objects. However, studies of the interactions of disks and planets have made it certain that planet migration occurs and has profound effects on the formation of planetary systems and the final assembly of their planets and smaller constituents. In our solar system, planetary and satellite migration has long been recognized as a major contributor to the formation and orbital architecture of planets, their moons, and other minor bodies. For instance, as shown by Greenberg (1972-3; [91, 92]), mean-motion resonances (i.e., commensurable orbital periods1) among the natural satellites of giant planets (e.g., Titan and Hyperion, satellites of Saturn) might have been the results of the radial migration of these objects due to their tidal interactions with their parent planets [93]. Similarly, the orbital architecture of Galilean satellites and their capture in a three-body resonance has been attributed to the migration of these objects first during their formation while interacting with Jupiter’s circum- planetary disk of satellitesimals [94], and subsequently by tidal forces after their formation [95]. The lack of irregular satellites between Callisto, the outermost Galilean satellite, and Themisto, the innermost irregular satellite of Jupiter can also be explained by a dynamical clearing process that occurred during the formation and migration of Galilean satellites [96]. The idea of the migration of planetary bodies was first proposed by Fernandez & Ip (1984; [97]). These authors suggested that after the dispersal of the nebular gas, giant planets may drift from their original orbits due to the exchange of angular momentum with the planetesimal debris disk, and scatter these objects to other regions of the solar system. This idea was later utilized by Malhotra (1993-5; [98, 99]) to explain the peculiar (highly eccentric, inclined, and long-term chaotic) orbit of Pluto, and by Malhotra (1996; [100]), and Hahn & Malhotra (2005; [101]) to explain the dynamical structure of Kuiper belt objects. Planetary migration has been used extensively to explain the existence of close-in Jupiter-like planets. In fact, it was the detection of the first hot Jupiter around the star 51 Pegasi [1] that prompted scientists to revisit theories of planet migration in our solar system, and apply them to extrasolar planets. At present, planet migration is well-developed and widely accepted as part of a comprehensive formation mechanism. As mentioned in the Introduction, depending on the physical and dynamical characteristics of planets and their circumstellar disks, migration occurs

1It is necessary to emphasize that orbital commensurability is necessary for two planets to be in a mean-motion resonance, however it is not sufficient. Other constraints have to exist between the angular elements of their orbits as well. For more details, the reader is referred to books on celestial mechanics. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 17

Figure 15. Graph of the formation of terrestrial planets during the migration of a Jupiter-sized body. Colors indicate water content as in figure 7. Simulations include gas drag as well. As shown here, while the giant planet migrates inwards, many protoplanetary bodies are either scattered out of the system, or their eccentricities are lowered due to the gas drag, and they stay in orbits at larger distances. The collisions among the latter embryos may result in the formation of terrestrial planets beyond the orbit of the giant body. Figure from [114] with the permission of AAS.

in different forms (e.g., Type I and Type II, see Figure 6). Numerous articles have been published on this subject which unfortunately makes it impossible to cite them all here. We refer the reader to papers by Nelson et al (2001;[102]), Masset & Snellgrove (2001; [103]), Papaloizou & Terquem (2006; [104]) and articles by Chambers (2009; [18]) and Armitage (2010; [19]) for a review on this topic and the effects of planet migration on the formation and dynamical evolution of planetary systems. The contribution of planet migration to the formation of close-in super-Earths may appear in different ways. A fully formed migrating giant planet affects the dynamics of interior protoplan- etary bodies by either increasing their orbital eccentricities and scattering them to larger dis- tances, or causing them to migrate to closer orbits. The migrated protoplanets may be shepherded by the giant planet into small close-in regions where they are captured in mean-motion reso- nances. As shown by Zhou et al (2005; [105]), Fogg & Nelson (2005-9; [106, 107, 108, 109, 110]), and Raymond et al (2008; [111]), around Sun-like stars, the shepherded protoplanets may also collide and grow to terrestrial-class and super-Earth objects. Also, as shown by Mandell & Sig- urdsson (2003; [112]), Raymond et al (2006; [113]), and Mandell et al (2007; [114]), in more massive protoplanetary disks around such stars, the out-scattered protoplanets may collide and grow to planetary-sized bodies (Figure 15). August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

18 Nader Haghighipour

Recent simulations by Haghighipour & Rastegar (2010; [115]) have shown that such accretion of protoplanets during giant planet migration may not be efficient around low-mass stars. Sim- ulating the dynamics of protoplanetary bodies at distances smaller than 0.2 AU around a 0.3 solar-mass star, these authors have shown that during the inward migration of one or several giant planets (the latter involves migrating planets in mean-motion resonances), the majority of the protoplanets leave the system and do not contribute to the formation of close-in Earth-sized and/or super-Earth bodies. Their results suggest that the currently known small planets around M stars might have formed at larger distances and were either scattered to their current close-in orbits, or migrated into their orbits while captured in mean-motion resonance with a migrating planet. In a protoplanetary disk, the interactions among cores of giant planets and planetary embryos may also result in the inward migration of the latter objects. While migrating, orbital crossing and collisional merging of these bodies may result in their growth to a few super-Earths, espe- cially in mean-motion resonances. Simulating the interactions of 25 protoplanetary objects with masses ranging from 0.1 to 1 Earth-masses, Terquem & Papaloizou (2007; [84]) have shown that a few close-in super-Earths may form in this way with masses up to 12 Earth-masses. The results of the simulations by these authors suggest that in systems where merging of migrating cores results in the formation of super-Earth and Neptune-like planets, such planets will always be accompanied by giant bodies and most likely in mean-motion resonances. Similar results have also been reported by Haghighipour & Rastegar (2010; [115]). Interestingly, unlike the scenarios explained above, there are several planetary systems that host small Naptune-sized objects and super-Earths but do not harbor giant planets (e.g., HD 69830, GL 581). The planets in these systems do not have a Jupiter-like companion that might have facilitated their formation. Such systems seem to imply that a different mechanism may be responsible for the formation of their super-Earth objects. Kennedy & Kenyon (2008; [82]) and Kenyon & Bromley (2009; [116]) suggested that the migration of protoplanetary embryos may be the key in facilitating the close-in accretion of these bodies. Similar to giant planets, planetary embryos can also undergo migration. Simulating the growth of planetary embryos in a circumstellar disk with a density enhancement at the region of its snow line, these authors have shown that during the collision and growth of planetary embryos, many of these objects may migrate towards the central star. Around a solar-type star, the time of such migrations for an Earth-sized planet at 1 AU is approximately 105–106 years–much smaller than the time of the chaotic growth of a typical moon- to Mars-sized embryos (108 years) [50]. This implies that most of the migration occurs prior to the onset of the final growth. Depending on their relative velocities, the interaction of the migrated embryos may result in the growth, scattering, and shepherding, as in the case of a migrating giant planet. Simulations by Kennedy & Kenyon (2008; [82]) and Kenyon & Bromley (2009; [116]) have shown that super-Earth objects with masses up to 8 Earth-masses may form in this way around stars ranging from 0.25 to 2 solar- masses (Figure 16).

2.2.2 Formation of Super-Earths Around Low-Mass Stars: Disk Instability As explained before, given the low masses of the circumstellar disks around M stars, the existence of giant planets around these stars suggests that they might have formed at large distances and migrated to their current orbits. This assumption is based on the fact that in a planet-forming nebula, more material is available at outer regions which can then facilitate the formation of a giant planet through the core-accretion model. The availability of more mass at the outer distances in a disk prompted researchers to look into the possibility of explaining the formation of super-Earths around M stars through the disk instability model. Recall that in this scenario, clumps, formed in an unstable gaseous disk, collapse and form gas-giant planets (e.g. [68, 70]). After the giant planets are formed, a secondary process is needed to remove their gaseous envelopes. Simulations by Boss (2006; [85]) have shown that such collapsing clumps August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 19

Figure 16. Migration of planetary embryos around a (from top to bottom) 0.25, 0.5, and 1 solar-mass star. The outer planets were initially 2, 3.2, and 6 Earth-masses, respectively. As the embryos migrate, they collide and grow to larger objects. The numbers next to the lines in each panel correspond to the final mass of that body in Earth-masses. Figure from [82] with the permission of AAS.

can form around a 0.5 solar-mass star at a distance of approximately 8 AU. This author sug- gests that, as most stars are formed in clusters and in high-mass star forming regions, intense FUV/EUV radiations from near-by O stars may rapidly (within 1 Myr) photo-evaporate the gaseous envelope around giant planets, leaving them with large super-Earth cores (Figure 17). Similar mechanism has been suggested for the formation of Uranus and Neptune in our solar system [117]. A subsequent migration, similar to that suggested by Michael et al (2011, [118]), may then move these cores to close-in orbits. The above-mentioned combination of the disk-instability and gas photo-evaporation presents August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

20 Nader Haghighipour

Figure 17. Formation of clumps (shaded regions) in a gravitationally unstable disk around a 0.5 solar-mass star, after 215 years. The outer edge of the disk is at 20 AU and its inner radius is 4 AU. The clumps’ densities are larger than 10−10 g/cm−3. Figure from [85] with the permission of AAS.

a possible scenario for the formation of super-Earths at large distances and their migration to their closer orbits. However, this mechanism does not seem to be able to explain the formation of the close-in 7.5 Earth-masses planet of the M star GJ 876 and its current 2-day orbit. According to the disk instability model, this object has to have 1) formed at a large distance where it also developed a gaseous envelop, 2) migrated inwards while its atmosphere was photo-evaporated, and finally 3) switched orbits with the two giant planets of the system–a scenario that (without switching orbits) may be applicable to the formation of the recently detected outermost super- Earth planet of this system [88], but is very unlikely to have happened to the innermost body.

As evident from the review presented in this section, it is generally accepted that super-Earths are formed through a combination of a core accumulation process and planetary migration. Modeling the formation of these objects requires the simulation of the collisional growth of planetary embryos, and their subsequent interactions with the protoplanetary disk. A realistic model requires a global treatment of the disk and inclusion of a large number of planetesimals and planetary embryos. In practice, such simulations are computationally expensive. To avoid such complications, most of the current models of super-Earth formation include only small numbers of objects (cores, progenitors, protoplanets, planetesimals, etc.). Recently McNeil & Nelson (2010, [119]) have shown that in systems with a large number of bodies (e.g. several thousand planetesimals and larger objects), the combination of the traditional core-accretion and type-I planet migration may not produce objects larger than 3-4 Earth-masses in close-in (e.g., ≤ 0.5 AU) orbits. Although the systems studies by these authors carry some simplifying assumption, their results point to an interesting conclusion: while the combination of core- accretion and planet migration seems to be a viable mechanism for the formation of close-in super-Earths, the formation of these objects is still an open question, and a comprehensive theory for their formation requires more sophisticated computational modeling. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 21

3 Habitability of Super-Earths

An important characteristic of super-Earths that differentiates them from other planetary bodies (i.e., terrestrial and giant planets) is the masses of these objects. The larger-than-terrestrial masses of these planets imply that super-Earths have the capability of developing and retaining atmospheres, and may also be able to have a dynamic interior. As super-Earths are formed (or dynamically evolved) in a region of a protoplanetary disk where the gas has a short lifetime, the amount of the gas accreted by these objects, or trapped in their interiors when they were fully formed, is much smaller than those of gas-giant planets. It is therefore natural to expect super- Earths to have thin to moderately thick atmospheres (e.g., see [120] and [121] for developments on modeling the atmosphere of super-Earth GJ 1214b, and its observational constraints). Around small and cool stars such as M dwarfs, where the liquid water habitable zone is at close distances, the thin to moderate atmospheres of close-in super-Earths and their probable dynamic interior make these objects prime candidates for habitability. Such close-in habitable super-Earths are potentially detectable by both the ground- and space-based telescopes. In this section, these unique characteristics of super-Earths are discussed in more detail. It is important to note that the notion of habitability is defined based on the life as we know it. Since Earth is the only habitable planet known to humankind, the orbital and physical characteristics of Earth are used to define a habitable planet. In other words, habitability is the characteristic of an environment which has similar properties as those of Earth, and the capability of developing and sustaining Earthly life. The statement above implies that the fact that the only habitable planet we know is Earth has strongly biased our understanding of the conditions required for life. From the astronomers’ point of view, and owing to the essential role that water plays on life on Earth, the definition of a habitable planet is tied to the presence of liquid water. However as simple this definition might be, it has strong connections to a variety of complex interdependent processes that need to be unraveled and understood to make predictions on which planets could be habitable. The basic principle is that the surface temperature and pressure of a planet should allow for liquid water. This is determined by the amount of irradiation that the planet receives from the star, and the response of the planet’s atmosphere. The latter delicately depends on the composition of the planet, and that in turn determines the heat transport mechanism, cloud presence, and many other atmospheric properties. The irradiation from the star is contingent on the type of the star and the planet’s orbital parameters. The atmospheric composition, on the other hand, depends on the in-gassing, out- gassing, and escape histories of the planet. The in-gassing and out-gassing accounts are intrin- sically connected to the interior dynamics of the planet, while atmospheric escape is related to a variety of thermal and non-thermal processes, which themselves are linked to the presence of a magnetic field. It is not clear how delicate the balance between these different processes could be. Nor is it evident if there are different pathways that could yield a habitable planet. However, the fact that Earth has succeeded in developing life indicates that our planet might have followed one, perhaps of many evolutionary paths that resulted naturally in a complex system by the series of steps and bifurcations that it encountered. It is important to note that the complexity and interdependence of these processes cannot be taken as evidence for the uniqueness of life on Earth. The road ahead is to understand which planetary characteristics are indispensable, which are facilitating, and which are a byproduct of evolution. For that purpose, and in order to assess the possibility that a planet (e.g., a super-Earth) may be habitable, a deep understanding of these processes (i.e. interior composition and dynamics, planet’s magnetic field, and atmospheric characteristics) is required. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

22 Nader Haghighipour

Figure 18. Graph of the mass-radius relationship for extrasolar planets OGLE-TR-10b [123], HD 209458b [125], OGLE-TR- 56b [126], OGLE-TR-132b [127], OGLE-TR-111b [128], OGLE-TR-113b [129, 130], TrES-1 [131]. The dotted lines represent curves of constant densities. Jupiter and Saturn are also shown. Figure from [123] with the permission of AAS.

3.1 Interior There are at least three aspects of the interior of a super-Earth that relate to its possible habitability: its composition, the manifestation of plate tectonics, and the presence of a magnetic field. 3.1.1 Composition As water is the most essential element for habitability, one might expect that it will have a significant contribution to the total mass of a habitable planet. However, on Earth, water constitutes less than 0.1% of Earth’s mass which places Earth among the rocky planets of our solar system. This suggests, in order to study the habitability of extrasolar planets, it is important to distinguish planetary type, and identify rocky planets with some liquid water. Unfortunately at the moment, the type of data available to infer the composition of extrasolar planets is limited. The first generation of data comprises masses and radii obtained from radial velocity and transit photometry searches. Neither of these quantities alone can lead to a definitive determination of the composition of a planet. However, for those planets whose masses and radii are known, a relationship between these quantities (known as the mass-radius relationship) can help to gain an insight on the possible materials that contributed to the formation of these objects. Among the currently known extrasolar planets, the knowledge of both mass and radius is limited to only a small number of these bodies. The majority of these planets are Jupiter-like with masses larger than 0.3 Jupiter-masses. To the zeroth order of approximation, one can assume that these planets are perfectly spherical and have uniform interiors1. The mass-radius relationship in this case will be of the simple form R ∼ M 1/3. Figure 18 shows a few of these extrasolar planets in a mass-radius diagram [see also figure 4 of Seager et al (2007, [122])]. The

1The mass and radius of a spherical body with uniform density ρ are related as M = (4π/3)ρR3 , where R is the radius and M is the mass of the object. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 23

Figure 19. Left: Graph of the density of planets with masses ranging from 1 to 10 Earth-masses. The core of each planet constitutes 32.59% of its mass. Each planet has 10% iron in its mantle and 30% Ferromagnesiowustite in its lower mantle. Right: Mass-radius power law for planets with ten different mineral compositions. The star denotes Earth. Note that the x-axis is logarithmic. See [132] for more details. Figure from [133] with permission.

masses of these objects are in the range of 0.5 to 1.5 Jupiter-masses. The figure shows some curves of constant density as well. As shown here, the assumption of a uniform interior places these objects close to the curves of constant density ranging from 0.4 to 1.3 g cm−3 [123]. As a point of comparison, Jupiter and Saturn are also shown. For more details, we refer the reader to the paper by Seager et al (2007, [122]) on the mass-radius relationship in massive extrasolar planets, and to the article by Sotin et al (2007, [124]) where these authors discuss the mass-radius relationship of ocean planets. Despite the apparent agreement between the values of the densities of the giant planets in Figure 18 and the assumption of a uniform interior, this assumption is not valid for super-Earth objects. As known from Earth, the large amount of internal pressure in terrestrial planets1 causes the interiors of these objects to not have a uniform composition. As a result, the mass- radius relationship for these planets deviates from the 1/3 power-law. Valencia et al (2006, 2007; [132, 133]) studied these deviations for objects with masses of 1 to 10 Earth-masses. Scaling Earth to larger sizes and assuming a layered structure with different values of density, temperature, and pressure for each layer, these authors modeled the composition of super-Earths by integrating the equation of state of each layer for different combinations of components such as iron, silicate, magnesium, alloys, and water. The results of their simulations show that super-Earths may be mainly composed of iron cores, silicate mantles, and water/ices (H2O and ammonia, methane in minor proportions). These authors suggested that the mass-radius relationship for super-Earths may be of the form R ∼ A(R) M β where the coefficient A(R) has different values for different compositions, and the exponent β varies in a small range between 0.262 and 0.274 (Figure 19). Although the results of the simulations by Valencia et al (2006, 2007; [132, 133]) portray a general picture of the components of which super-Earths might have formed, the mere knowledge of the mass and radius of these objects is not sufficient to determine their actual compositions. Since the above-mentioned mass-ratio relation is model-dependent, many combinations of dif- ferent components may result in the same mass and radius. In other words, the mass-radius relationship suffers from a degeneracy that stems from the existing trade-offs between compo- nents with different densities (iron cores, silicate mantles, water/icy layers, hydrogen envelope) [134]. This degeneracy does not allow for the definite determination of the composition of super- Earths. Despite this degeneracy, it may still be possible to attribute a set of probable compositions to a super-Earth once its radius is determined from observation. Integrating the equation of state

1In case of super-Earths, this pressure may amount to approximately 60 Mbars August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

24 Nader Haghighipour

Figure 20. Ternary Diagrams for a 5 Earth-mass planet (e.g., GL 581 c) showing its possible compositions and different radii. Each diagram has three axes indicating the amount of water, iron (core), and silicate (mantle). Each point inside a diagram corresponds to a unique combination of the three end-members. The vertices correspond to a 100% composition, and the opposite line corresponds to 0% of a particular component. A few examples are shown in the top panel. Many different combinations of the three end-member components for a given mass can have the same radius. The bottom panel shows this in more detail. Labels on isoradius curves are radii in km. A ternary diagram exists for each value of planetary mass. The color bar spans the range of sizes of rocky and icy super-Earths from 1 Earth-mass pure iron planet (of 5000 km), to a 10 Earth-masses snowball planet (16000 km). Figure from [135] with the permission of AAS.

for different combinations of silicate, iron, and water, and for different values of the radius of a super-Earth with a known mass, Valencia et al (2007; [135]) have developed an archive that can be used for this purpose. Figure 20 shows the results of one of their simulations. Known as a ternary diagram, each panel of this figure shows the connection between different combinations of a 5 Earth-mass super-Earth and its radius. Each vertex of a triangle represents an object with a 100% composition of the vertex’s material. Each side depicts the amount of the two components on its two vertices that compose the planet. A point inside the triangle uniquely specifies a composition and its corresponding radius. As shown by the bottom panel, super-Earths with August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 25

30000 Uranus

Neptune 20000

planet icate m] sil Mg- -like arth d E e ntiate th-lik 12000 CoRoT-7b iffere Ear ius [k Und

Rad 10000 63% core %, lanet tle 37 Iron p te man 8000 silica Mg-

6000

1 2 3 4 5 6 8 10 15 20

Mass [MEarth]

Figure 21. Mass-Radius relations for rocky super-Earths. The shaded area shows the data for CoRoT-7b. Five different compositions are shown. Orange: a Mg-Si planet (or super-Moon planet) has little or no content of iron; Green: an Earth- like composition (differentiated planet with 33% core, 67% mantle, and 0.1 molar iron fraction in the mantle); Blue: undifferentiated planet with the same elemental bulk composition as Earth (Fe/Si = 2); Pink: A super-Mercury composition; Black: a pure iron composition. The pure Mg-Si and pure Fe compositions correspond to the lightest and densest rocky compositions, although they are highly unlikely given that Mg, Si, and Fe condense at similar temperatures. Compositions with increasing amounts of iron lie progressively below the Mg-Si planet composition. Any mass-radius combination that lies above the Mg-Si planet line necessarily implies a volatile content. Figure from [136] with permission from A&A.

similar masses but different compositions may have identical radii. Using a ternary diagram, it is possible to identify the extreme sizes that a planet might have. For instance, from Figure 20, the maximum value of the radius of a 5 Earth-masses super-Earth corresponds to a planet that is formed entirely of pure ice and water (left corner of the ternary diagram). A radius larger than that of such a snowball planet would indicate the presence of an atmosphere which could probably be made of hydrogen/helium. The minimum value of the radius of a super-Earth, on the other hand, corresponds to a planet that is made of pure iron or heavy alloys (right corner of the ternary diagram). There is also a maximum size for a rocky planet corresponding to a pure silicate composition devoid of an iron core. Any radius above this critical size would indicate the presence of volatiles. By volatiles we refer to water and other ices (ammonia, methane), as well as hydrogen and helium. The progression of the radius from the dry side (mantle-core connection) to the wet side suggests that for a given planet, there is a threshold radius beyond which the planet contains a substantial amount of water (e.g., an ocean planet). This threshold corresponds to the largest isoradius curve that intersects the terrestrial side of the ternary diagram. For a 5 Earth-mass super-Earth, as shown in the lower panel of Figure 20, this radius is equal to 10400 km (not shown in the figure) [135]. As mentioned earlier, in order to obtain an insight into possible scenarios for the composition of a super-Earth, the values of its mass and radius have to be known. Among the currently known super-Earths, CoRoT-7b [5] and GJ 1214b [6] are two planets for which these values have been determined. The knowledge of the orbital elements and mass-radius of these planets has made it possible to obtain a better understanding of the compositions of these bodies. For instance, as shown by Valencia et al (2010; [136]), and following the numerical modeling of Valencia et al (2006, 2007; [132, 133]), the best fit to the size and mass of CoRoT-7b points to a composition with 3% water vapor above a rocky interior. Within a one-sigma uncertainty, the composition could range from at most 10% vapor to an Earth-like composition with 67% silicate mantle and 33% iron core (Figure 21, also see Swift et al 2010 [137]). In addition, given its proximity to its Sun-like star, CoRoT-7b is highly irradiated. Such strong irradiation causes significant atmospheric and mass loss [138]. Given that the evaporating flow of an exoplanet has August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

26 Nader Haghighipour

Figure 22. Artistic illustration of conduction cells in Earth’s mantle.

already been observed as in the case of the transiting planet HD 209458b [139], in the future, it might be possible to detect the nature of the evaporating flow of CoRoT-7b as either silicate- or vapor-based, since the limiting factor is not the size of the planet but the star brightness.

3.1.2 Plate Tectonics Given the similarity between the formation of super-Earths and terrestrial planets, it is ex- pected that these objects are formed hot, and have hot interiors. As in Earth, the internal heat of these bodies may be due to the radioactivity of unstable elements, as well as processes such as the impact of planetesimals and planetary embryos during the formation of these objects, their gravitational contraction during their accretional growth, and frictional heating due to the settling of heavy elements in their cores (the differentiation process). Although the actual composition of super-Earths is unknown, those that are potentially hab- itable are expected to be mainly made of rocks. As the surface layers of these planets cool from above and form a crust, the heat generated by the above-mentioned processes will be trapped inside and produce large convection cells within the mantles of these bodies1. These convection cells cycle hot material through the mantle and gradually cool the planet. The cooling of the mantle through convection also controls the cooling of the core. Figure 22 shows an artistic conception of this process. Convection may operate in two different modes: mobile and non-mobile. In the non-mobile mode, the material cycled by convection cells forms a rigid and immobile layer at the surface of the mantle known as stagnant-lid [140, 141, 142]. During stagnant-lid convection, the surface plate thickens with time and acts as a lid to the interior. Compared to the mobile mode, the stagnant-lid regime is an inefficient mode of cooling a planet. The interior heat in this mode may transfer out through volcanism as in the moon of Jupiter, Io [142], or may gradually be trans- ported by the lid through conduction. In the mobile mode, on the other hand, the lithosphere or plate actively participates in convection by being formed at mid-ocean ridges and subducted at trenches. Known as plate tectonics, this mechanism allows for a more efficient heat and chem- ical transport from the interior to the surface of the planet (Figure 22). The recently suggested remote detection of volcanism on exoplanets by Kaltenegger et al (2010, [143]) may be useful in identifying possible modes of cooling of a planet and to determine if the planet undergoes plate tectonics.

1The internal heat of a planet is transported out through both conduction and convection. However, because rock is a poor conductor, the amount of the heat transported through conduction is negligibly small. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 27

The mode of convection (stagnant-lid with volcanic activity like Io or without profuse vol- canism vs. plate tectonics) has a profound effect on the thermal evolution (and consequently the habitability) of a rocky planet. Among the terrestrial planets in our solar system, Earth is the only one with an active plate tectonics. While Venus has a similar mass, its heat has been transported out through a stagnant-lid process for at least 500 Myr. Mars, with its small size, also has stagnant-lid convection, although it might have had plate tectonics sometime in the past [140]. The reason for plate tectonics on Earth and not on other solar system objects is still under debate. It is widely accepted that this mechanism has played an important role in the geophysical evolution of our planet, and is associated with its geochemical cycles. As a result, plate tectonics has been recognized as an important mechanism for the habitability of Earth [144]. However, whether this process exists (or should exist) in any habitable planet is unknown. Although it seems natural to assume that, similar to Earth, any habitable planet has to have a dynamic interior and maintain plate tectonics, it is not clear whether that is entirely true. In regard to super-Earths, as explained below, the matter is even more complicated. The subject of plate tectonics on rocky super-Earths is controversial. Much research has been done on this topic and depending on the approach to mantle convection, results point to two different schools of thought: favoring plate tectonics based on scaling mantle convection in Earth to larger planets, and favoring a stagnant-lid regime based on numerical modeling of convection in the mantles of super-Earth objects. On the scaling mantle convection, Valencia et al (2007, 2009; [145, 146]) proposed that massive terrestrial planets would have more favorable conditions for subduction, which is an essential part of plate tectonics. In their model, these authors used a parameterized convection scheme described in terms of the surface heat flux, and included the effects of compression in the structure parameters (mantle thickness, gravity, etc). They concluded that while faults’ strength increases with mass, the convective stresses increase even more, so that deformation can happen more easily in massive planets. This is due to the canceling effect between increasing the gravity and decreasing the thickness of the plate which causes the pressure-temperature regime of the plate to be almost invariant with size. Valencia et al. (2007, 2009; [145, 146]) also suggested that unlike small planets such as Earth, where plate tectonics would depend on the presence of water, larger terrestrial planets have sufficiently large convective stresses and would not need weakening agents to lower their yield stress in deformation. This implies that the one Earth-mass regime seems to be the lower threshold for active-lid tectonics. Another approach to plate tectonics in super-Earths is given by O’Neill & Lenardic (2007; [147]). These authors suggested that at most, massive Earth-analogs would be in an episodic regime in which episodes of plate tectonics and stagnant-lid occur at different times. They assumed that planets are in a mixed heated state with different proportions of radioactive to basal heating for each planet, and adapted the numerical model of Moresi & Solomatov (1998; [148]), which has been developed to reproduce plate tectonics on Earth, to model mantle convection in super-Earths. Despite that Valencia et al (2007, 2009; [145, 146]) and O’Neill & Lenardic (2007; [147]) agree on considering the Byerlee criterion (an empirical relation to determine the minimum amount of stress that is required to fracture a planet’s crust along its faults, [149]) for plate boundary creation and the need for convection-induced stresses, results by O’Neill & Lenardic (2007; [147]) conclude that owing to higher gravity, faults are locked due to increased pressure and thus deformation is halted. Other recent studies on this topic have arrived at different results. For instance, Tackley & van Heck (2009; [150]) used numerical modeling and constant density scaling, and show that planets that are internally heated as well as those heated from below (and maintain a temperature difference between top and bottom), are more likely to have plate tectonics as in Earth. Sotin & Schubert (2009; [151]) have also attempted to explain the difference between the results obtained by Valencia et al. (2007, 2009; [145, 146]) and O’Neill & Lenardic (2007; [147]). Utilizing a parameterized convection approach and using the results of their structure-scaling August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

28 Nader Haghighipour

model, these authors have shown that despite an overestimate of the ratio of the driving to resistive forces in the model by Valencia et al (2007; [145]), this ratio is weakly dependent on the size of a terrestrial planet, and other compositional and/or geophysical properties may have to be considered in order to determine the probability of the occurrence of plate tectonics in super-Earths. Sotin & Schubert (2009; [151]) also considered a 3D spherical scaling and assumed an increase in the heat flux of a planet with increasing its size, and showed that planets such as super-Earths may be marginally in the plate-tectonic regime. In conclusion, whether plate tectonics occur in super-Earths or not is still under debate. Although there seems to be better qualitative agreements between models, there are still dis- crepancies that have to be resolved.

3.1.3 Magnetic Fields One important characteristic of Earth, that is a consequence of having a molten dynamic iron core and an active and on-going plate tectonics, is its magnetic field. Earth’s magnetic field plays an important role in its habitability. It protects our planet from harmful radiations and maintains its atmospheric composition by preventing non-thermal escape of different elements and components [152, 153, 154]. As such, the presence of a magnetic field has been considered essential for habitability. Whether and how magnetic fields are developed around super-Earths is an active topic of research. In general, in order for a magnetic field to be in place around an Earth-like planet, a dynamo action has to exist in the planet’s core. In order for this dynamo to develop and sustain, the planet has to have a core of liquid iron (or an alloy [155]) with a vigorous and on-going convection process. The latter can be sustained by maintaining a temperature difference across core-mantle boundary which itself depends on the efficiency of transporting heat and cooling the planet. On Earth, the core has cooled enough as to yield a freezing inner core which releases latent heat into the liquid outer core that drives Earth’s dynamo. Also, thanks to plate tectonics, the mantle is cooling effectively to allow for the core to sustain it super-adiabaticity (the hotter part of the core becomes less dense and rises to the cooler part in a fast pace). We refer the reader to Planetary Magnetism, a special issue of Space Science Review by Christensen et al [156] for a complete review of the current state of research on planetary magnetism. The appearance of a magnetic field around a super-Earth and its lifetime are different from those of Earth. Studies of the internal heating and cooling of these objects suggest that large super-Earths will not be able to develop magnetic fields. Modeling the internal evolution of hot super-Earths (i.e. super-Earths in close-in orbits) and studying their cooling histories, Tachinami et al (2009; [157]) have shown that planets more massive than 5 Earth-masses would not be able to develop a dynamo for most of their evolution1. Recent study by Gaidos et al [158] lowers this limit to 2 Earth-masses. As shown by these authors, planets larger than 1.5-2 Earth-masses with stagnant lids do not generate a dynamo. Only if in these planets, the cooling of the core is supported by a mobile lid, they can produce magnetic fields that may last a long time. Figure 23 shows the results of some the simulations by these authors. As shown here, CoRoT-7b might have maintained a magnetic field for the duration of its lifetime. Stamenkovic et al (2010; [159]) and Stamenkovic & Breuer (2010; [160]) have also indicated that the possibility of developing a magnetic field decreases as the planet’s mass becomes larger. These authors studied the thermal evolution of planetary bodies with masses ranging from 0.1 to 10 Earth-masses, and showed that when a pressure-dependent viscosity is included in their models, results suggest that mantle convection and the growth of a low-lid in the core-mantle boundary will be ineffective. According to these authors, the heat-transport through convection will eventually cease and the cooling of the core will be only through conduction. Since conduction

1The melting curve of iron shows a steep increases with pressure. s a result, in order for large planets to have a molten core, the temperature in the core has to rise to values of the order 104 K. These values are too high to be reached August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 29

Figure 23. Averaged surface magnetic fields of super-Earths. PT and SL in the graph of a 1 Earth-mass planet stand for plate tectonics and stagnant lid. The temperature of each graph corresponds to the effective surface temperature of the planet. As shown here, planets larger than 2.5 Earth-masses with stagnant lids do not develop magnetic fields. For those with plate tectonics, the lifetime of the magnetic field decrease as the mass of the planet increases. Note that for simulations of CoRoT-7b, the surface temperature of the planet was set to 1810 K [5]. Figure from [158] with the permission of AAS.

is not an effective way to transport heat from the core, the thermally generated magnetic field will be strongly suppressed. The results of the simulations by Stamenkovic et al (2010; [159]) and Stamenkovic & Breuer (2010; [160]) also suggest that the scaling laws, as used by Valencia et al (2007; [145]) and O’Neill & Lenardic (2007; [147]), cannot be used for pressure-dependent viscosity models such as those for studying the interior of super-Earths.

3.2 Atmosphere The presence of an atmosphere around a terrestrial planet has profound effects on its capability in developing and maintaining life. While the chemical properties of the atmosphere point to the planet’s possible biosignatures [161, 162, 163] as well as the materials of which the planet is formed (the latter can be used to infer information about the origin of the planet, its formation August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

30 Nader Haghighipour

Figure 24. Habitable zones of stars with different masses based on the model by Kasting et al (1993; [164]).

mechanism, as well as its orbital evolution and interior dynamics), its greenhouse effect prevents the planet from rapid cooling, and its cloud circulations enable the planet to maintain global uniformity in its surface temperature. As such, the planet’s atmosphere plays an important role in the determination of the habitable zone of its central star. In general, the habitable zone of a star is defined as a region where an Earth-sized planet can maintain liquid water on its surface [164] (Figure 24). In the absence of planetary atmosphere, the width of this region is small and the locations of its inner and outer boundaries are determined by the amount of the radiation that planet receives from the central star. When the planet is surrounded by an atmosphere, the greenhouse effect causes these boundaries to move to larger distances. In this case, the outer edge of the habitable zone is defined as a distance beyond which CO2 clouds can no longer keep the surface of the planet warm and runaway glaciation may occur. Correspondingly, the inner edge of the habitable zone is defined as a distance closer than which runaway greenhouse effect may increase the surface temperature and pressure of the planet to values higher than those accommodating life [165, 166, 167]. Whether or not a super-Earth can have an atmosphere, and what the chemical composition of this atmosphere would be are directly linked to the properties of the environment where the super-Earth was formed, and it’s subsequent interior dynamics and orbital evolution. As explained in section 2, the fact that super-Earths are smaller than giant planets suggests that these objects might have either formed in the low-mass and gas-poor region of a protoplanetary disk, or were formed in its outer regions where the disk is more massive and the lifetime of the gas is longer, but were scattered to the inner orbits before they accreted a large amount of gas. As a result, one can think of three mechanisms for the formation of an atmosphere around a super-Earth:

• direct accretion of the gas from circumstellar disk, • out-gassing during the formation of the planet, and • out-gassing due to the planet’s active interior and plate tectonics.

Different mechanisms of the formation of an atmosphere, combined with different scenarios for the formation of super-Earths lead to a range of atmospheres with different masses and elemental- abundances. For instance, as shown by Elkins-Tanton & Seager (2008; [168]), a super-Earth accreted from planetesimals of different primitive and differentiated chondritic and achondritic meteorites can out-gas an atmosphere with an initial mass ranging from approximately 1% to a few percent of the total mass of the planet (in some extreme cases the mass of the atmosphere August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 31

Figure 25. Graph of thermal velocity of different elements (solid lines) and escape velocities of planets (shown by symbols) in terms of temperature. Planets of solar system are shown by their initials. The triangles represent a 10 Earth-masses planet with similar differentiated composition as Earth, and the squares represent a 10 Earth-masses planet with 25% water in the outer layer, 52.5% silicate mantle, and 22.5% iron core [122]. The exobase temperatures at 1000 K and 10,000 K correspond to those of super Earths in close-in orbits around M dwarfs and Sun-like stars, respectively. As shown here, a planet will retain all gases that have thermal velocities below its escape velocity. Figure from [168] with the permission of AAS.

may even increase to over 20% of the planet’s total mass). Considering an object with a mass of 1 to 30 Earth-masses, these authors have shown that this atmosphere may be primarily made of water, hydrogen, and/or carbon compounds that include oxygen. The abundance of helium in such an atmosphere will be very small, and its nitrogen concentration will be equivalent to the amount of nitrogen in Earth’s atmosphere. Although the initial mass and elemental abundance of an out-gassed atmosphere is driven by the material of which the planet is formed and by planet’s geochemical and geodynamical history, its final chemical composition is determined by a series of processes such as thermal and non- thermal atmospheric escapes, interactions of molecules with the light of the star (photolysis), and the rates of such chemical interactions. Among these processes, atmospheric escape plays a more important role. Molecules such as hydrogen, for instance, may undergo hydrodynamics escape and carry off larger elements with them. This process may, however, be counterbalanced by the surface gravity of the planet. The gravitational attraction of a massive planet may prevent molecules from escaping its atmosphere, or at least slow down the rate of their escape. In regard to super-Earths, the latter implies that super-Earths more massive than Earth should be able to retain hydrogen in their atmospheres and avoid its erosion through (thermal) atmospheric escape. Figure 25 shows this by comparing a planet’s escape velocity with the thermal velocity of different elements in the models studied by Elkins-Tanton & Seager (2008; [168]). As shown here, a 10 Earth-mass planet in a short period orbit around a Sun-like star or an M dwarf can retains its hydrogen and prevents it from thermal escaping. Simulations by these authors also suggested that the initial amount of hydrogen in the out-gassed atmosphere of a (massive) super- Earth may not exceed ∼ 6% implying that if more hydrogen is detected around a super-Earth, it August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

32 Nader Haghighipour

must have come through other processes (e.g., as shown by Alibert et al [169], a 10 Earth-mass planet, similar to that in a 0.08 AU orbit around the star HD 69830, may retain equivalent to 2 Earth-masses of H/He while migrating to a short-period orbit and subject to atmospheric evaporation). Other processes that erode an atmosphere include intense radiation from the central star (e.g, extreme ultraviolet) [170], stellar wind, and coronal mass ejection [171]. Around M stars, in particular, these processes are strong, and compared to solar-type stars, they last longer [172, 173]. As a result, many authors have assumed that close-in super-Earths around M stars may have small to no atmospheres [122, 124, 132, 133, 135, 171, 174, 175, 176]. Whether massive super-Earths, or those subject to small stellar radiations can have hydrogen- rich atmospheres, and how to identify the signature(s) of such an atmosphere have been subjects of active research for the past few years. The majority of these studies rely on the computational simulations of the accumulation and erosion of a gaseous envelope around an Earth-sized or larger body in order to estimate the amount of hydrogen that may exist in the atmosphere of a super- Earth. These simulations themselves require detailed modeling of processes such as atmospheric escape, photochemistry, and the pressure/temperature distribution in the lower part of planet’s atmosphere (near the surface of the planet). Since different models use different assumptions on the formation, evolution, and interior dynamics of a super-Earth, the results of these simulations are different. Some point to a hydrogen-rich atmosphere, whereas some suggest moderate to low levels of H/He, and some even present the possibility of other chemical compositions. An interesting case that has been studied by several researchers is the case of the transiting super- Earth GJ 1214b. The larger-than-Earth radius of this planet (2.7 Earth-radii) combined with − its low density (∼ 1870 kg m 3) [6] suggests that this planet has to be surrounded by a gaseous envelope. As shown by Miller-Ricci & Fortney (2010; [120]), a hydrogen-rich atmosphere around this super-Earth may not be unrealistic, and can produce up to 0.3% variations (as a function of the wavelength) in the depth of the primary transit of this body compared to the background light of its parent M star. As shown by Rogers & Seager (2010a; [177]), on the other hand, a hydrogen-rich atmosphere may not be the only possibility. Analyzing the bulk composition of GJ 1214b using the models developed by the same authors [178], Rogers & Seager (2010a; [177]) considered three possible scenarios for the accumulations of an atmosphere around this planet; accreting gas from the protoplanetary nebula, ice sublimation, and out-gassing. These authors have shown that if GJ 1214b accreted its gaseous envelope from the primordial nebula, its interior would be primarily composed of iron, silicate, and ice, and its gaseous envelope would contain primordial H/He with a mass equivalent to 0.01% to 5% of the total mass of the planet. An atmosphere formed mainly from ice sublimation, on the other hand, would contain a massive amount of water, equal to at least 47% of the planet’s mass. Such an atmosphere would be able to account for the measured values of mass and radius of GJ 1214b without requiring a layer of H/He. Finally, an out-gassed atmosphere would have to be rich in hydrogen in order to be able to account for the observed value of the planet’s radius. Observational uncertainties on the measurements of the radius of the star GJ 1214, on the other hand, have put strong constraints on the predictions of these models. As suggested by Charbonneau et al (2009, [6]), the M star GJ 1214 may be 15% smaller in radius which suggest that the radius of GJ 1214b will be also smaller with the same amount, removing the necessity for a large atmosphere to explain its observed radius [6]. As shown in this section, the uncertainties in the internal composition of super-Earths result in different models for the atmospheres of these objects. A test of the validity of these models can come from the observation of the transmission and/or emission features in the spectra of these bodies. As suggested by Miller-Ricci et al (2009; [179]), super-Earths with massive hydrogen atmospheres will show strong water features in their emission spectra whereas those that lack hydrogen show signatures of CO2. Also, a hydrogen-dominated atmosphere will show significant August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 33

spectral lines in its transmission spectrum since such an atmosphere has a large scale height1. Recent observations of GJ 1214b have not been able to detect such transmission features in the planet’s spectrum implying that GJ 1214b does not have a cloud-free hydrogen-rich atmosphere. The lack of features in the transmission spectrum of this planet seems to instead point to a dense steam atmosphere [121]. In conclusion, it would not be unrealistic to assume that super-Earths carry gaseous envelopes. Around low-mass stars, some of such atmosphere-bearing super-Earths may even have stable orbits in the habitable zones of their host stars. As we explain in the next section, many of such habitable super-Earths are potentially detectable by different observational techniques. The recently detected super-Earth GL 581 g [2] with its possible atmospheric circulation [180] in the habitable zone of its host star may in fact be one of such planets. More advanced telescopes are needed to identify the biosignatures of these bodies and the physical and compositional characteristics of their atmospheres.

4 Detection of Super-Earths

4.1 Radial Velocity Technique Although the current sensitivity of the radial velocity technique has reached a level that allows for the detection of super-Earths around solar-mass stars, low-mass stars such as M dwarfs present the most promising targets for searching for Earth-sized planets and super-Earths. This is primarily due to the fact that as the least massive stars, M dwarfs show the greatest reflex acceleration due to an orbiting planet. Also, simulations of planet formation suggest that planets formed around M dwarfs are generally smaller than gas-giants and more probably in close-in orbits (note that Jovian-type planets have in fact been detected around M stars. Also note that precision Doppler surveys are optimally sensitive to small orbits). It is therefore not surprising that during the past few years, 23 extrasolar planets have been detected around 17 M stars. More than half of these planets are of the size of Neptune or smaller and the majority of them are in close-in orbits with orbital periods as small as 1.3 days (e.g., HD 41004 B b, [181]). There are currently several radial velocity surveys that search for super-Earths around M stars. Among these surveys, the Lick-Carnegie Exoplanet Survey [2, 89, 182], the M2K Planet Search Project [183, 184], and the HARPS Search for Southern Extrasolar Planets [185, 186, 187]1 have been successful in detecting several prominent super-Earths.

4.2 Transit Photometry Similar to Doppler spectroscopy, transit photometry also shows great sensitivity to large planets in close-in orbits. Since in this technique, the decrease in the light of a star is the quantity that leads to the detection of a planetary companion, low-mass stars such as M dwarfs present more favorable targets for searching for transiting super-Earths. There are currently several transit photometry surveys that use ground- and space-based telescopes to search for Earth-like planets and super-Earths2. Among the ground-based surveys, the MEarth project3, a robotically controlled set of eight 40 cm telescopes at the F. L. Whipple Observatory on Mt. Hopkins in Arizona, has been able to detect the first super-Earth transiting an M star (GJ 1214b, a 5.69 Earth-mass planet around the M dwarf GJ 1214) [6, 188, 189, 190]. In space, the two telescopes CoRoT4 and Kepler5 have succeeded in detecting several super-Earths

1Transmission features in a planet’s spectrum are direct indication of the scale height of its atmosphere 1http://www.eso.org/sci/facilities/lasilla/instruments/harps/ 2We refer the reader to exoplanet.eu and exoplanets.org for more details. 3https://www.cfa.harvard.edu/∼zberta/mearth/ 4http://smsc.cnes.fr/COROT/index.htm 5http://kepler.nasa.gov/ August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

34 Nader Haghighipour

Figure 26. Variation of transit timing. The perturbation of an unseen planet causes the center of transit in the observed (perturbed) system to shift from its one-planet theoretical value (black).

[10, 11] and are continuing their progress.

4.3 Transit Timing Variation Method (TTV) The detection of the dimming of the light of a star due to a transiting planet requires monitoring the star for a long time. Given the complexities in extracting information about a possible planetary companion from transit photometry data, and the fact that even for planets as large and as close as hot Jupiters, the dimming is only a few percent, it is not surprising that until recently, in almost all currently known transiting systems, the number of the detected planets was only one. This is, however, unlike what the simulations of planet formation suggest. In general, models of giant and terrestrial planet formation around different stars indicate that planets tend to form in multiples. In other words, many of the currently known transiting systems may in fact harbor additional bodies [191]. Among these (unseen) planets, those that are close to the orbit of the transiting one will perturb its orbit and cause variations in the time and duration of its transits (Figure 26). Examples of such variations can also be observed in the transit timing of the terrestrial planets of our solar system [192]. Measurements of the variations in the time of transit may be used to infer the existence of the August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

Contemporary Physics 35

10

5 L sec H

C 0 - O

-5

-10

0 50 100 150 200 250 300 350 Transit Number

200

100 L sec H 0 C - O -100

-200

0 50 100 150 200 Transit Number Figure 27. Graphs of transit timing variations for a transiting Jupiter-mass planet in a 3-day (0.0407 AU) circular orbit around a solar-mass star. The planet is perturbed by an Earth-sized object at 0.06 AU (top) and 0.0257 AU (bottom) where it is in an interior 2:1 MMR with the transiting planet. The orbit of the Earth-sized planet has an eccentricity of 0.1. The quantity O–C represents (Observed – Calculated). Figure from [197].

perturbing planet. As shown by [192, 193, 194, 195, 196], in a system consisting of a transiting Jupiter-like planet and a small Earth-like perturber, the perturbations due to the small planet create variations in the transit timing of the hot Jupiter. The top panel of Figure 27 shows such variations for a Jupiter-sized planet in a 3-day orbit (semimajor axis of 0.047 AU) around a solar-mass star [197]. The perturber in this case is Earth-sized and in an exterior orbit with a semimajor axis of 0.06 AU and eccentricity of 0.1. The amplitude of the variations in transit timing is approximately 10 seconds. Since the deviations of the orbit of a transiting planet from pure Keplerian is caused by the gravitational force of the perturbing body, the geometrical arrangement of the orbits of these two planets significantly affects their mutual interactions and the amplitude of the transit timing variations (TTVs). When the two planets are in or near a mean-motion resonance, the TTV signal is greatly enhanced. This can be seen in the bottom panel of Figure 27 where the TTVs are shown for the same transiting planet as in the upper panel, but now the transiting planet and the perturber are in an interior 2:1 mean-motion resonance (the orbital period of the perturber is 1.5 days) [197]. A comparison between the two panels indicates that the signal in the resonant August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

36 Nader Haghighipour

system is more than 20 times stronger. This signal is, for instance, within the range of the detection sensitivity of Kepler space telescope. As shown by Ford et al (2011, [198]), the median transit timing uncertainty for Kepler is approximately 10 minutes (in the system of Kepler 9 where two Saturn-sized planets are in a near 2:1 mean-motion resonance, the amplitude of the TTVs of the inner planet reaches to ∼ 50 minutes) and its minimum detection capability is ∼ 20 seconds. Recently Payne et al (2010, [199]) and Haghighipour & Kirste (2011, [200]) studied the variation of transit timing for two-planet systems around stars with different masses and identified regions of the parameter-space for which the amplitudes of TTVs can be detected by Kepler space telescope. Although a perturber may be able to create large disturbances in the motion of a transiting planet, extracting information about its orbital and physical characteristics from the measure- ments of the variations in the duration and intervals of transits is an extremely complicated task. In general, observational measurements of TTVs are compared with the TTVs obtained from numerical simulations of different planetary systems [199, 200] and/or those calculated an- alytically [201, 202]. The goodness-of-fit is then used to determine the most probable planetary system. Unfortunately, as one can imagine, both these approaches suffer from strong degeneracy (different combinations of mass and orbital elements may produce similar results). Follow-up ob- servations, in particular with radial velocity technique, will be necessary to confirm the detection and determine the mass and orbital parameters of the perturber. Despite these difficulties, Kepler has been able to detected several super-Earths using the transit timing variations method [11] (see Table 1). The study by Ford et al [198], in which the transit timing of more than 1200 potential planetary candidates in Kepler’s target list were analyzed, suggests that as this telescope continues its successful operation, several tens of such TTV-detected super-Earths will be identified in the near future.

4.4 Microlensing While the radial velocity, transit photometry, and TTV methods are more sensitive to detect- ing planets in close-in orbits, microlensing is capable of detecting planets in large distances. This technique has been recognized as one of the most promising detection methods for finding low-mass planets beyond the snow line, as well as super-Earths that reside further from their host stars [203]. Microlensing also has the unique capability of discovering what is known as free-floating planets. Many of these planets are Earth-sized or super-Earths1. The Wide-Field Infrared Survey Telescope (WFIRST)2, NASA’s future large space mission, will use microlensing to search for extrasolar planets. This mission has been recognized as the top-ranked large space mission for the next decade in the New Worlds, New Horizon Decadal Survey of Astronomy and Astrophysics3. There are currently 7 different active microlensing search projects among which MicroFUN (Microlensing Follow-Up Network)4, MOA (Microlensing Observations in As- trophysics)5, OGLE (Optical Gravitational Lensing Experiment)6, PLANET(Probing Lensing Anomalies NETwork)7, and Robonet8 have been successful in detecting several planets.

1As the simulations of planet formation indicate, migrating planets, in particular the giant ones, cause the orbits of many smaller bodies such as Earth-sized planets and super-Earths to become unstable. These objects may be ejected from their systems and freely float in space. 2http://wfirst.gsfc.nasa.gov/ 3http://sites.nationalacademies.org/bpa/BPA 049810 4http://www.astronomy.ohio-state.edu/∼microfun/ 5http://www.phys.canterbury.ac.nz/moa/ 6http://ogle.astrouw.edu.pl/ 7http://planet.iap.fr/ 8http://robonet.lcogt.net/ August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

REFERENCES 37

Acknowledgments

This manuscript is the result of discussions as well as oral and poster presentations in two NAI (NASA Astrobiology Institute) supported events: A workshop on the habitability of super-Earths held at the Aspen Center for Physics in August 2008, and a session entitled Building Habitable Planets held at the Goldschmidt 2009 conference. I am indebted to D. Valencia for her invaluable contribution to this manuscript and for her constructive comments and suggestions. I am also thankful to E. Agol, S. Gaudi, L. Kaltenegger, S. Kenyon, as well as R. Joseph and a second (anonymous) referee for their critically reading of the original draft of this paper and for their valuable suggestions that greatly improved this manuscript. Supports are also acknowledged from NASA Astrobiology Institute under Cooperative Agreement NNA04CC08A at the Institute for Astronomy, University of Hawaii, and NASA EXOB grant NNX09AN05G.

References

[1] M. Mayor, and D. Queloz, A Jupiter-mass companion to a solar-type star, Nature 378 (1995), pp. 355-359. [2] S. S. Vogt, R. P. Butler, E. J. Rivera, N. Haghighipour, G. W. Henry, and M. H. Williamson, The Lick-Carnegie Exoplanet Survey: A 3.1 M⊕ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581, Astrophys. J., 723 (2010), pp. 954-965. [3] A. Wolszczan, and D. A. Frail, A planetary system around the millisecond pulsar PSR1257 + 12, Nature 355 (1992), pp. 145–147. [4] D. Charbonneau, T. M. Brown, D. W. Latham, and M. Mayor, Detection of Planetary Transits Across a Sun-like Star, Astrophys. J. 529 (2000), pp. L45-L48. [5] A. L`eger et al., Transiting Exoplanets From the CoRoT Space Mission. VIII. CoRoT-7b: The First Super-Earth With Measured Radius, Astron. Astrophys. 506 (2009), pp. 287-302. [6] D. Charbonneau et al., A Super-Earth Transiting a Nearby Low-Mass Star, Nature 462 (2009), pp. 891-894. [7] G. Tinetti et al., Water vapor in the atmosphere of a transiting extrasolar planet, Nature 448 (2007), pp. 169-171. [8] A. Gould et al., Microlens OGLE-2005-BLG-169 Implies That Cool Neptune-like Planets Are Common, Astrophys. J. 644 (2006), pp. L37-L40. [9] J.-P. Beaulieu et al.,Discovery of a cool planet of 5.5 Earth-masses through gravitational microlensing, Nature 439 (2006), pp. 437-440. [10] M. J. Holman, et al Kepler-9: A System of Multiple Planets Transiting a Sun-Like Star, Confirmed by Timing Vari- ations, Science, 330 (2010), pp. 51-54. [11] J. J. Lissauer et al A closely packed system of low-mass, low-density planets transiting Kepler-11, Nature, 470 (2011), pp. 53-58. [12] P. Kalas, J. R. Graham, E. Chiang, M. P. Fitzgerald, M. Clampin, E. S. Kite, K. Stapelfeldt, C. Marois, J. Krist, Optical Images of an Exosolar Planet 25 Light-Years from Earth, Science 322 (2008) Vol. 322. no. 5906, pp. 1345-1348. [13] M. C. Marois, B. Macintosh, T. Barman, B. Zuckerman, I. Song, J. Patience, D. Lafreni`ere, and R. Doyon, Direct Imaging of Multiple Planets Orbiting the Star HR 8799, Science 322 (2008), pp. 1348-1352. [14] M. C. Marois, B. Zuckerman, Q. M. Konopacky, B. Macintosh, and T. Barman, Images of a Fourth Planet Orbiting HR 8799, Nature, 468 (2010), pp. 1080-1083. [15] D. Queloz, et al The CoRoT-7 Planetary System: Two Orbiting Super-Earths, Astron. Astrophys. 506 (2009), pp. 303-319 [16] A. P. Hatzes, R. Dvorak, G. Wuchterl, P. Guterman, M. Hartmann, M. Fridlund, D. Gandolfi, E. Guenther, and M. P´’atzold, An Investigation into the Radial Velocity Variations of CoRoT-7, Astron. Astrophys. 520 (2010), pp. 93-108. [17] A. P. Hatzes, et al, Oon the Mass of CoRoT-7b, arXiv:1105.3372v1 (2011) [18] J. E. Chambers, Planetary Migration: What Does It Mean for Planet Formation?, Ann. Rev. Earth. Planet. Sci. 37 (2009), pp. 321-344. [19] P. J. Armitage, Lecture Notes on the Formation and Early Evolution of Planetary Systems, In: Astrophysics of Planet Formation (2010), Cambridge University Press, Cambridge, UK [20] M. V. Smoluchowski, Drei Vortrage uber Diffusion, Brownsche Bewegung und Koagulation von Kolloidteilchen, Physik. Zeit. 17 (1916), pp. 557-585. [21] C. Dominik and A. G. G. M. Tielens, The Physics of Dust Coagulation and the Structure of Dust Aggregates in Space, Astrophys. J. 480 (1997), pp. 647-673. [22] J. Blum, G. Wurm, T. Poppe, and L.-O. Heim, Aspects of Laboratory Dust Aggregation With Relevance to the For- mation of Planetesimals, Earth Moon & Planets 80 (1998), pp. 285-309. [23] G. Wurm, and J. Blum, Experiments on Preplanetary Dust Aggregation, Icarus 132 (1998), pp. 125-136. [24] J. Blum, and G. Wurm, Experiments on Sticking, Restructuring, and Fragmentation of Preplanetary Dust Aggregates, Icarus 143 (2000), pp. 138-146. [25] M. Krause, and J. Blum, Growth and Form of Planetary Seedlings: Results from a Sounding Rocket Microgravity Aggregation Experiment, Phys. Rev. Let. 93 (2004), pp. 021103. [26] J. Blum, Dust agglomeration, Adv. Phys. 55 (2006), pp. 881-947. [27] K. Wada, H. Tanaka, T. Suyama, H. Kimura, and T. Yamamoto, Numerical Simulation of Dust Aggregate Collisions. I. Compression and Disruption of Two-Dimensional Aggregates, Astrophys. J. 661 (2007), pp. 320-333. [28] J. Blum, et al., Growth and Form of Planetary Seedlings: Results from a Microgravity Aggregation Experiment, Phys. Rev. Lett. 85 (2000), pp. 2426-2429. [29] J. Blum, and G. Wurm, The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks, Ann. Rev. Astron. Astrophys. 46 (2008), pp. 21-56. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

38 REFERENCES

[30] D. Paszun, and C. Dominik, The Influence of Grain Rotation on the Structure of Dust Aggregates, Icarus 182 (2006), pp. 274-280. [31] S. J. Weidenschilling, Aerodynamics of Solid Bodies in the Solar Nebula, Mon. Not. Roy. Ast. Soc. 180 (1977), pp. 57-70. [32] G. Wurm, J. Teiser, A. Bischoff, H. Haack, and J. Roszjar, Experiments on the Photophoretic Motion of Chondrules and Dust Aggregates-Indications for the Transport of Matter in Protoplanetary Disks, Icarus 208 (2010), pp. 482-491. [33] A. Johansen, J. S. Oishi, M.-M. Mac Low, H. Klahr, Hubert; T. Henning, and A. Youdin, Rapid planetesimal formation in turbulent circumstellar disks, Nature 448 (2007), pp. 1022-1025. [34] J. N. Cuzzi, R. C. Hogan, and K. Shariff, Toward Planetesimals: Dense Chondrule Clumps in the Protoplanetary Nebula, Astrophys. J. 687 (2008), pp. 1432-1447. [35] S. J. Weidenschilling, Particles in the nebular midplane: Collective effects and relative velocities, Meteorit. Planet. Sci. 45 (2010), pp. 276-288. [36] E. Chiang, and A. N. Youdin, Forming Planetesimals in Solar and Extrasolar Nebulae, Ann. Rev. Earth. Planet. Sci 38 (2010), pp. 493-522. [37] V. S. Safronov, Evolution of Protoplanetary Cloud and Formation of the Earth and Planets, Nauka, Moscow (1969) [38] R. Greenberg, W. K. Hartmann, C.R. Chapman, and J. F. Wacker, Planetesimals to Planets–Numerical Simulations of Collisional Evolution, Icarus 35 (1978), pp. 1-26. [39] G. W. Wetherill, and G. R. Stewart, Accumulation of a Swarm of Small Planetesimals, Icarus 77 (1989), pp. 330-357. [40] G. W. Wetherill, and G. R. Stewart, Formation of Planetary Embryos–Effects of Fragmentation, Low Relative Velocity, and Independent Variation of Eccentricity and Inclination, Icarus 106 (1993), pp. 190-209. [41] E. Kokubo, and S. Ida, On Runaway Growth of Planetesimals, Icarus 123 (1996) pp. 180-191. [42] S. Ida, and J. Makino, Scattering of planetesimals by a protoplanet-Slowing down of runaway growth, Icarus 106 (1993), p. 210-227. [43] S. J. Weidenschilling, D. Spaute, D. R. Davis, F. Marzari, and K. Ohtsuki, Accretional Evolution of a Planetesmial Swarm, Icarus 128 (1997), pp. 429-455. [44] E. Kokubo, and S. Ida, Formation of Protoplanets from Planetesimals in the Solar Nebula, Icarus 143 (2000) pp. 15-27. [45] J. B. Pollack, O. Hubickyj, P. Bodenheimer, J. J. Lissauer, M. Podolak, and Y. Greenzweig, Formation of the Giant Planets by Concurrent Accretion of Solids and Gas, Icarus 124 (1996), pp. 62-85. [46] O. Hubickyj, P. Bodenheimer, and J. J. Lissauer, Accretion of the Gaseous Envelope of Jupiter Around a 5-10 Earth- Mass Core, Icarus 179 (2005), pp. 415-431. [47] J. J. Lissauer, O. Hubickyj, G. DAngelo,´ and P. Bodenheimer, Models of Jupiter’s Growth Incorporating thermal and hydrodynamic Constraints, Icarus 199 (2009), pp. 338-350. [48] N. Movshovitz, P. Bodenheimer, M. Podolak, and J. J. Lissauer, Formation of Jupiter Using Opacities Based on Detailed Grain Physics, Icarus 209 (2010), pp. 616-624. [49] B. C. Bromley, and S. J. Kenyon, Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth, Astron. J. 131 (2006), pp. 1837-1850. [50] P. Goldreich, Y. Lithwick, and R. Sari, Final Stages of Planet Formation, Astrophys. J. 614 (2004), pp. 497-507. [51] E. Kokubo, and S. Ida, Orbital evolution of protoplanets embedded in a swarm of planetesimals, Icarus 114 (1995), pp. 247-257. [52] E. Kokubo, and S. Ida, Orbital Oligarchic Growth of Protoplanets, Icarus 131 (1998), pp. 171-178. [53] A. Morbidelli, J. Chambers, J. I. Lunine, J. M. Petit, F. Robert, G. B. Valsecchi, G. B., and K. E. Cyr, Source Regions and Time Scales for the Delivery of Water to Earth, Meteor. Planet. Sci. 35 (2000), pp. 1309-1320. [54] J. E. Chambers, and G. W. Wetherill, Planets in the asteroid belt, Meteorit. Planet. Sci. 36 (20010, pp. 381-399. [55] D. P. O’Brien, A. Morbidelli, and H. F. Levison, Terrestrial Planet Formation with Strong Dynamical Friction, Icarus 184 (2006), pp. 39-58. [56] S. N. Raymond, T. Quinn, and J. I. Lunine, High Resolution Simulations of the Final Assembly of Earth-like Planets I: Terrestrial Accretion and Dynamics, Icarus (2006) 183, pp. 265-282. [57] E. Kokubo, J. Kominami, and S. Ida, Formation of Terrestrial Planets from Protoplanets. I. Statistics of Basic Dynamical Properties, Astrophys. J. 642 (2006), pp. 1131-1139. [58] E. Kokubo, and S. Ida, Formation of Terrestrial Planets from Protoplanets. II. Statistics of Planetary Spin, Astrophys. J. 671 (2007), pp. 2082-2090. [59] S. E. Strom, S. Edwards, and M. F. Skrutskie, Evolutionary Time scales For Circumstellar Disks Associated With Intermediate- and Solar-Type Stars, In: E. H. Levy and J. I. Lunine (Eds) Protostars and Planets III (1993), Univ. of Arizona Press, Tucson, pp. 837-866. [60] K. E. Haisch Jr., E. A. Lada, and C. J. Lada, Disk Frequencies and Lifetimes in Young Clusters, Astrophys. J. Lett. 553 (2001), pp. L153-L156. [61] C. H. Chen, and I. Kamp, Are Giant Planets Forming Around HR 4796A?, Astrophys. J. 602 (2004), pp. 985-992. [62] M. Maercker, M. G. Burton, and WC.M. right, L-band (3.5 µm) IR-excess in Massive Star Formation. II. RCW 57/NGC 3576, Astron. Astrophys. 450 (2006), pp. 253-263. [63] T. Guillot, THE INTERIORS OF GIANT PLANETS: Models and Outstanding Questions, Ann. Rev. Earth. Planet. Sci. 33 (2005), pp. 493-530. [64] B. Militzer, W. B. Hubbard, J. Vorberger, I. Tamblyn, S. A. Bonev, A Massive Core in Jupiter Predicted from First- Principles Simulations, Astrophys. J. 688 (2008), pp. L45-L48. [65] M. Podolak, The Contribution of Small Grains to the Opacity of Protoplanetary Atmospheres, Icarus 165 (2003), pp. 428-437. [66] N. Movshovitz, and M. Podolak, The Opacity of Grains in Protoplanetary Atmospheres, Icarus 194 (2008), pp. 368-378. [67] B. C. Bromley, and S. J. Kenyon, A New Hybrid N-body-coagulation Code for the Formation of Gas Giant Planets, Astrophys. J. 731, id.101 (2011) [68] A. P. Boss, Possible Rapid Gas-Giant Planet Formation in the Solar Nebula and Other Protoplanetary Disks, Astro- phys. J. 536 (2000), pp. L101-L104. [69] A. P. Boss, Formation of Extrasolar Giant Planets: Core-Accretion or Disk Instability?, Earth Moon Planets 81 (2000), pp. 19-26. [70] L. Mayer, T. Quinn, J. Wadsley, and J. Stadel, Formation of Giant Planets by Fragmentation of Protoplanetary Disks, Science 298 (2002), pp. 1756-1759. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

REFERENCES 39

[71] A. P. Boss, Rapid Formation of Outer Giant Planets by Disk Instability, Astrophys. J. 599 (2003), pp. 577-581. [72] L. Mayer, T. Quinn, J. Wadsley, J. Stadel, The Evolution of Gravitationally Unstable Protoplanetary Disks: Fragmen- tation and Possible Giant Planet Formation, Astrophys. J. 609 (2004), pp. 1045-1064. [73] R.H. Durisen, A. P. Boss, L. Mayer, A. F. Nelson, T. Quinn, and W. K. M. Rice, Gravitational Instabilities in Gaseous Protoplanetary Disks and Implications for Giant Planet Formation, In: K. K. B. Reipurth, D. Jewitt, ed., Protostars and Planets V (2007), pp. 607-622. [74] L. Mayer, G. Lufkin, T. Quinn, and J. Wadsley, Fragmentation of Gravitationally Unstable Gaseous Protoplanetary Disks with Radiative Transfer, Astrophys. J. 661 (2007), pp. L77-L80. [75] A. C. Boley, The Two Modes of Gas-Giant Planet Formation, Astrophys. J. 695 (2009), pp. L53-L57. [76] A. C. Boley, T. Hayfield, L. Mayer, and R. H. Durisen, Clumps in the Outer Disk by Disk Instability: Why They Are Initially Gas-Giants and the Legacy of Disruption, Icarus 207 (2010) pp. 509-516. [77] K. Cai, M. K. Pickett, R. H. Durisen, and A. M. Milne, Giant Planet Formation by Disk Instability: A Comparison Simulation with an Improved Radiative Scheme, Astrophys. J. 716 (2010), pp. L176-L180. [78] G. Laughlin, P. Bodenheimer, and F. C. Adams, The Core-Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs, Astrophys. J. 612 (2004), pp. L73-L76. [79] G. M. Kennedy, S. J. Kenyon, and B. C. Bromley, Planet Formation Around Low-Mass Stars: The Moving Snow Line and Super-Earths, Astrophys. J. 650 (2006), pp. L139-L142. [80] G. M. Kennedy, S. J. Kenyon, and B. C. Bromley, Planet Formation Around M Stars: The Moving Snow Line and Super-Earths, Astrophys. Space. Sci. 311 (2007), pp. 9-13. [81] G. M. Kennedy, and S. J. Kenyon, Planet Formation around Stars of Various Masses: The Snow Line and the Frequency of Giant Planets, Astrophys. J. 673 (2008), pp. 502-512. [82] G. M. Kennedy, and S. J. Kenyon, Planet Formation Around Stars of Various Masses: Hot Super-Earths, Astrophys. J. 682 (2008), pp. 1264-1276. [83] S. Ida, and D. N. C. Lin, Toward a Deterministic Model of Planetary Formation. V. Accumulation Near the Ice Line and Super-Earths, Astrophys. J. 685 (2008), pp. 584-595. [84] C. Terquem, and J. C. B Papaloizou, Migration and the Formation of Systems of Hot Super-Earths and Neptunes, Astrophys. J. 654 (2007), pp. 1110-1120. [85] A. P. Boss, Rapid Formation of Super-Earths around M Dwarf Stars, Astrophys. J. 644 (2006), pp. L79-L82. [86] C. J. Lada, and E. A. Lada, Embedded Clusters in Molecular Clouds, Ann. Rev. Astron. Astrophys. 41 (20003), pp. 57-115. [87] F. C. Adams, D. Hollenbach, G. Laughlin, and U. Gorti, Photoevaporation of Circumstellar Disks Due to External Far-Ultraviolet Radiation in Stellar Aggregates, Astrophys. J. 611 (2004) pp. 360-379. [88] E. J. Rivera, Eugenio, G. Laughlin, R. P. Butler, S. S. Vogt, N. Haghighipour, S. Meschiari, The Lick-Carnegie Exoplanet Survey: A Uranus-Mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration, Astrophys. J. 719 (2010), pp. 890-899. [89] N. Haghighipour, S. S. Vogt, R. P. Butler, E. J. Rivera, G. Laughlin, S. Meschiari, and G. W. Henry, The Lick-Carnegie exoplanet Survey: A Saturn-Mass Planet in the Habitable Zone of the Nearby M4V star HIP 57050, Astrophys. J. 715 (2010), pp. 271-276. [90] C. Hayashi, Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula, Prog. Theor. Phys. Suppl.,70 (1981), pp. 35-53. [91] R. J. Greenberg, C. C. Counselman, and I. I. Shapiro, Orbit-Orbit Resonance Capture in the Solar System, Science 178 (1972), pp. 747-749. [92] R. J. Greenberg, Evolution of Satellite Resonances by Tidal Dissipation, Astron. J. 78 (1973), pp. 338-346. [93] P. Goldreich, An Explanation of the Frequent Occurrence of Commensurable Mean Motions in the Solar System, Mon. Not. Roy. Astro. Soc. 130 (1965), pp. 159-181. [94] R. M. Canup, and W. R. Ward, Formation of the Galilean Satellites: Conditions of Accretion, Astron. J. 124 (2002),pp. 3404-3423. [95] S.J. Peale, and M. H. Lee, A Primordial Origin of the Laplace Relation Among the Galilean Satellites, Science 298 (2002), pp. 593-597. [96] N. Haghighipour, and D. Jewitt, A Region Void of Irregular Satellites around Jupiter, Astron. J. 136 (2008), pp. 909-918. [97] J. A. Fernandez, and W. -H. Ip, Some Dynamical Aspects of the Accretion of Uranus and Neptune - The Exchange of Orbital Angular Momentum With Planetesimals, Icarus 58 (1984), pp. 109-120. [98] R. Malhotra, The Origin of Pluto’s Peculiar Orbit Nature 365 (1993), pp. 819-821. [99] R. Malhotra, The Origin of Pluto’s Orbit: Implications for the Solar System Beyond Neptune, Astron. J. 110 (1995), pp. 420-429. [100] R. Malhotra, The Phase Space Structure Near Neptune Resonances in the Kuiper Belt, Astron. J. 111 (1996), pp. 504-516. [101] J. M. Hahn, and R. Malhotra, Neptune’s Migration into a Stirred-Up Kuiper Belt: A Detailed Comparison of Simu- lations to Observations, Astron. J. 130 (2005), pp. 2392-2414. [102] R. P. Nelson, J. C. B. Papaloizou, F. Masset, and W. Kley, The Migration and Growth of Protoplanets in Protostellar Discs, Mon. Not. Roy. Astron. Soc. 318 (2001), pp. 18-36. [103] F. Masset, and M. Snellgrove, Reversing Type II Migration: Resonance Trapping of a Lighter Giant Protoplanet, Mon. Not. Roy. Astron. Soc. 320 (2001), pp. L55-L59. [104] J. C. B. Papaloizou, and C. Terquem, Planet Formation and Migration, Rept. Prog. Phys. 69 (2006), pp. 119-180. [105] J.-L. Zhou, S. J. Aarseth, D. N. C. Lin, and M. Nagasawa, Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential Accretion Theory of Planet Formation, Astrophys. J. 631 (2005), pp. L85-L88. [106] M. J. Fogg, and R. P. Nelson, Oligarchic and Giant Impact Growth of Terrestrial Planets in the Presence of Gas-Giant Planet Migration, Astron. Astrophys. 441 (2005), pp. 791-806. [107] M. J. Fogg, and R. P. Nelson, On the Possibility of Terrestrial Planet Formation in Hot-Jupiter Systems, Inter. J. Astrobio. 5 (2006), pp. 199-209. [108] M. J. Fogg, and R. P. Nelson, On the Formation of Terrestrial Planets in Hot-Jupiter Systems, Astron. Astrophys. 461 (2007), pp. 1195-1208. [109] M. J. Fogg, and R. P. Nelson, The Effect of Type I Migration on the Formation of Terrestrial Planets in Hot-Jupiter Systems, Astron. Astrophys. 472 (2007), pp. 1003-1015. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

40 REFERENCES

[110] M. J. Fogg, and R. P. Nelson, Terrestrial Planet Formation in Low-Eccentricity Warm-Jupiter Systems, Astron. Astrophys. 498 (2009), pp. 575-589. [111] S. N. Raymond, R. Barnes, and A. M. Mandell, Observable Consequences of Planet Formation Models in Systems With Close-in Terrestrial Planets, Mon. Not. Roy. Astron. Soc., 384 (2008), pp. 663-674. [112] A. M. Mandell, and S. Sigurdsson, Survival of Terrestrial Planets in the Presence of Giant Planet Migration, Astro- phys. J. 599 (2003), pp. L111-L114. [113] S. N. Raymond, A. M. Mandell, and S. Sigurdsson, Exotic Earths: Forming Habitable Worlds with Giant Planet Migration, Science 313 (2006), pp. 1413-1416. [114] A. M. Mandell, S. N. Raymond, and S. Sigurdsson, Formation of Earth-like Planets During and After Giant Planet Migration, Astrophys. J. 660 (2007), pp. 823-844. [115] N. Haghighipour, and S. Rastegar, Implications of the TTV-Detection of Close-In Terrestrial Planets Around M Stars for Their Origin and Dynamical Evolution, in ”Detection and Dynamics of Transiting Exoplanets” (ed. F. Bouchy, R. F. Diaz, C. Moutou), 2010 [116] S. J. Kenyon, and B. C. Bromley, Rapid Formation of Icy Super-Earths and the Cores of Gas-Giant Planets, Astro- phys. J. 690 (2009), pp. L140-L143. [117] A. P. Boss, G. W. Wetherill, and N. Haghighipour, Rapid Formation of Ice Giant Planets, Icarus 156 (2002), 291-295. [118] S. Michael, R. H. Durisen, and A. C. Boley, Migration of Gas Giant Planets in Gravitationally Unstable Disks, arXiv:1106.1617 (2011) [119] D. S. McNeil, and R. P. Nelson, On the Formation of Hot Neptunes and Super-Earths, Mon. Not. Roy. Astron. Soc. 401 (2010), pp. 1691-1708. [120] E. Miller-Ricci, and J. J. Fortney, The Nature of the Atmosphere of the Transiting Super-Earth GJ 1214b, Astrophys. J. 716 (2010), pp. L74-L79. [121] J. L. Bean, E. Miller-Ricci Kempton, and D. Homeier, A Ground-Based Transmission Spectrum of the Super-Earth Exoplanet GJ 1214b, Nature 468 (2010), pp. 699-672. [122] S. Seager, M. Kuchner, C. A. Hier-Majumder, and B. Militzer, Mass-Radius Relationship for Solid Exoplanets, 669 (2007), pp. 1279-1297. [123] M. Konacki, G. Torres, D. D. Sasselov, and S. Jha, A Transiting Extrasolar Giant Planet Around the Star OGLE- TR-10, The Astrophys. J. 624 (2005), pp. 372-377. [124] C. Sotin, O. Grasset, and A. Mocquet, Mass-Radius Curve for Extrasolar Planets and Ocean Planets, Icarus, 191 (2007), pp. 337-351. [125] T. M. Brown, D. Charbonneau, R. L. Gilliland, R. W. Noyes, and A. Burrows, Hubble Space Telescope Time-Series Photometry of the Transiting Planet of HD 209458, Astrophys. J. 552 (2001), pp. 699-709. [126] G. Torres, M. Konacki, D. D. Sasselov, and S. Jha, New Data and Improved Parameters for the Extrasolar Transiting Planet OGLE-TR-56b, Astrophys. J. 609 (2004), pp. 1071-1075. [127] C. Moutou, F. Pont, F. Bouchy, and M. Mayor, Accurate Radius and Mass of the Transiting Exoplanet OGLE-TR- 132b, Astron. Astrophys. J. 424 (2004), pp. L31-L34. [128] F. Pont, F. Bouchy, D. Queloz, N. C. Santos, C. Melo, M. Mayor, and S. Udry, The “Missing Link”: A 4-day Period Transiting Exoplanet Around OGLE-TR-111, Astron. Astrophys. J. 426 (2004), pp. L15-L18. [129] F. Bouchy, F. Pont, F., N. C. Santos, C. Melo, M. Mayor, Q. Queloz, and S. Udry, Two Mew “Very Hot Jupiters” Among the OGLE Transiting Candidates, Astron. Astrophys. 421 (2004), pp. L13-L16. [130] M. Konacki, G. Torres, D. D. Sasselov, G. Pietrzy`nski, A. Udalski, S. Jha, M. T. Ruiz, W. Gieren, D. Minniti, Dante, The Transiting Extrasolar Giant Planet Around the Star OGLE-TR-113, Astrophys. J. 609 (2004), pp. L37-L40. [131] A. Sozzetti, D. Yong, G. Torres, D. Charbonneau, D. W. Latham, C. Allende Prieto, T. M. Brown, B. W. Carney, J. B. Laird, High-Resolution Spectroscopy of the Transiting Planet Host Star TrES-1, Astrophys. J. 616 (2004), pp. L167-L170. [132] D. Valencia, R. J. OConnell,´ and D. D. Sasselov, Internal Structure of Massive Terrestrial Planets, Icarus 181 (2006), pp. 545-554. [133] D. Valencia, D. D. Sasselov, and R. J. OConnell,´ Radius and Structure Models of the First Terrestrial Super-Earth Planets, Astrophys. J. 656 (2007), pp. 545-551. [134] E. R. Adams, S. Seager, L. Elkins-Tanton, Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres, Astrophys. J. 673 (2008), pp. 1160-1164. [135] D. Valencia, D. D. Sasselov, and R. J. OConnell,´ Detailed Models of Super-Earths: How well Can We Infer Bulk Properties?, Astrophys. J. 665 (2007), pp. 1413-1420. [136] D. Valencia, M. Ikoma, T. Guillot, and N. Nettelmann, Composition and Fate of Short-Period Super-Earths. The Case of CoRoT-7b, Astron. Astrophys. 516 (2010), id.A20. [137] D. C. Swift, J. Eggert, D. G. Hicks, S. Hamel, K. Caspersen, E. Schwegler, G. W. Collins, and G. J. Ackland, Mass-Radius Relationships for Exoplanets, (arXiv:1001.4851) [138] B. Jackson, N. Miller, R. Barnes, S. N. Raymond, J. J. Fortney, and R. Greenberg, The Roles of Tidal Evolution and Evaporative Mass Loss in the Origin of CoRoT-7 b, to appear in Mon. Not. Roy. Ast. Soc. (2010), 407, pp. 910-922. [139] A. Vidal-Madjar, A. Lecavelier des Etangs, J.-M. Dsert, G. E. Ballester, R. Ferlet, G. H´ebrard, and M. Mayor, An Extended Upper Atmosphere Around the Extrasolar Planet HD209458b, Nature 422 (2003), 143-146 [140] A. Lenardic, F. Nimmo, and L. Moresi, Growth of the Hemispheric Dichotomy and the Cessation of Plate Tectonics on Mars, J. Geophys. Res. 109 (2004), E02003. [141] C.C. Reese, V.S. Solomatov, and J.R. Baumgardner, Scaling Laws for Time-Dependent Stagnant-Lid Convection in a Spherical Shell, Phys. Earth. Planet. Inter. 149 (2005), pp. 361-370. [142] A. Loddoch, C. Stein, and U. Hansen, Temporal Variations in the Convective Style of Planetary Mantles, Earth Planet. Sci. Let. 251 (2006), pp. 79-89. [143] L. Kaltenegger, W. G. Henning, and D. D. Sasselov, Detecting Volcanism on Extrasolar Planets, Astron. J. 140 (2010), pp. 1370-1380. [144] J. C. G. Walker, P. B. Hays, J. F. Kasting, A Negative Feedback Mechanism for the Long-Term Stabilization of the Earth’s Surface Temperature, J. Geophys. Res. 86 (1981), pp. 9776-9782. [145] D. Valencia, R. J. OConnell,´ and D. D. Sasselov, Inevitability of Plate Tectonics on Super-Earths, Astrophys. J. 670 (2007) pp. L45-L48. [146] D. Valencia, and R. J. OConnell,´ Convection Scaling and Subduction on Earth and Super-Earths, Earth Planet. Sci. Lett. 286 (2009), pp. 492-502. August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

REFERENCES 41

[147] C. O’Neill, and A. Lenardic, Geological Consequences of Super-Sized Earths, Geophys. Res. 34 (2007), L19204 [148] L. Moresi, and V. Solomatov, Mantle Convection With a Brittle Lithosphere: Thoughts on the Global Tectonic Styles of the Earth and Venus, Geophys. J. Int. 133 (1998), pp. 669-682. [149] J. D. Byerlee, Friction of Rocks, Pure Appl. Geophys. 116 (1978), pp. 615-626. [150] P.J. Tackley, and H. van Heck, Mantle Convection, Stagnant Lids and Plate Tectonics on Super-Earths, Geochim. Cosmoch. Acta Suppl. 73 (2009), p. A1303. [151] C. Sotin, and G. Schubert, Mantle Convection and Plate Tectonics on Earth-Like Exoplanets, Americ. Geo. U. Fall Meeting 2009, abstract P42B-02. [152] J.-M. Griessmeier, A. Stadelmann, T. Penz, H. Lammer, F. Selsis, I. Ribas, E. F. Guinan, U. Motschmann, H. K. Biernat, and W. W. Weiss, The Effect of Tidal Locking on the Magnetospheric and Atmospheric Evolution of “Hot Jupiters”, Astron. Astrophys., 425 (2004), pp. 753-762. [153] R. Lundin, H. Lammer, and I. Ribas, Planetary Magnetic Fields and Solar Forcing: Implications for Atmospheric Evolution, Sp. Sci. Rev., 129 (2007), pp. 245-278. [154] V. Dehant, H. Lammer, Y. N. Kulikov, J.-M. Griessmeier, D. Breuer, O. Verhoeven, O.¨ Karatekin, T. van Hoolst, O. Korablev, and P. Lognonn´e, P., Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars, Sp. Sci. Rev., 129 (2007), pp. 279-300. [155] D. J. Stevenson, Planetary Magnetic Fields: Achievements and Prospects, Sp. Sci. Rev., 152 (2010), pp. 651-664 [156] U. R. Christensen, A Balogh, D. Breuer, and K.-H. Glassmeier, Planetary Magnetism, Special issue of Sp. Sci. Rev., 152 (2010) [157] C. Tachinami, H. Senshu, and S. Ida, The Thermal Evolution and Magnetic Field of Hot Super-Earths, In: EXO- PLANETS AND DISKS: THEIR FORMATION AND DIVERSITY. AIP Conference Proceedings 1158 (2009), pp. 267-268. [158] E. Gaidos, C. P. Conrad, M. Manga, and J. Hernlund, Thermodynamic Limits on Magnetodynamos in Rocky Exo- planets, ApJ, 718 (2010), pp. 596-609. [159] V. Stamenkovic, L. Noack, and D. Breuer, Low-Lid Formation on Super-Earths and Implications for the Habitability of Super-Earths and Sub-Earths, European Geophysical Union, Geophys. Res. Abst. 12 (2010), Abstrac # 9380. [160] V. Stamenkovic, and D. Breuer, Magnetic Field Generation on Super-Earths and Sub-Earths, European Geophysical Union, Geophys. Res. Abst. 12 (2010), Abstrac # 8995-1. [161] L. Kaltenegger, W. A. Traub, and K. W. Jucks, Spectral Evolution of an Earth-like Planet, Astrophys. J., 658 (2007), pp. 598-616. [162] L. Kaltenegger, and W. A. Traub, Transits of Earth-like Planets, Astrophys. J., 698 (2009), pp. 519-527 [163] L. Kaltenegger, and D. Sasselov, Detecting Planetary Geochemical Cycles on Exoplanets: Atmospheric Signatures and the Case of SO2, Astrophys. J., 708 (2010), pp. 1162-1167. [164] J. F. Kasting, D. P. Whitmire, and R. T. Reynolds, Habitable Zones Around Main Sequence Stars, Icarus 101 (1993), pp. 108-128. [165] F. Forget, and R.T. Pierrehumbert, Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radia- tion, Science, 278 (1997), pp. 1273-1276. [166] D. M. Williams, and J. F. Kasting, Habitable Planets with High Obliquities, Icarus, 129 (1997), pp. 254-267. [167] M. A. Mischna, J. F. Kasting, A. Pavlov, and R. Freedman, Influence of Carbon Dioxide Clouds on Early Martian Climate, Icarus, 145 (2000), pp. 546-554. [168] L. T. Elkins-Tanton, and S. Seager, Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets, Astrophys. J., 685 (2008), pp. 1237-1246. [169] Y. Alibert, I. Baraffe, W. Benz, G. Chabrier, C. Mordasini, C. Lovis, M. Mayor, F. Pepe, F. Bouchy, D. Queloz, and S. Udry, Formation and Structure of the Three Neptune-Mass Planets System Around HD 69830, Astron. Astrophys., 455 (2006), pp. L25-L28. [170] I. Baraffe, Y. Alibert, G. Chabrier, and W. Benz, Birth and Fate of Hot-Neptune Planets, Astron. Astrophys., 450 (2006), pp. 1221-1229. [171] H. Lammer et al, Coronal Mass Ejection (CME) Activity of Low Mass M Stars as an Important Factor for the Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones, Astrobio. 7 (2007), pp. 185-207. [172] J. Scalo et al., M Stars as Targets for Terrestrial Exoplanet Searches and Biosignature Detection, Astrobio. 7 (2007), pp. 85-166. [173] A. A. West, S. L. Hawley, J. J. Bochanski, K. R. Covey, I. N. Reid, S. Dhital, E. J. Hilton, and M. Masuda, Constraining the Age-Activity Relation for Cool Stars: The Sloan Digital Sky Survey Data Release 5 Low-Mass Star Spectroscopic Sample, Astron. J. 135 (2008), pp. 785-795. [174] J. J. Fortney, M. S. Marley, and J. W. Barnes, Planetary Radii Across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits, Astrophys. J. 659 (2007), pp. 1661-1672. [175] F. Selsis, J. F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse, Habitable planets around the star Gliese 581?, Astrono. Astrophys., 476 (2007), pp. 1373-1387. [176] S. Seager, and D. Deming, On the Method to Infer an Atmosphere on a Tidally Locked Super Earth Exoplanet and Upper Limits to GJ 876d, Astrophys. J. 703 (2009), pp. 1884-1889. [177] L. A. Rogers, and S. Seager, Three Possible Origins for the Gas Layer on GJ 1214b, Astrophys. J., 716 (2010), pp. 1208-1216. [178] L. A. Rogers, and S. Seager, A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions, Astrophys. J., 712 (2010), pp. 974-991. [179] E. Miller-Ricci, S. Seager, and D. Sasselov, The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres, The Astrophys. J., 690 (2009), pp. 1056-1067. [180] K. Heng, and S. S. Vogt, Gliese 581g as a Scaled-Up Version of Earth: Atmospheric Circulations Simulations, Mon. Not. Roy. Astron. Soc. (2011), tmp..738H. [181] S. Zucker, T. Mazeh, N. C. Santos, S. Udry and M. Mayor, Multi-Order TODCOR: Application to Observations Taken With the CORALIE Echelle Spectrograph, II. A Planet in the System HD 41004 Astron. Astrophys. 426 (2004), pp. 695-698. [182] E.J. Rivera, R. P. Butler, S. S. Vogt, G. Laughlin, G. W. Henry, and S. Meschiari, A Super-Earth Orbiting the Nearby Sun-like Star HD 1461, Astrophys. J. 708 (2010), pp. 1492-1499. [183] K. Clubb, D. Fischer, A. Howard, G. Marcy, and G. W. Henry, M2K: A Search for Planets Orbiting Early M and August 10, 2018 11:21 Contemporary Physics Haghighipour-Manuscript

42 REFERENCES

Late K Dwarf Stars, Bull. Am. Astron. Soc. 41 (2009), p.192. [184] K. Apps, et al., M2K: I. A Jupiter-Mass Planet Orbiting the M3V Star HIP 79431, Pub. Astron. Soc. Pac. 122 (2010), pp. 156-161. [185] X. Bonfils, T. Forveille, X. Delfosse, S. Udry, M. Mayor, C. Perrier, F. Bouchy, F. Pepe, D. Queloz, and J.-L. Bertaux, The HARPS Search for Southern Extrasolar Planets. VI. A Neptune-mass Planet Around the Nearby M-Dwarf Gl 581, Astron. Astrophys., 443 (2005), pp. L15-L18. [186] S. Udry, X. Bonfils, X. Delfosse, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and J.-L. Bertaux, The HARPS Search for Southern Extrasolar Planets. XI. Super-Earths (5 and 8 M⊕) in a 3-Planet System, Astron. Astrophys., 469 (2007), pp. L43-L47. [187] M. Mayor, X. Bonfils, T. Forveille, X. Delfosse, S. Udry, J.-L. Bertaux, H. Beust, F. Bouchy, C. Lovis, F. Pepe, C. Perrier, D. Queloz, and N. C. Santos, The HARPS Search for Southern Extrasolar Planets. XVIII. An Earth-Mass Planet in the GJ 581 Planetary System, Astron. Astrophys., 507 (2009), pp. 487-494. [188] P. Nutzman and D. Charbonneau, Design Considerations for a Ground-Based Transit Search for Habitable Planets Orbiting M Dwarfs, PASP, 120 (2008) pp. 317-327. [189] J. Irwin, D. Charbonneau, P. Nutzman, and E. Falco, The MEarth Project: Searching for Transiting Habitable Super- Earth Planets Around Nearby M-Dwarfs, in COOL STARS, STELLAR SYSTEMS AND THE SUN: Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun. AIP Conference Proceedings, 1094 (2009), pp. 445-448. [190] J. Irwin, D. Charbonneau, P. Nutzman, and E. Falco, The MEarth Project: Searching for Transiting Habitable Super- Earth Planets Around Nearby M-Dwarfs, in Transiting Planets, Proceedings of the International Astronomical Union, IAU Symposium, 253 (2009), pp. 37-43. [191] J. H. Steffen et al, Five Kepler Target Stars That Show Multiple Transiting Exoplanet Candidates, Astrophys. J., 725 (2010), pp. 1226-1241. [192] M. J. Holman, and N. W. Murray, The Use of Transit Timing to Detect Terrestrial-Mass Extrasolar Planets, Science 307 (2005), pp. 1288-1291. [193] A. R. Dobrovolskis, and W. J. Borucki, Influence of Jovian Extrasolar Planets on Transits of Inner Planets, Bull. Am. Astron. Soc. 28 (1996), p. 1112. [194] J. Miralda-Escud`e, Orbital Perturbations of Transiting Planets: A Possible Method to Measure Stellar Quadruples and to Detect Earth-Mass Planets, Astrophys. J. 564 (2002), pp. 1019-1023. [195] E. Agol, J. Steffen, R. Sari, and W. Clarkson, On Detecting Terrestrial Planets With Timing of Giant Planet Transits, Mon. Not. Roy. Astron. Soc., 359 (2005), pp. 567-579. [196] J. H. Steffen, and E. Agol, An analysis of the transit times of TrES-1b, Mon. Not. Roy. Astron. Soc. 364 (2005), pp. L96-L100. [197] N. Haghighipour, E. Agol,and J. Steffen, Detection of Super-Earth Planets via the Transit Timing Variation Method, American Geophysical Union (2008), abstract P14B-04. [198] E. B. Ford et al, Transit Timing Observations from Kepler: I. Statistical Analysis of the First Four Month, arXiv:1102.0544v1. [199] M. J. Payne, E. B. Ford, and D. Veras, Transit Timing Variations for Inclined and Retrograde Exoplanetary Systems, The Astrophys. J. 712 (2010), pp. L86-L92. [200] N. Haghighipour, and S. Kirste On the Detection of (Habitable) Around Low-Mass Stars using Kepler and Transit Timing Variation Method, Celest. Mech. Dynamic. Astron., (2011), in press (arXiv:1107.2885) [201] D. Nesvor`y, Transit Timing Variations for Eccentric and Inclined Exoplanets, Astrophys. J. 701 (2009), pp. 1116- 1122. [202] D. Nesvorn`y, and C. Beaug`e, Fast Inversion Method for Determination of Planetary Parameters from Transit Timing Variations, Astrophys. J. 709 (2010), pp. L44-L48. [203] A. W. Mann, E. Gaidos, and B. S. Gaudi, The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants, Astrophys. J. 719 (2010), pp. 1454-1469.