DOCSLIB.ORG

5 Habitable Zones and Planetary Atmospheres

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
5 Habitable Zones and Planetary Atmospheres 5 Habitable zones and Planetary atmospheres 5.1 Introduction Questions to answer: How does star affect the emergence and sustainability of life on a planet? What are the main properties of planetary atmospheres in the Solar system? How and what can we learn about atmospheres of extrasolar planets? 5.2 Habitable zones Unique Earth’s properties enabling the life support: liquid water planetary environment (atmosphere, etc.) A circumstellar habitable zone is defined as encompassing the range of distances from a star for which liquid water can exist on a planetary surface. Pure water exists as a liquid between 273 K and 373 K, unless the pressure is too low. Therefore, the primary factor in determining a planet’s habitability is temperature. The planet temperature is defined by the balance between absorption of stellar radiation (neglecting internal sources) re-radiation of it The stellar energy absorbed by the planet is L ER 2 * (1 a ) , ap4 d 2 where L* is stellar luminosity, Rp is radius of the planet, d is distance between the planet and the star, and a is albedo. The amount of energy to be radiated by the planet can be expressed as 24 Lp 4 RTpe , where Te is effective temperature of the planet. The balance between Lp and Ea defines the effective temperature of the planet. The effective temperature of the Earth is 255 K. It is lower than 273 K because the Earth is not a perfect black body and it traps some energy because of the greenhouse effect . If the Sun would be more luminous, to maintain the same effective temperature the Earth should be further away, and vice versa: Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-1 S.V. Berdyugina, University of Freiburg Figure 1: Left: the distance to the habitable zone versus stellar luminosity calculated from the balance between Lp and Ea. Right: the evolutionary track of a solar-type star starting on the Main Sequence and continued through the Red Giant and Horizontal Branch phases to the Asymptotic Giant Branch. The luminosity of the star increases by several orders of magnitude. 4 Gyr ago the solar luminosity was lower by 30%, as main-sequence stars move up along the sequence and further to the red giant phases habitable zone was closer to the Sun habitable zone is moving away from the Sun The continuous habitable zone (CHZ) is the region, in which a planet may reside and maintain liquid water throughout most of a star’s life. Figure 2: Continuous habitable zone: yellow region is the habitable zone in the beginning of the main sequence, blue region is the habitable zone at late stages of the stellar evolution, and the green region is an intersection of the other two. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-2 S.V. Berdyugina, University of Freiburg Figure 3: Evolution of the solar luminosity (left) and the HZ (right). The most realistic model of the HZ is depicted by long dashes (Kasting et al. 1993). To estimate realistically the CHZ for the Sun, one needs to take into account albedo greenhouse effect The inner edge of the CHZ is determined by photodissociation of water and loss of hydrogen. 0.95 AU The outer edge of the CHZ is mainly defined by formation of CO2 clouds. 1.15 AU This contradicts however to the fact that surface of Mars was once carved by streams of some flowing liquid. The used climate model should be modified by adding other greenhouse gases (e.g. CH4) and perhaps by including a more dense cloud cover on Mars. Tidal heating of satellites around giant planets, such as Jupiter’s satellite Europa, raises the possibility of liquid water existing below the surface of ice-covered satellites. Sun in time To understand the early phase of the planet formation and the evolution of the habitable zone in the Solar system, we need to study the effects of the young Sun on planets. This is possible with a sample of stars of about the same mass as the Sun but at different evolutionary stages. A study of such stars shows that the solar radiation has undergone dramatic variations since it arrived to the Main Sequence. For instance, the X-ray luminosity of the young Sun (~108 years) was 1000 times of the present level. Fig. 4 shows the X-ray luminosity of solar-type stars versus the age. In Fig. 5 this is compared with the radiation in other wavelengths. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-3 S.V. Berdyugina, University of Freiburg Figure 4: The X-ray luminosity of solar-type stars versus the age. Note the decrease by three orders of magnitude over 10 Gyr. Figure 5: The X-ray, UV and total luminosity of the Sun versus age as deduced from solar analogues. The young Sun had UV radiation factor of 10 to 100 higher than the present level, while its total luminosity was in fact only 70% of the present. Magnetic activity of the Sun (flares, coronal mass ejections, solar wind, etc.) was also much more prominent as compared to the Sun today. These factors have significantly influenced the planetary atmospheres, their composition and density. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-4 S.V. Berdyugina, University of Freiburg Effects of Solar magnetic activity on terrestrial planets Mercury 0.39 AU from the Sun Variation of the mean density with diameter of the terrestrial planets as well as the Moon (shown on the right) indicates that Mercury has a much higher mean density than expected given its size. This is due to the relative size of its iron core which is significantly larger than for any other terrestrial planet. One possible explanation is that Mercury’s lighter mantle/crust was eroded away by the strong (~1,000 times present values) winds and the early Sun’s higher extreme ultraviolet fluxes. The eroded and ionized material has been carried away with the solar wind. Figure 6: The mean density of terrestrial planets versus the diameter. Mercury significantly deviates from the common tendency. Venus 0.71 AU from the Sun No water or oxygen. The explanation is in photochemistry/photoionization effects: Venus has a slow rotation period (P= 243 days) and a very weak magnetic dynamo Venus is thus not protected from the Sun’s plasma by planetary magnetic field Perhaps the young Sun’s enhanced activity and UV flux played a major role: e.g. Photodissociation of water H2O H + OH, and subsequent loss of H. Earth 1 AU from the Sun A Young active Sun may have played a major role in the evolution of the Earth’s atmosphere and possibly the origin and evolution of life. Destruction of methane (CH4) by the early Sun’s strong UV radiation Formation of ozone (O3) via photoionization of O2 Photochemical reactions leading to the formation of organic molecules Erosion of the atmosphere due to enhanced solar wind: loss of H+, O++, N+ Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-5 S.V. Berdyugina, University of Freiburg Mars 1.52 AU from the Sun Today, Mars is a cold dry planet with a thin atmosphere rich in CO2. Mars also possesses a very weak magnetic field. There is also geological evidence of running water and possibly a permanent layer of permafrost. It is important to study the effects of the active young Sun on Mars: Loss of water and atmosphere Soil oxidation Possible early life Early Mars: liquid iron core produced a magnetic field strong enough to protect the young Martian atmosphere and surface water from the punishing effects of the young Sun’s intense solar wind. (Lammer et al. 2003). Roughly 3.5 Billion years ago, Mars’ core solidified, shutting down the Martian magnetic dynamo. Consequences: Without a magnetic field, the outer Martian atmosphere was subjected to the ionizing effects and strong winds of the sun, and began to erode. At this time, water disassociates into 2H+O, where the lighter hydrogen is lost to the space while the heavier oxygen combines with iron on its surface Habitable zones around other stars In principle low-mass stars (dwarfs) are prime candidates for searches of planets in habitable zones: Long life times on the Main Sequence (>10Gyr, see figure below) Very abundant in the solar neighborhood (>70%) Better contrast star/planet for detection 4 1.4 M.; F4-51.4 M 1.0 M.;1.0 G2 M 3 F 4-5 G2 2 “stable” lifetime ~2.5 Gyr “stable” lifetime 1 L/L . Figure 7: Variations of ~8 Gyr the total luminocity for log stars of different masses 0 and spectral classes. “stable” lifetime >20 Gyr 0.70.7 M M.;, K K2-3 2-3 Low-mass K-M dwarfs have long periods of -1 “stable” lifetime >40 Gyr 0.4 M, M 1-2 constant luminosity and 0.4 M.; M1-2 are therefore prime -2 candidates for searches 0.0 5.0e+9 1.0e+10 1.5e+10 2.0e+10 of planets in habitable Age (years) zones. Astrobiology: 5. Habitable zones and Planetary Atmospheres 5-6 S.V. Berdyugina, University of Freiburg Stars later than F0 have main sequence lifetimes exceeding 2 Gyr. They are therefore potential candidates for harboring habitable planets. The HZ around an F star is larger and occurs farther out than for our Sun. The CHZ of F stars are narrower (log distance) than for the Sun because they evolve more rapidly. the HZ around K and M stars is smaller and occurs closer to the star.
Load more

Recommended publications
  • Exploring Exoplanet Populations with NASA's Kepler Mission
    SPECIAL FEATURE: PERSPECTIVE PERSPECTIVE SPECIAL FEATURE: Exploring exoplanet populations with NASA’s Kepler Mission Natalie M. Batalha1 National Aeronautics and Space Administration Ames Research Center, Moffett Field, 94035 CA Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board June 3, 2014 (received for review January 15, 2014) The Kepler Mission is exploring the diversity of planets and planetary systems. Its legacy will be a catalog of discoveries sufficient for computing planet occurrence rates as a function of size, orbital period, star type, and insolation flux.The mission has made significant progress toward achieving that goal. Over 3,500 transiting exoplanets have been identified from the analysis of the first 3 y of data, 100 planets of which are in the habitable zone. The catalog has a high reliability rate (85–90% averaged over the period/radius plane), which is improving as follow-up observations continue. Dynamical (e.g., velocimetry and transit timing) and statistical methods have confirmed and characterized hundreds of planets over a large range of sizes and compositions for both single- and multiple-star systems. Population studies suggest that planets abound in our galaxy and that small planets are particularly frequent. Here, I report on the progress Kepler has made measuring the prevalence of exoplanets orbiting within one astronomical unit of their host stars in support of the National Aeronautics and Space Admin- istration’s long-term goal of finding habitable environments beyond the solar system. planet detection | transit photometry Searching for evidence of life beyond Earth is the Sun would produce an 84-ppm signal Translating Kepler’s discovery catalog into one of the primary goals of science agencies lasting ∼13 h.
    [Show full text]
  • Formation of TRAPPIST-1
    EPSC Abstracts Vol. 11, EPSC2017-265, 2017 European Planetary Science Congress 2017 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2017 Formation of TRAPPIST-1 C.W Ormel, B. Liu and D. Schoonenberg University of Amsterdam, The Netherlands ([email protected]) Abstract start to drift by aerodynamical drag. However, this growth+drift occurs in an inside-out fashion, which We present a model for the formation of the recently- does not result in strong particle pileups needed to discovered TRAPPIST-1 planetary system. In our sce- trigger planetesimal formation by, e.g., the streaming nario planets form in the interior regions, by accre- instability [3]. (a) We propose that the H2O iceline (r 0.1 au for TRAPPIST-1) is the place where tion of mm to cm-size particles (pebbles) that drifted ice ≈ the local solids-to-gas ratio can reach 1, either by from the outer disk. This scenario has several ad- ∼ vantages: it connects to the observation that disks are condensation of the vapor [9] or by pileup of ice-free made up of pebbles, it is efficient, it explains why the (silicate) grains [2, 8]. Under these conditions plan- TRAPPIST-1 planets are Earth mass, and it provides etary embryos can form. (b) Due to type I migration, ∼ a rationale for the system’s architecture. embryos cross the iceline and enter the ice-free region. (c) There, silicate pebbles are smaller because of col- lisional fragmentation. Nevertheless, pebble accretion 1. Introduction remains efficient and growth is fast [6]. (d) At approx- TRAPPIST-1 is an M8 main-sequence star located at a imately Earth masses embryos reach their pebble iso- distance of 12 pc.
    [Show full text]
  • Water, Habitability, and Detectability Steve Desch
    Water, Habitability, and Detectability Steve Desch PI, “Exoplanetary Ecosystems” NExSS team School of Earth and Space Exploration, Arizona State University with Ariel Anbar, Tessa Fisher, Steven Glaser, Hilairy Hartnett, Stephen Kane, Susanne Neuer, Cayman Unterborn, Sara Walker, Misha Zolotov Astrobiology Science Strategy NAS Committee, Beckmann Center, Irvine, CA (remotely), January 17, 2018 How to look for life on (Earth-like) exoplanets: find oxygen in their atmospheres How Earth-like must an exoplanet be for this to work? Seager et al. (2013) How to look for life on (Earth-like) exoplanets: find oxygen in their atmospheres Oxygen on Earth overwhelmingly produced by photosynthesizing life, which taps Sun’s energy and yields large disequilibrium signature. Caveats: Earth had life for billions of years without O2 in its atmosphere. First photosynthesis to evolve on Earth was anoxygenic. Many ‘false positives’ recognized because O2 has abiotic sources, esp. photolysis (Luger & Barnes 2014; Harman et al. 2015; Meadows 2017). These caveats seem like exceptions to the ‘rule’ that ‘oxygen = life’. How non-Earth-like can an exoplanet be (especially with respect to water content) before oxygen is no longer a biosignature? Part 1: How much water can terrestrial planets form with? Part 2: Are Aqua Planets or Water Worlds habitable? Can we detect life on them? Part 3: How should we look for life on exoplanets? Part 1: How much water can terrestrial planets form with? Theory says: up to hundreds of oceans’ worth of water Trappist-1 system suggests hundreds of oceans, especially around M stars Many (most?) planets may be Aqua Planets or Water Worlds How much water can terrestrial planets form with? Earth- “snow line” Standard Sun distance models of distance accretion suggest abundant water.
    [Show full text]
  • Last Time: Planet Finding
    Last Time: Planet Finding • Radial velocity method • Parent star’s Doppler shi • Planet minimum mass, orbital period, semi- major axis, orbital eccentricity • UnAl Kepler Mission, was the method with the most planets Last Time: Planet Finding • Transits – eclipse of the parent star: • Planetary radius, orbital period, semi-major axis • Now the most common way to find planets Last Time: Planet Finding • Direct Imaging • Planetary brightness, distance from parent star at that moment • About 10 planets detected Last Time: Planet Finding • Lensing • Planetary mass and, distance from parent star at that moment • You want to look towards the center of the galaxy where there is a high density of stars Last Time: Planet Finding • Astrometry • Tiny changes in star’s posiAon are not yet measurable • Would give you planet’s mass, orbit, and eccentricity One more important thing to add: • Giant planets (which are easiest to detect) are preferenAally found around stars that are abundant in iron – “metallicity” • Iron is the easiest heavy element to measure in a star • Heavy-element rich planetary systems make planets more easily 13.2 The Nature of Extrasolar Planets Our goals for learning: • What have we learned about extrasolar planets? • How do extrasolar planets compare with planets in our solar system? Measurable Properties • Orbital period, distance, and orbital shape • Planet mass, size, and density • Planetary temperature • Composition Orbits of Extrasolar Planets • Nearly all of the detected planets have orbits smaller than Jupiter’s. • This is a selection effect: Planets at greater distances are harder to detect with the Doppler technique. Orbits of Extrasolar Planets • Orbits of some extrasolar planets are much more elongated (have a greater eccentricity) than those in our solar system.
    [Show full text]
  • Formation of the Solar System (Chapter 8)
    Formation of the Solar System (Chapter 8) Based on Chapter 8 • This material will be useful for understanding Chapters 9, 10, 11, 12, 13, and 14 on “Formation of the solar system”, “Planetary geology”, “Planetary atmospheres”, “Jovian planet systems”, “Remnants of ice and rock”, “Extrasolar planets” and “The Sun: Our Star” • Chapters 2, 3, 4, and 7 on “The orbits of the planets”, “Why does Earth go around the Sun?”, “Momentum, energy, and matter”, and “Our planetary system” will be useful for understanding this chapter Goals for Learning • Where did the solar system come from? • How did planetesimals form? • How did planets form? Patterns in the Solar System • Patterns of motion (orbits and rotations) • Two types of planets: Small, rocky inner planets and large, gas outer planets • Many small asteroids and comets whose orbits and compositions are similar • Exceptions to these patterns, such as Earth’s large moon and Uranus’s sideways tilt Help from Other Stars • Use observations of the formation of other stars to improve our theory for the formation of our solar system • Use this theory to make predictions about the formation of other planetary systems Nebular Theory of Solar System Formation • A cloud of gas, the “solar nebula”, collapses inwards under its own weight • Cloud heats up, spins faster, gets flatter (disk) as a central star forms • Gas cools and some materials condense as solid particles that collide, stick together, and grow larger Where does a cloud of gas come from? • Big Bang -> Hydrogen and Helium • First stars use this
    [Show full text]
  • Survival of Satellites During the Migration of a Hot Jupiter: the Influence of Tides
    EPSC Abstracts Vol. 13, EPSC-DPS2019-1590-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license. Survival of satellites during the migration of a Hot Jupiter: the influence of tides Emeline Bolmont (1), Apurva V. Oza (2), Sergi Blanco-Cuaresma (3), Christoph Mordasini (2), Pierre Auclair-Desrotour (2), Adrien Leleu (2) (1) Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, CH-1290 Sauverny, Switzerland ([email protected]) (2) Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, 3012, Bern, Switzerland (3) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Abstract 2. The model We explore the origin and stability of extrasolar satel- lites orbiting close-in gas giants, by investigating if the Tidal interactions 1 M⊙ satellite can survive the migration of the planet in the 1 MIo protoplanetary disk. To accomplish this objective, we 1 MJup used Posidonius, a N-Body code with an integrated tidal model, which we expanded to account for the migration of the gas giant in a disk. Preliminary re- Inner edge of disk: ain sults suggest the survival of the satellite is rare, which Type 2 migration: !mig would indicate that if such satellites do exist, capture is a more likely process. Figure 1: Schema of the simulation set up: A Io-like satellite orbits around a Jupiter-like planet with a solar- 1. Introduction like host star. Satellites around Hot Jupiters were first thought to be lost by falling onto their planet over Gyr timescales (e.g. [1]). This is due to the low tidal dissipation factor of Jupiter (Q 106, [10]), likely to be caused by the 2.1.
    [Show full text]
  • Lunar & Planetary Surface Power Management & Distribution
    NASA SBIR 2020 Phase I Solicitation Z1.05 Lunar & Planetary Surface Power Management & Distribution Lead Center: GRC Participating Center(s): GSFC, JSC Technology Area: TA3 Space Power and Energy Storage Scope Title Innovative ways to transmit high power for lunar & Mars surface missions Scope Description The Global Exploration Roadmap (January 2018) and the Space Policy Directive (December 2017) detail NASA’s plans for future human-rated space missions. A major factor in this involves establishing bases on the lunar surface and eventually Mars. Surface power for bases is envisioned to be located remotely from the habitat modules and must be efficiently transferred over significant distances. The International Space Station (ISS) has the highest power (100kW), and largest space power distribution system with eight interleaved micro-grids providing power functions similar to a terrestrial power utility. Planetary bases will be similar to the ISS with expectations of multiple power sources, storage, science, and habitation modules, but at higher power levels and with longer distribution networks providing interconnection. In order to enable high power (>100kW) and longer distribution systems on the surface of the moon or Mars, NASA is in need of innovative technologies in the areas of lower mass/higher efficiency power electronic regulators, switchgear, cabling, connectors, wireless sensors, power beaming, power scavenging, and power management control. The technologies of interest would need to operate in extreme temperature environments, including lunar night, and could experience temperature changes from -153C to 123C for lunar applications, and -125C to 80C for Mars bases. In addition to temperature extremes, technologies would need to withstand (have minimal degradation from) lunar dust/regolith, Mars dust storms, and space radiation levels.
    [Show full text]
  • University of Groningen Evaluating Galactic Habitability Using High
    University of Groningen Evaluating galactic habitability using high-resolution cosmological simulations of galaxy formation Forgan, Duncan; Dayal, Pratika; Cockell, Charles; Libeskind, Noam Published in: International Journal of Astrobiology DOI: 10.1017/S1473550415000518 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Final author's version (accepted by publisher, after peer review) Publication date: 2017 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Forgan, D., Dayal, P., Cockell, C., & Libeskind, N. (2017). Evaluating galactic habitability using high- resolution cosmological simulations of galaxy formation. International Journal of Astrobiology, 16(1), 60–73. https://doi.org/10.1017/S1473550415000518 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal.
    [Show full text]
  • Exploring Exoplanet Populations with NASA's Kepler Mission
    SPECIAL FEATURE: PERSPECTIVE PERSPECTIVE SPECIAL FEATURE: Exploring exoplanet populations with NASA’s Kepler Mission Natalie M. Batalha1 National Aeronautics and Space Administration Ames Research Center, Moffett Field, 94035 CA Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board June 3, 2014 (received for review January 15, 2014) The Kepler Mission is exploring the diversity of planets and planetary systems. Its legacy will be a catalog of discoveries sufficient for computing planet occurrence rates as a function of size, orbital period, star type, and insolation flux.The mission has made significant progress toward achieving that goal. Over 3,500 transiting exoplanets have been identified from the analysis of the first 3 y of data, 100 planets of which are in the habitable zone. The catalog has a high reliability rate (85–90% averaged over the period/radius plane), which is improving as follow-up observations continue. Dynamical (e.g., velocimetry and transit timing) and statistical methods have confirmed and characterized hundreds of planets over a large range of sizes and compositions for both single- and multiple-star systems. Population studies suggest that planets abound in our galaxy and that small planets are particularly frequent. Here, I report on the progress Kepler has made measuring the prevalence of exoplanets orbiting within one astronomical unit of their host stars in support of the National Aeronautics and Space Admin- istration’s long-term goal of finding habitable environments beyond the solar system. planet detection | transit photometry Searching for evidence of life beyond Earth is the Sun would produce an 84-ppm signal Translating Kepler’s discovery catalog into one of the primary goals of science agencies lasting ∼13 h.
    [Show full text]
  • The Search for Exomoons and the Characterization of Exoplanet Atmospheres
    Corso di Laurea Specialistica in Astronomia e Astrofisica The search for exomoons and the characterization of exoplanet atmospheres Relatore interno : dott. Alessandro Melchiorri Relatore esterno : dott.ssa Giovanna Tinetti Candidato: Giammarco Campanella Anno Accademico 2008/2009 The search for exomoons and the characterization of exoplanet atmospheres Giammarco Campanella Dipartimento di Fisica Università degli studi di Roma “La Sapienza” Associate at Department of Physics & Astronomy University College London A thesis submitted for the MSc Degree in Astronomy and Astrophysics September 4th, 2009 Università degli Studi di Roma ―La Sapienza‖ Abstract THE SEARCH FOR EXOMOONS AND THE CHARACTERIZATION OF EXOPLANET ATMOSPHERES by Giammarco Campanella Since planets were first discovered outside our own Solar System in 1992 (around a pulsar) and in 1995 (around a main sequence star), extrasolar planet studies have become one of the most dynamic research fields in astronomy. Our knowledge of extrasolar planets has grown exponentially, from our understanding of their formation and evolution to the development of different methods to detect them. Now that more than 370 exoplanets have been discovered, focus has moved from finding planets to characterise these alien worlds. As well as detecting the atmospheres of these exoplanets, part of the characterisation process undoubtedly involves the search for extrasolar moons. The structure of the thesis is as follows. In Chapter 1 an historical background is provided and some general aspects about ongoing situation in the research field of extrasolar planets are shown. In Chapter 2, various detection techniques such as radial velocity, microlensing, astrometry, circumstellar disks, pulsar timing and magnetospheric emission are described. A special emphasis is given to the transit photometry technique and to the two already operational transit space missions, CoRoT and Kepler.
    [Show full text]
  • Why Should We Care About TRAPPIST-1?
    PUBLISHED: 27 MARCH 2017 | VOLUME: 1 | ARTICLE NUMBER: 0104 news & views EXOPLANETS Why should we care about TRAPPIST-1? The discovery of a system of six (possibly system we’ve found with terrestrial planets seven) terrestrial planets around the tightly packed together and in resonance. TRAPPIST-1 star, announced in a paper Their orbits are also all co-planar, as the by Michaël Gillon and collaborators image shows, and luckily for us this orbital (Nature 542, 456–460; 2017) attracted a plane passes through the line of sight great deal of attention from researchers between us and TRAPPIST-1: all these as well as the general public. Reactions planets are ‘transiting’. This is much more ranged from the enthusiastic — already than a simple technical detail, as it allows us hypothesizing a string of inhabited to characterize all their atmospheres — an planets — to those who felt that it essential step to understand their climate didn’t add anything substantial to our and propensity for habitability. current knowledge of exoplanets and Finally and maybe most importantly, was ultimately not worth the hype. The these planets are distributed over all of cover of the Nature issue hosting the the three zones related to different phases paper (pictured), made by Robert Hurt, (IPAC) HURT NASA/JPL-CALTECH/ROBERT of water, as cleverly represented by the a visualization scientist at Caltech, is a illustration. However, this information, fine piece of artwork, but it also helps us They can thus be called ‘terrestrial’ without linked to the concept of habitable or in assessing the actual relevance of the any qualms about terminology.
    [Show full text]
  • Can Brown Dwarfs Survive on Close Orbits Around Convective Stars? C
    A&A 589, A55 (2016) Astronomy DOI: 10.1051/0004-6361/201527100 & c ESO 2016 Astrophysics Can brown dwarfs survive on close orbits around convective stars? C. Damiani1 and R. F. Díaz2 1 Université Paris-Sud, CNRS, Institut d’Astrophysique Spatiale, UMR8617, 91405 Orsay Cedex, France e-mail: [email protected] 2 Observatoire astronomique de l’Université de Genève, 51 ch. des Maillettes, 1290 Versoix, Switzerland e-mail: [email protected] Received 1 August 2015 / Accepted 23 February 2016 ABSTRACT Context. The mass range of brown dwarfs extends across the planetary domain to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM-type stars compared to exoplanets for orbital periods of less than a few years, but most of the short-period brown dwarf companions that are fully characterised by transits and radial velocities are found around F-type stars. Aims. We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convec- tive envelopes because the tides and angular momentum loss, the result of magnetic braking, would lead to a rapid orbital decay with the companion being quickly engulfed. Methods. We use a classical Skumanich-type braking law and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. Results. We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with an orbital period less than five days.
    [Show full text]