Exoplanet Status Report: Observation, Characterization and Evolution of Exoplanets and Their Host Stars1 H
ISSN 0038 0946, Solar System Research, 2010, Vol. 44, No. 4, pp. 290–310. © Pleiades Publishing, Inc., 2010. Original Russian Text © H. Lammer, A. Hanslmeier, J. Schneider, I.K. Stateva, M. Barthelemy, A. Belu, D. Bisikalo, M. Bonavita, V. Eybl, V. Coudé du Foresto, M. Fridlund, R. Dvorak, S. Eggl, J. M. Grieβmeier, M. Güdel, E. Günther, W. Hausleitner, M. Holmström, E. Kallio, M.L. Khodachenko, A.A. Konovalenko, S. Krauss, L.V. Ksanfomality, Yu.N. Kulikov, K. Kyslyakova, M. Leitzinger, R. Liseau, E. Lohinger, P. Odert, E. Palle, A. Reiners, I. Ribas, H.O. Rucker, N. Sarda, J. Seckbach, V.I. Shematovich, A. Sozzetti, A. Tavrov, M. Xiang Grüβ, 2010, published in Astronomicheskii Vestnik, 2010, Vol. 44, No. 4, pp. 314–335.
Exoplanet Status Report: Observation, Characterization and Evolution of Exoplanets and Their Host Stars1 H. Lammer1, A. Hanslmeier2, J. Schneider3, I. K. Stateva4, M. Barthelemy5, A. Belu6, D. Bisikalo7, M. Bonavita8, V. Eybl9, V. Coudé du Foresto10, M. Fridlund11, R. Dvorak9, S. Eggl9, J. M. Grieβmeier12, M. Güdel9,13, E. Günther14, W. Hausleitner1, M. Holmström15, E. Kallio16, M. L. Khodachenko1, A. A. Konovalenko17, S. Krauss1, L. V. Ksanfomality18, Yu. N. Kulikov19, K. Kyslyakova22, M. Leitzinger2, R. Liseau21, E. Lohinger9, P. Odert2, E. Palle13, A. Reiners22, I. Ribas23, H. O. Rucker1, N. Sarda24, J. Seckbach25, V. I. Shematovich7, A. Sozzetti26, A. Tavrov16, M. Xiang Grüβ27 1 Space Research Institute (IWF), Austrian Academy of Sciences, Graz, Austria 2 Institute for Geophysics and Meteorology (IGAM), University of Graz, Austria 3 Observatoire Paris Site de Meudon LUTH, Paris, France 4 European Science Foundation, Standing Committee for Physical and Engineering Sciences, Strasbourg, France 5 Laboratoire de Planétologie de Grenoble CNRS UJF, Grenoble, France 6 Université de Bordeaux, France 7 Institut for Astronomy (INASAN), Russian Academy of Sciences, Moscow, Russia 8 Osservatorio Astronomico di Padova, INAF, Italy 9 Institute for Astronomy, University of Vienna, Vienna, Austria 10 Observatoiré de Paris—LESIA, Paris, France 12 ASTRON, Dwingeloo, The Netherlands 13 ETH Zürich Astrophysics Institute of Astronomy, Zürich, Switzerland 14 Instituto de Astrofisica de Canarias, Tenerife, Spain 15 The Swedish Institute of Space Physics, Kiruna, Sweden 16 Finish Meteorological Institute (FMI), Helsinki, Finland 17 Institute of Radio Astronomy in Kharkov of the Ukrainian Academy of Sciences, Ukraine 18 Space Research Institute (IKI), Russian Academy of Sciences, Moscow, Russia 19 Solar Geophysical Institute (PGI), Russian Academy of Sciences, Murmansk, Russia 20 Institute for Applied Physics (IAP), Russian Academy of Sciences, Nizhny Novgorod, Russia 21 Onsala Space Observatory Chalmers University of Technology, Onsala, Sweden 22 Universitát Göttingen, Institut für Astrophysik, Göttingen, Germany 23 Institut d’Estudis Espacials de Catalunya, Barcelona, Spain 24 Astrium Ltd, Stevenage, United Kingdom 25 The Hebrew University of Jerusalem, Jerusalem, Israel 26 INAF—Osservatorio Astronomico di Torino, Torino, Italy 27 Institut für Theoretische Physik und Astrophysik, Universitát zu Kiel, Kiel, Germany Received February 18, 2010
Abstract—After the discovery of more than 400 planets beyond our Solar System, the characterization of exoplanets as well as their host stars can be considered as one of the fastest growing fields in space science during the past decade. The characterization of exoplanets can only be carried out in a well coordinated interdiscipli nary way which connects planetary science, solar/stellar physics and astrophysics. We present a status report on the characterization of exoplanets and their host stars by reviewing the relevant space and ground based projects. One finds that the previous strategy changed from space mission concepts which were designed to search, find and characterize Earth like rocky exoplanets to: A statistical study of planetary objects in order to get information about their abundance, an identification of potential target and finally its analysis. Spectral analysis of exoplanets is mandatory, particularly to identify bio signatures on Earth like planets. Direct charac terization of exoplanets should be done by spectroscopy, both in the visible and in the infrared spectral range. The way leading to the direct detection and characterization of exoplanets is then paved by several questions, either concerning the pre required science or the associated observational strategy. DOI: 10.1134/S0038094610040039
290 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 291
Fig. 1. The workshop participants in front of the ESF workshop location in Hotel Legenstein, in Bairisch Kölldorf, Austria. From left 3th row: Alessandro Sozzetti, Alexander Tavrov, Ignasi Ribas, Enric Palle, Rene Liseau, Nicola Sarda, Martin Leitzinger, Siegfried Eggl, Ansgar Reiners, Eike Günther, Dmitry Bisikalo, Rudolf Dvorak, Jean Schneider; From left 2th row: Leonid Ksan 5 fomality, Ivanka K. Stateva, Elke Lohinger, Jean Mathias Grieömeier, Kristina Kyslyakova, Helmut Lammer, Adrian Belu, Mats Holmström, Valery I. Shematovich, Mathieu Barthelemy; From left 1th row: Mariangela Bonavita, Manuel Güdel, Joseph Seck bach, Yuri N. Kulikov, Arnold Hanslmeier.
1 INTRODUCTION potentially habitable rocky exoplanets will be discov ered during the not so distant future. The European Science Foundation (ESF) is an The workshop addressed three main aspects: association of 80 member organizations devoted to —The first objective focused on the discussion of scientific research in 30 European countries with the the present status related to exoplanet research, exist aim to advance and connect European research cen ing and future mission concepts, including space and tres and to explore new directions for research at the ground based projects. highest scientific level. During the year 2009 the Inter —The second objective was related to the charac national Year of Astronomy was celebrated. One of the terisation of the host stars of potential terrestrial most important and interesting problems in modern exoplanets. Because it is from fundamental impor astrophysics is the discovery of exoplanets. Because tance—related to habitability and atmosphere evolu exoplanet research becomes more and more impor tion—to observe and analyze the activity, radiation tant an ESF sponsored Exploratory Workshop was and plasma environment and the ages of exoplanetary held in the village of Bairisch Kölldorf, Austria, 29th host stars various methods for obtaining observational November–1st December 2009 (see Fig. 1). The char evidence of the radiation environment, Coronal Mass acterisation of these discoveries has evolved dramati Ejections (CMEs) and stellar winds as well as the stel cally since the first detection of the first Jupiter type lar age were addressed. object more than a decade ago. Now more than 400 —The third objective was a survey of exoplanet or exoplanets are known and the near future observing host star characterisation relevant projects/research facilities become extremely powerful. After the recent carried out at present, in non EU Eastern European discoveries of transiting small and low mass exoplanets countries especially the Russian Federation and the like CoRoT 7b with 1.65REarth by the CNES led Ukraine, so that exoplanet science activities become European CoRoT space observatory and the discov more known to researchers in these countries and new ery by the ground based MEarth project in the USA of scientific cooperation’s can be established. the 2.68REarth size planet GJ1214b, one can expect that Besides these objectives discussions and plans to establish a coordinated effort, which bring together 1 The article is published in the original. different research groups and project leaders so that
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 292 LAMMER et al. ground based and space based observations of mission. A 3 month extension of the Euclid mission exoplanets and their host stars can be coordinated in would be sufficient to undertake a unique microlens the best way, were also carried out. This status report ing survey of the Galactic Bulge, reaching detection on exoplanet projects and research summarizes the limits which would include planets similar to those in main points of the two day workshop. our solar system with masses larger than Mercury. In 2008 ESA has initiated an expert advisory team of advising the space agency on the best scientific and SPACE MISSIONS DESIGNED technological roadmap to address questions related to FOR THE DETECTION the characterization of terrestrial exoplanets up to the AND CHARACTERISATION possible detection of biomarkers on Earth like planets OF HABITABLE EXOPLANETS and the preparation of an exoplanet roadmap in order We are in an era where the field of exoplanetology to fulfill the scientific goals of the Cosmic Vision evolves very rapidly in three aspects: new scientific 2015–2025 scientific plan with respect to exoplanets results, new scientific ideas, new technological idea and especially terrestrial bodies orbiting stars other and development. This makes the science political than our Sun. situation evolving permanently. It is therefore difficult Shortly after the Exoplanet Roadmap Advisory to develop a definitive roadmap; here we describe the Team (EPRAT) was formed, an open Call for White situation as it is in early 2010. On the US side, the Papers for the European science community was Decadal Survey Astro2010 is currently working on US issued. During August 2008 and 2010 the EPRAT plans. The outcome is foreseen for late 2010. On the team evaluates the papers as well as suggestions by European side, in September 2004 the European interested scientists and takes them into account in the Space Agency ESA organized its Cosmic Vision preparation of a final report. An open workshop orga 2015–2025 workshop, at UNESCO in Paris, France nized by ESA as part of the EPRAT process will be which brought together scientists from all over Europe held in UCL, London, UK during April 7–8, 2010, where they could present and outline what they were the exoplanet roadmap will be discussed between thought should be the major issues of space explora participating scientist and the EPRAT team. Feed tion during 2015–2025, focusing on four major backs from the scientific community will be included themes such as: in the final report, which is intended to be finished —What are the conditions for planet formation during summer 2010 and the final roadmap will be and the emergence of life? presented to ESA, the ESA advisory bodies and the —from gas and dust to stars and planets; world wide scientific community in general. —from exoplanets to biomarkers; Besides the exoplanet roadmap exercises of the big —life and habitability in the Solar System; space agencies, the exoplanet science community established the so called Blue Dots team, which is —How does the Solar system work? organized around a core of coordinating groups of —What are the Fundamental Physical Laws of the experts in several themes which cover: Universe? —exoplanet targets and their environments; —How did the Universe originate and what is it —formation and evolution of planetary systems; made of? —habitability criteria; This exercise showed that Europe is richer than —observations of exoplanetary atmospheres and ever in ideas for what should be done in space science relevant detection techniques: in the coming years. ESAs Cosmic Vision 2015–2025 —single aperture imaging; programme aims at furthering Europe’s achievements in space science and is based on a massive response by —multiple aperture imaging; the scientific community to ESA’s call for themes, —microlensing; issued in April 2004. —radial velocities; Finally after advises and stepwise evaluations of the —astrometry; ESA advisory bodies such as the Astronomy Working —transits; Group (AGW) one M class exoplanet mission, —exoplanet modeling, including habitability studies. namely PLATO and the low cost mission of opportu Besides these scientific subjects a main concern is nity contribution to the Japanease SPICA telescope related to public outreach and education, the stepwise were selected 2007 and will be further studied in com building of a well organized and structured European petition with other M class missions for a possible exoplanet community, European networking and launch date around 2017. One should also mention funding search, acting as an interface with the industry that the Dark Energy Cosmology mission Euclid and observers/contact points for international collab which is also studied within the Cosmic Vision pro oration. In first steps scientists involved to the Blue gramme has a so called Extra Euclid Survey Science Dots team are also connected to the EU FP7 funded Programme (EESSP) where a microlensing survey Europlanet project within the Na2 working groups could be planned during the extensions of the Euclid Exoplanets and other planetary systems (WG5) and
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 293
Meilleurs Voeux pour 2007 Best Wishes for 2007
de la pert de toute l‘equipe CoRoT from all the CoRoT Team
Launch The CoRoT Soyuz Rocket on the launch December 27 th 2006 pad of the Baikonour Cosmodrome 14:23 UT
Fig. 2. Left, the CoRoT space observatory at the top of a Soyuz rocket on the launch path of the Baikonour Cosmodrome, short before the launch (insert) and right an illustration of the satellite in orbit.
Magnetic worlds, the Sun planet connection (WG4). discovery of the first hot Jupiter in the mid 90ies—the The Blue Dot team works, like the ExoPTF in the search for exoplanets was included in the mission pro USA and the EPRAT in Europe on a report, which will gram. CoRoT was successfully launched on December also be released during the first half of 2010 to the science 27 from the Baikonour Cosmodrome, and the first community, space agencies and decision makers. observations started during February 2007 (Fig. 2). However, one of the most important aspects will be Besides the discovery of several exosolar gas giants international cooperation with space agencies besides (e.g., Dvorak et al., 2010; see also special issue Astron. ESA, NASA and JAXA (e.g. in Russia, China, India, Astrophys., Vol. 506, 1, Oct. IV, 2009; Lammer et al., etc.). Given the budgetary limitations in any coun 2010; article on CoRoT discoveries in this issue), tries, cooperations in larger space missions will be CoRoT detected the first transiting rocky super essential. ESA, JAXA and NASA cannot in the com Earth type planet with a size of about 1.68 REarth ing decade build a large mission by their own. (Léger et al., 2009). The life time of the mission was scheduled for 3 years of observations and recently the extension for another three years was approved. EXISTING, PLANNED, AND FUTURE SPACE PROJECTS RELEVANT The CoRoT satellite has a length of about 4 m, a FOR THE CHARACTERISATION diameter of about 2 m and weights about 630 kg. The OF EXOPLANETS accuracy of the pointing is 0.5 arcsec with a capacity of the telemetry of 1.5 Gbit/day. Three systems are oper At present (March 2010), there are two space ating on the satellite. CoRoTel is an afocal telescope observatories, namely CoRoT (CNES led European) with 2 parabolic mirrors and aperture of 0.27 m and a and Kepler (NASA) in orbit, which search exoplanets cylindric baffle with a length of 2 m. CoRoTcam is a with the transit method. Transits of Earth size wide field camera consisting of a dioptric objective of exoplanets produce a small change in the host starts 4 6 lenses where the focal unit is equipped with 4 frame brightness of about 1/10 lasting for 2–16 hours. This transfer CCD 2048 × 4096. Two of the four CCDs are change must be absolutely periodic if it is caused by a for the exoplanet program and two are for the seismol planet and if possible at least three times confirmed. In ogy program. Finally CoRoTcase is hosting the elec addition, all transits produced by the same planet must tronics and the software managing the aperture pho be of the same change in brightness and last the same tometry processing. A total number of 12000 stars and amount of time, thus providing a highly repeatable sig magnitudes between 11 and 16 are observed in the nal and robust detection method. The size of the exoplanet mode. CoRoT polar orbit of the satellite planet is found from the depth of the transit and the allows observing 150 days, the long run, and between size of the star. 20 and 30 days the short run, both in the direction of the center respectively anticenter of the galaxy. The CoRoT and Kepler Space Observatories Radial velocity measurements to confirm planet The CoRoT (Convection, Rotation and planetary candidates are usually performed with the 3.6 m Transits) space observatory is led by the French space HARPS spectrograph in La Silla, the 1.93 m SOPHIE agency CNES, in a cooperation with ESA, Austria, spectrograph in the Observatoire de Haute Provence, Brazil, Belgium, Germany and Spain. Initially the the CORALIE spectrograph in La Silla, the Coud’e goal of the CoRoT mission was the observation and echelle spectrograph from the 2 m telescope in Tau study of variable stars via asteroseismology. After the tenburg (TLS), Germany and the UVES and
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 294 LAMMER et al. FLAMES spectrographs in Chile, as well as spectro searches, for the definition of the volume of space graphs at McDonald observatory. needed for the search. Based form the mission design, The discovery of CoRoT 7b, the first small rocky planet formation models and discovered exoplanets, exoplanet with measured radius and mass, and there the Kepler team estimated that the mission may dis fore with a known density, has opened up a new era, in cover about 50 planets with the size of the Earth, about which the CoRoT extended mission and Kepler are 185 exoplanets where most of them have sizes of about now playing a major role. While CoRoT has demon 1.3 REarth and more than 600 exoplanets with sizes strated that exoplanets can be observed via transits around 2.2 REarth. Kepler’s first discoveries of 4 from space, due to the mission design CoRoT cannot exoplanets in the hot Jupiter/Saturn domain, and one discover Earth size exoplanets within the habitable hot Neptune was announced at the American Astro zones of their host stars. This is a task where NASAs nomical Society in Washington DC, on January 4, this Kepler satellite will take over. year. The scientific objective NASAs Kepler satellite However, both telescopes, CoRoT and Kepler tar which was successfully launched on March 6, 2009 is get rather faint stars, up to mV = 15 and beyond. Their to explore the structure and diversity of planetary sys ground based follow up, in particular in radial veloc tems. The Kepler space observatory surveys a large ity monitoring, is made difficult by this relative faint sample of stars with the following main scientific ness. As a consequence, ground based confirmation goals: and mass measurements are restricted to the largest of —Determination of the percentage of terrestrial the CoRoT and Kepler planets, which impacts defi and larger planets there are in or near the habitable nitely the scientific return of these two missions. In zone of a wide variety of stars; case Kepler will discover Earth size planets within the —Determination of the distribution of sizes and habitable zones of Sun type stars the mass determina shapes of the orbits of these exoplanets; tion will be very difficult. —Estimate how many planets there are in multi ple star systems; PLATO: The Next Generation Exoplanet Transit —Determine the variety of orbit sizes and planet Mission reflectivity, sizes, masses and densities of short period giant exoplanets; ESAs PLATO mission is proposed as a next gener —Identify additional planets of each discovered ation exoplanet transit mission with the main focus to planetary system by using other techniques; detect and by including follow up programs to charac terize exoplanets and their host stars in the solar and neighborhood. The main differences between —Determine the properties of those stars that har CoRoT/Kepler and PLATO will be its strong focus on bor planetary systems. bright stellar targets, typically with mV ~ 11, including Kepler is a 0.95 m diameter telescope which has a a large number of very bright and nearby stars, with very large field of view of about 105 square degrees. mV ~ 8. Because of this PLATO would fit into the pre The fields of view of most telescopes are less than one characterization aim of rocky exoplanets close enough to square degree. Kepler needs this large a field in order the Solar System. If this is possible future missions could to observe continuously and simultaneously monitor spectroscopically study the atmospheres of the discov the brightnesses of more than 100 000 stars during the ered planets. The prime science goals of PLATO are: nominal mission life time of about 3.5 years. —The detection of exoplanets of all kinds reaching The design of the entire system is such that the down to small, terrestrial planets in the habitable zone; combined differential photometric precision over a —The study of the host stars (age, activity, etc.); 6.5 hour integration is less than 20 ppm (one sigma) for a 12th magnitude solar like star including an assumed —The pre characterization of the discovered stellar variability of 10 ppm. This is a conservative, worse exoplanetary systems, planets + host stars; case assumption of a grazing transit. A central transit of —The identification of suitable targets for future, the Earth crossing the Sun lasts 13 hours. And about more detailed characterization, including a spectro 75% of the stars older than 1 Gyr are less variable than scopic search for biomarkers in nearby habitable the Sun on the time scale of a transit. exoplanets. The science case of the Kepler fits like that of During ESAs assessment study as shown in Fig. 3, CoRoT into the discussed exoplanet roadmap steps three concepts for the PLATO telescope have been and supports also the objectives of the future ExoPTF studied. In the first concept, the chosen solution has recommended NASA Space Interferometry Mission 12 reflective telescopes with a field of view of about (SIM Lite) and the Terrestrial Planet Finder (TPF 1800 deg2, and a total collecting area for each observed C/I) projects. Although these future missions will not star of 0.15 m2, while the second concept uses re observe the Kepler discoveries, the CoRoT/Kepler 54 refractive telescopes with a field of view of 625 deg2, results can be used for the identification of common and a total collecting area for each observed star of stellar characteristics of host stars for future planet 0.29 m2. The third concept proposes a different
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 295 arrangement with 42 refractive telescopes. PLATO is planned to be launched by a Soyuz 2 1b launcher around 2017, which will inject the spacecraft in a direct transfer orbit and transfer the telescope to the second Lagrange point. These goals will be reached by ultra high precision, long (few years), uninterrupted photometric monitor ing in the visible band of very large samples of (pre selected) bright stars. The obtained space based data will be comple Fig. 3. Illustration of the three studied PLATO telescope mented by ground based follow up observations, in concepts. Right (first concept) 4 groups of 3 telescopes (brown) mounted on an angled optical bench. The deploy particular very precise radial velocity monitoring, able sunshield is shown in yellow/orange. Middle (second which will be used to confirm the planetary nature of concept) 54 telescopes mounted on the tilted base plate. the detected events and to measure the planet masses. The sun shield is shown in blue. FEE boxes are visible at Planetary rings and large moons around exo Jupiter’s the bottom. Each telescope has a hexagonal radiator attached to the barrel segment which is shown in yellow. or exo Saturn’s can also be detected by their modifi Right (third concept) telescopes mounted in groups on a cation on the shape and duration of the transit light staircase shaped optical bench (courtesy of the PLATO curve. For example, for a Saturn analogue at 1 AU team). from the parent star, the ingress and egress take one hour for the planet and two hours for the ring. In addi tion, the planet ingress (egress) starts (ends) steeper tions and for other future space projects/program ded for the planet than for the ring. Finally, the projected icated to the search for bio markers. inclination of the ring with respect to the planet’s Asteroseismic data from PLATO will be used to orbital plane and the ring optical depth can be derived measure the stellar masses and ages, to confirm and from the transit shape, which provides valuable infor improve the star’s radius already known from the Gaia mation for the formation and evolution studies of the mission, which will be discussed in the following Sec system. tion, as well as to study the internal structure and Several host stars of discovered exoplanets will be internal angular momentum of planet host stars. amongst the brightest. Therefore, they will be the nat ural targets for atmospheric studies, either through: Gaia —transmission spectroscopy; or The ESA Gaia all sky survey space mission, due to —reflection spectroscopic studies. be launched in Spring 2012, will monitor astrometri The second program of PLATO focuses on aster cally, during its 5 yr nominal mission lifetime, all point oseismology and stellar evolution. It is planed that the sources (stars, asteroids, quasars, extragalactic superno instrument will perform a detailed seismic analysis of vae, etc.) in the visual magnitude range 6–20 mag, a the planet host stars, allowing a precise determination huge database encompassing ~109 objects. Using the of their radii, masses and ages, from which the radii, continuous scanning principle first adopted for Hip masses and ages of the exoplanets will be derived. This parcos, Gaia will determine the five basic astrometric will provide a complete characterization of the parameters (two positional coordinates α and δ, two exoplanetary systems, including their evolutionary proper motion components μα and μδ, and the paral status. The obtained results will add to the knowledge lax ω ) for all objects, with end of mission precision of planets related to the age of their host star, activity, between 6 μs (at V = 6 mag) and 200 μas (at V = 20 mag). etc. If one obtains atmospheric knowledge by follow Gaia astrometry, complemented by on board spec up observations one can study the role of the host star trophotometry and (partial) radial velocity informa (age) activity to particular atmosphere species during tion, will have the precision necessary to quantify the the evolution of the planet in time. early formation, and subsequent dynamical, chemical To fulfill the whole program the mission relies on and star formation evolution of the Milky Way Galaxy. an extensive ground based follow up programme. The broad range of crucial issues in astrophysics that PLATO discoveries should be complementary with can be addressed by the wealth of the Gaia data is sum ESOs 42 m E ELT telescope. Phase A studies of the marized by e.g., Perryman et al. (2001). likely first generation suite of instruments are cur One of the relevant areas in which the Gaia obser rently underway. This includes the Exo Planet Imag vations will have great impact is the astrophysics of ing Camera and Spectrograph (EPICS), which will be planetary systems (e.g., Casertano et al., 2008), in par optimized for visible and near IR photometric, spec ticular when seen as a complement to other techniques troscopic and polarimetric observations. Interesting for planet detection and characterization (e.g., discovered exoplanets will be considered as prime tar Sozzetti, 2009). For multiple systems, a trade off will gets by E ELT follow up characterization observa have to be found between accuracy in the determina
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 296 LAMMER et al. tion of the mutual inclination angles between pairs of RELEVANT PROJECTS IN ORBIT, planetary orbits, single measurement precision and DEVELOPMENT OR STUDY: SPITZER/HST, redundancy in the number of observations with WSU UV, JWST, TESS, SPICA, SIM LITE respect to the number of estimated model parameters. It will constitute a challenge to correctly identify sig The Hubble Space Telescope (HST) and nals with amplitude close to the measurement uncer SPITZER space telescope detected recently atmo tainties, particularly in the presence of larger signals spheric species in the hydrogen rich atmospheres of a induced by other companions and/or sources of astro few transiting hot gas giants with a resolution of about physical noise of comparable magnitude. Finally, in 5. The telescopes can be used for transit spectroscopy cases of multiple component systems where dynami by taking a photometric measurement of the exoplanet cal interactions are important, fully dynamical New and its host star during the transit and later another tonian fits involving an n body code might have to be measurement when the star is occulting the exoplanet. used to properly model the Gaia astrometric data and By subtracting first signal from the second, the to ensure the short and long term stability of the remained signal from the planet can be analyzed. solution (Sozzetti, 2005). Because the signal obtained from the exoplanet is very faint compared to that of its host star, such observa All the above issues could have a significant impact tions demand a very high photometric stability over on Gaia’s capability to detect and characterize plane the observing time. Theoretical models indicate that tary systems. For these reasons, within the pipeline of transit spectroscopy may be possible for super Earth’s Coordination Unit 4 (object processing) of the Gaia if they orbit around M type dwarf stars which are Data Processing and Analysis Consortium (DPAC), in cooler and therefore fainter than our Sun. charge of the scientific processing of the Gaia data and production of the final Gaia catalogue to be released The HST follower, James Webb Space Telescope sometime in 2020, a Development Unit (DU) has (JWST), currently constructed by ESA/NASA and been specifically devoted to the modelling of the astro launched in 2014 is a large infrared telescope with a 6.5 meter primary mirror. Originally planned for metric signals produced by planetary systems. The DU observations related to star and planet formation, one is composed of several tasks, which implement multi of its key scientific objectives will be the characteriza ple robust procedures for (single and multiple) astro tion of exoplanets. This capability was recently recog metric orbit fitting (such as Markov Chain Monte nized after the recent observations by SPITZER and Carlo and genetic algorithms) and the determination HST. The JWST has three instruments which are rele of the degree of dynamical stability of multiple com vant for exoplanet characterization. The NIRCAM ponent systems. covers wavelengths between 2–5 μm, the TFI 2–5 μm Gaia holds promise for crucial contributions to and MIRI 5–28 μm. By using the transit spectroscopy many aspects of planetary systems astrophysics, in method as described above, JWST will make also a combination with present day and future extrasolar contribution to the detection of atmospheric species in planet search programs. For example, the Gaia data, exoplanet atmospheres. There are model simulations over the next decade, will allow us to: and plans to investigate the possibility that, if transit ing super Earth’s orbit nearby red cool, faint dwarf —significantly refine of our understanding of the stars, atmospheric spectra, including bio markers may statistical properties of extrasolar planets; be obtained. —carry out crucial tests of theoretical models of Hydrogen rich gas giants or exoplanets with gas giant planet formation and migration; expanded hydrogen thermospheres (Fig. 4) can be detected in the Lyman α line at 121.567 nm with a UV —achieve key improvements in our comprehen spectrograph, as neutral hydrogen atoms absorb sion of important aspects of the formation and dynam Lyman α photons. This is visible as a dip in the light ical evolution of multiple planet systems; curve of the transit signal. The presence of an extended —provide important contributions to the under hydrogen cloud around a planet can be explained by standing of direct detections of giant extrasolar planets; several scenarios, depending on the composition of the atmosphere. Stellar wind produces additional —collect essential supplementary information for absorption features, making it possible to distinguish the optimization of the target lists of future observato between hydrodynamic, hot atom and stellar wind ries aiming at the direct detection and spectroscopic induced mass loss and additionally observing stellar characterization of terrestrial, habitable planets in the wind conditions at the location of extrasolar planets. vicinity of the Sun; and For instance the hot Jupiter HD209458b is an —will enhance our knowledge on potential exoplanet found to transit the disk of its parent star. exoplanet host star parameters, which is important for Observations have shown a broad absorption signature asteroseismic studies yielding the age of the star for about the Lyman α stellar line during a transit, sug instance. gesting the presence of a thick cloud of atomic hydro
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 297 gen around the HD 209458b (Vidal Madjar et al., 2003; Holmström et al., 2008; Eckenbäck et al., 2010). These Lyman α observations compared with ther mospheric modeling, including hydrodynamic expan sion and out flow models, photochemical produced hot planetary hydrogen atoms and stellar wind plasma interaction modeling can be a useful diagnostic for: —exomagnetospheric modeling; —for obtaining a better understanding of the ther mospheric exospheric structure of hydrogen rich exoplanets; and Fig. 4. This illustration shows the size relation of exoplan —for inferring stellar wind plasma properties ets with extended hydrogen coronae around G type stars around the exoplanets location. and M dwarfs, respectively (all radii to scale). On the left However, the HST is at present time the only UV there is a Jupiter like exoplanet with the size of 1RJup with space observatory in operation and its follower the an extended hydrogen thermosphere of 4.3 RJup orbiting a Sun type star, on the right an Earth analogue with an JWST does not have the capability to observe the UV atmosphere extending up to 10 R around an M star universe. From all near future UV projects the Rus Earth (0.35 RSun and 0.55 RSun, respectively). In the observation sian led WSO UV space telescope is the best suited for of HD209458b the hydrogen thermosphere exosphere the observations of hydrogen clouds around exoplan resulted in a dip in the light curve of 15%. It is reasonable ets after HST stops its operation in 2013. The WSO to assume that Lyman α intensity can be similar for the M UV is grown out the needs of the Astronomical com star. Hence as a rough estimate, the occultation of the munity to have access to the Ultraviolet (UV) range of 0.35 RSun M dwarf by an Earth size exoplanet would show the spectrum and is planned to be launched in 2012. a dip of about 6%, and 2.5% for the 0.55 RSun M dwarf. The WSO UV project consists at present of partners from Russia, Spain, Germany, China, Ukraine, and Khazakhstan. It is an international space observatory planets around the closest and brightest stars could be for observation in UV spectral range (>100–350 nm) carried out. The scientists involved in the study of the and it includes a telescope with primary mirror of TESS project expect that the capabilities of this space 170 cm and scientific instruments—imaging field observatory are such that it could discover hundreds of cameras and 3 spectrometers (resolving power ranges rocky exoplanets from Earth size to super Earth from 2000 to 55000). sizes. Further TESS may detect giant exolanets at large orbital radii with a photometric precision comparable The WSO UV illustrated in Fig. 5 incorporates a to that of CoRoT with periods up to several tens of primary mirror of 1.7 m diameter which is half of the days around bright stars. TESS would have the capa collecting area of the HST, but taking advantage of the bility to do an all sky survey to study bright nearby modern technology for astronomical instrumentation, potential exoplanet hosts. In combination with JWST and of a high altitude, high observational efficiency the detected exoplanets could then be a very good can orbit. The WSO UV will provide UV optical astro didate for spectroscopic follow up. nomical data quantitatively and qualitatively compa rable to the exceptional data base collected by the In the laboratory of the Russian Space Research HST. Institute (IKI) a small three dimensional common The science tasks of the WSO UV space observa path interferometer for achromatic nulling is currently tory will be related to the study of thermal and chemi investigated (Fig. 6) related to problems of stellar cal evolution of the Universe, including the search for coronography (Tavrov, 2008). In Bracewell’s method, dark matter, supernovae. Besides these astrophysical a long baseline interferometer increases the resolution themes the WSO UV will be a powerful tool for stellar by using two telescopes. In this interferometer, the physics, the study of accretion discs, stellar wind light from the background on axis source (star) has a detection related to stellar activity, and finally the phase shift by π radians and interferes with a phase dif study of the outer layers of exoplanetary atmospheres. ference of π. At the same time, the light from the off The study with the WSO UV of transiting exoplanets axis source (planet) interferes with another phase dif will provide most of the information related to stellar ference, so that the off axis source is attenuated only planet interaction discussed in the previous section. slightly and has a signal level sufficient for photodetec Besides the ESA studied European transit mission tion. The suppression of the on axis source (star) PLATO, a Transiting Exoplanet Survey Satellite allows the observation of the off axis source (TESS) is planned in the USA by scientists at MIT, the (exoplanet). The theoretical parameters agree with Harvard Smithsonian Center for Astrophysics, and their laboratory experiment, in which the demon NASA Ames. The TESS satellite would use a set of six strated achromatic nulling of the on axis signal with a wide angle cameras with large, high resolution elec nulling contrast of 10–3 in a visible spectral range tronic CCDs so that an all sky survey of transiting 300 nm in width.
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External Light protective Optical Cover of the Baffle cover Bench Instrumental Compartment
Instrumental Compartment
Secondary Mirror Tube Primary Mirror External Module Module Instrumental Panel
Fig. 5. Illustration of the Russian WSO UV telescope. The observatory includes a single 1.7 m aperature telescope capable of high resolution spectroscopy, long slit, low resolution spectroscopy, and deep UV and optical imaging. The Russian Federal Space Agency ROSCOSMOS has confirmed its readiness to lead the WSO UV project so that the project was included in the Federal Space Program of the Russian Federation and is planned to be launched around 2012 (http://wso.inasan.ru/).
PM
z v
Fig. 6. Left laboratory test bed of the three dimensional common path interferometer for achromatic nulling, which is planned to be tested on a 0.7 m telescope (right) which will be installed at the Russian ISS segment around 2015.
In astronomical observations, exosolar planets existing implementation could be applied in the IR, have a total brightness that is lower than the brightness where the required nulling contrast is 10–6 (Tavrov, of a star by 6–10 orders of magnitude, depending on 2008). The method is planned to be tested on a 0.6 m the wavelength range, respectively, from the IR to the telescope which will be brought to the Russian seg visible. The theory is developed for the dependence of ment of the ISS around 2015. Besides various scien the nulling contrast due to partial spatial coherence tific goals the ISS T 600 telescope the three dimen caused by the finite size of an extended source. It sional common path interferometer for achromatic shows that the method of nulling interferometry in the nulling will be tested related to trial methods of coro
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Fig. 7. Illustration of the JAXA/ESA SPICA telescope which may be launched around 2015–2017 (JAXA/ESA). nagraphic observation of exoplanets and other low compatibility with the SPICA system—the final contrast targets. detector selection will be made by mid 2010. The SPICA (SPace Infrared telescope for Cosmol Data obtained by SPICA has several solar system ogy and Astrophysics) space observatory as illustrated science connections for instance related to the chem in Fig. 7 is a proposed Japanese JAXA mission with a istry of protoplanetary discs, which is the major reser large 3 m class infrared telescope where ESA would voir of key species with prebiotical relevance, such as contribute within Cosmic Vision 2010–2025 by a mis oxygen, ammonia (NH3), methane (CH4) or water sion of opportunity to the telescope assembly, a ground (H2O) to be found later in exoplanets, asteroids and station, and the science operations. The European comets. Within SPICA’s capabilities is the study of the part of the mission is the SAFARI instrument. ice lines in early planetary systems which will enhance our understanding how the water is delivered to the Besides astrophysical and cosmological science Earth, maybe early Mars, early Venus and terrestrial questions (formation studies on the evolution of galax exoplanets within habitable zones. Information of dust ies) SPICA also addresses solar system and exoplanet and mineralogy in planetary nebulae will be obtained science relates issues related to: via MIR/FIR mapping. SAFARI will study Kuiper —the formation and evolution of planetary sys Belt analogues of other planetary systems out to tems (dust and gas of planetary nebulae, protoplane 150 pc, during the epoch at >10 Myr when the transition tary discs, etc.); form protoplanetary to debris discs occurs. If SPICA is —the study of the ice line in other star systems; approved the satellite will be launched after 2015 with JAXA’s H IIB from the Tanegashima Space Centre and —detailed characterization of Kuiper Belt Objects is planned as a nominal three year mission with a goal of in the solar system and exosolar systems; 5 years orbiting at L2. —atmosphere characterization by transit spectros SIM Lite is a NASA space based differential copy of known exoplanets in MIR (H2O, CH4, NH3, astrometry project that could be launched around CO2, etc.). 2015–2020. It consists of a white light interferometer The SAFARI instrument, which is to be built by a with a small aperture that could measure astrometric consortium of European institutes, with Canadian and signatures of less than 0.6 micro arcseconds at nearby Japanese participation, is an imaging Fourier Trans stars. This would allow seeing the deflection in the form Spectrometer designed to provide continuous plane of the sky caused by any accompanying Earth coverage in photometry and spectroscopy from 34 to mass planets for about the 60–80 nearest solar type 210 μm, with a field of view of 2′ × 2′ and spectral res stars. olution modes R = 2000 (at 100 μm), R ~ few hundred and 20 < R < 50. Four detector technologies are under going development to meet these requirements, where EXOPLANET MISSIONS BEYOND 2020 several of these detector technologies have already By evaluating the present space projects and demonstrated that they should be capable of delivering planned project until 2020, by assuming no rise in the required sensitivity for SAFARI and all are under budgets on can see that the characterization of going a development program aimed at proving their exoplanet, including rocky Earth like exoplanets is a
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 300 LAMMER et al. together with the uninterrupted visibility and NIR sensitivity, be ideal for such imaging capabilities. They will provide detections of microlensing events using as sources G and K stars for rocky exoplanets down to 0.1–1 MEarth from orbits of 0.5 AU. It is expected that such a space based microlensing survey is a good way to enhance our understanding of the nature of plane tary systems and their host stars, which is required to understand planet formation and habitability. Besides the microlensing capability of an extended Euclid mission exoplanet projects related to spectro scopically characterization are studied. The Super Earth Explorer Coronagraphic Off Axis Space Tele scope (SEE COAST or SEE, Fig. 8) was proposed during ESAs Cosmic Vision 2010–2025 programme as a M class project but was not selected by ESAs AWG for further studies such as the exoplanet transit mission PLATO. SEE COAST or a similar mission could be aimed for direct detection and characterizing exoplanets from the super Earth (~5 Earth masses) to Fig. 8. Illustration of the SEE COAST space observatory, gas giant regime. Such missions would be dedicated to which is designed for direct characterization of exoplanet investigate and study star systems known to have atmospheres. One may expect that the first generation of such type of missions may be developed after 2020. exoplanets. Given the rapid advances in the methods employed to find exoplanets which can be character ized by a SEE COAST type mission, mainly by complicated task, which is dependent of various small ground based RV and astrometric measurements sev to medium class space observatories which have to eral interesting target planets should be observable. compete with other non exoplanet related space mis SEE COAST as shown in Fig. 8 consists of a space sions. However, it is unlikely that there will be a space telescope, a coronagraph to suppress the starlight and mission before 2020 for direct characterization of a spectropolarimeter to take full advantage of the exoplanet atmospheres. information embedded in the light that is reflected by As mentioned before, the Euclid mission which is the exoplanet. The photons from the exoplanets will dedicated to map the geometry of the dark Universe be submitted to an extensive analysis which should was ranked scientifically high and is studied further give as much information as possible about the inves within the Cosmic Vision 2010–2015 projects. It is tigated targets: planned that the science return of Euclid can be fur —spatial information from the imager; ther enhanced through an extension of the Euclid mis —near infrared spectral fluxes from the Integral sion after 2020 which can undertake additional surveys Field Spectrograph; and use microlensing for the detection of low mass exoplanets. This microlensing signal originates from a —visible spectral fluxes and polarization from the temporary magnification of a galactic bulge source star high precision polarimeter. by the gravitational potential of an intervening lens All that can be learnt from the faint exoplanet sig star passing near the line of sight, at a distance less then nal will be acquired to an unprecedented level of high the Einstein ring radius. A planet which orbits the lens contrast imaging spectro polarimeter. One may expect star generates a caustic structure in the source plane. that a SEE COAST type mission may belong to the The source star transiting or passing next to one of first generation of direct characterization missions for these caustics will have an altered magnification com exoplanets (Fig. 9). pared to a single lens, showing a brief flash or a dip in This first generation is expected to be in the 1.5–2 m the observed light curve. coronagraph class which could study atmospheres of The duration of such planet lensing anomalies giant exoplanets, Neptune class and even super Earth scales with the square root of its mass, lasting typically atmospheres (e.g., Schneider et al., 2009). After 2020 one hour for Mars mass, a few hours for Earth mass, these missions may be followed by a generation of an and up to 2–3 days for Jupiter mass planets. The interferometer (Cockel et al., 2009, Lawson et al., Euclid science team plans a 3 month extension of the 2009), an external occulter (Glassman et al., 2009), a nominal mission period for undertaking a unique large 8 m class coronagraph (visible), a Fresnel Inter microlensing survey of the Galactic Bulge, so that ferometric Imager (Koechlin et al., 2009) or a 20 m exoplanets with masses > Mercury can be detected. segmented coronagraph which would resample a Euclid has high angular resolutions which will super JWST for the NIR (Lillie et al., 2001).
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Transit spectroscopy SPITZER/HST, JWST, SPICA
Transits (space based) TESS
Astrometry SIM Lite
Super Earth Explorer, Coronographs, External occulter, Interferometer etc. CoRoT, Kepler Gaia PLATO 2010 2012 2014 2016 2018 2020
Fig. 9. Illustration of orbiting developed and planned S and M class exoplanet related space missions between 2010–2020. One can see that during the next ten years the characterization of exoplanet atmospheres will be a complex and time consuming inter play between transits, astrometry and transit spectroscopy. The current budgets available to the space agencies do not allow a tech nologically feasible direct atmosphere characterization space mission. It seems that direct characterization space missions will not be developed before 2020–2025.
The characterization of exoplanet atmospheres the diversity of terrestrial exoplanets by using spectro with these 2020+ space observatories will be focused polarimetry (Bühl et al., 2010). on spectroscopy and polarimetric approaches. Addi Since the host star’s light scattered from an tionally to the detection of atmospheric gases, the exoplanet is going to be polarized to a certain degree, polarimetric approach could also be used for the study depending on surface and atmospheric features, polari of clouds, surface, rings, etc. (Stam, 2008). metric measurements contain valuable information on these characteristics not accessible by other methods. Examples are cloud coverage, cloud particle size and SEARCH A FUTURE SPACE MISSION shape, ocean coverage, atmospheric pressure among CONCEPT? others. Also the change in the degree of polarization The study of a future space mission concept with over one orbital period contains valuable information the name SEARCH originated during the FFG ESA on the orbital inclination, which is not accessible by ESO ISSI Austrospace sponsored Summer School in radio velocity measurements. By combining this Alpbach, Austria between July 21–30, 2009, whose method with a spectral measurement, it should be pos theme was Exoplanets: Discovering and Characteriz sible to characterize the atmospheric composition and ing Earth Type Planets, and further developed during to detect molecules like H2O, O2, O3, NH3 and CH4. a Post Alpbach workshop by an international team of In order to achieve these ambitious goals two main young European scientists and former Summer obstacles have to be overcome, being the high contrast School participants at the Space Research Institute of ratio between the exoplanet and its host star and the the Austrian Academy of Science in Graz, Austria the need to spatially resolve these two with the spacecraft’s week before the ESF Exploratory workshop was held. telescope. To meet those two requirements, new The SEARCH mission concept is intended to study design concepts have been developed an analyzed.
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Secondary mirror
Primary mirror
Bus
Sun shield
Fig. 10. Illustration of the SEARCH concept with its folded 9 × 3.7 m2 mirror which fit well into an Ariane 5 rocket. Bottom left the SEARCH team at the Space Research Institute (IWF) of the Austrian Academy of Sciences during the Post Alpbach workshop (Nov. 24–29, 2009), where they developed this mission concept. Key: 1. Secondary mirror; 2. Primary mirror; 3. Bus; 4. Sun shied
The proposed mirror is an f/1 parabolic mirror with because of the host star’s light being almost completely elliptical rim the size of 9 × 3.7 m2 in a Cassegrain lay unpolarized. For the mission discussed, a half wave out. It is cut in half and folded towards the secondary achromatic four quadrant phase mask can be chosen mirror to fit into Ariane 5 as illustrated in Fig. 10. as both, a coronograph and a polarimeter. In addition Thus, with the SEARCH concept problems regarding to that, the use of an integral field spectrograph is pro the symmetric distribution of weight at launch can be posed for separation of light from the star and the overcome. Also the on axes configuration signifi planet as well as the reduction of speckle noise. It is cantly decreases the polarization losses introduced by clear that there are further technological develop the system. This design allows for an angular resolu ments to be made before a mission concept like tion high enough to resolve planets as close to its host SEARCH can be realized. Nevertheless, since nearly star as 0.5 AU in a distance of 10 pc and as close as all of those are key developments for other planned or 1.4 AU in a distance of 30 pc, being able to actually already scheduled space missions, the SEARCH con characterize Earth type exoplanets in a significant tar cept may be able to use these developments to achieve get sample. Given the possibility to overcome techni the ambitious goal of characterizing Earth like planets cal problems in the manufacturing of such large mir beyond 2020. rors, it is thinkable to launch a 20 × 3.7 m2 mirror within an Ariane 5 rocket. The mirrors surface smoothness is crucial to the quality of the optical sys PRESENT AND FUTURE GROUND BASED tem. Since it is not possible to meet the required EXOPLANET PROJECTS smoothness over the whole area of the mirrors, active Besides the before mentioned space based optics has been introduced correcting the wave front exoplanet projects there exist many ground based aberrations due to deviations on the mirror surface. exoplanet programs dedicated to Radial Velocity (RV) Regarding contrast ratio, it has been shown, that surveys/observations, transit searches, etc. where we polarimetry itself can reduce the contrast ratio can only mention a few of them. From the more than between host star and planet by 5 orders of magnitude, 400 exoplanets, more than about 80% were detected
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 303 by the RV method, mostly of them are gas giants, around the small, faint star GJ 1214 (Charbonneau et about a dozen are hot Neptune’s and a handful of al., 2009). In Europe the WTS/UKIRT survey will tar super Earth’s. From this fact one can see that the get a large sample of low mass stars, searching for ground based exoplanet programs are important for transiting rocky planets with periods of a few days. example for the determination of the constraints on Another European project related to the detection the exoplanet mass function, and orbit distribution. of exoplanets around very low mass stars uses the pre The most prominent present day RV telescopes are cision of measurements in the near infrared which is Coralie, Sophie, HARPS, HIRES, Lick, and AAPS. compared to optical wavelength from different instru These instruments are capable of long term precision ments (ESO VLT/UVES, Keck/HIRES) (Reiners and studies with RVs of < 1 m s–1, allowing the detection of Basri, 2010; Reiners et al., 2010). It shows that for Neptune’s and super Earth’s on short orbital periods early ~M stars only no advantage is existent. From and gas giants on long periods up to 10 years. Around ~M4 type stars on, the near Infrared gains in precision 2014, NAHUL and GTC, SPIROU and CFHT towards cooler types. Optical together with near Infra should improve the exoplanet detection limit to about red high precision spectroscopy will provide a power 1 m s–1 around cool M type dwarf stars. ful tool for the search for exoplanets orbiting very low Several instruments are being planned for large mass stars. telescopes currently in operation. Specifically, the In the Russian Federation a project of exoplanet next generation of ultra high precision RV instru observations is planned for the 2010–2014 period, ments can on a dedicated 8 m telescope, take the pre with a new 2 m telescope (observatory Terskol, Fig. 11) cision currently available on telescopes like the ESO and with a participation of the Crimea astrophysical 3.6 m + HARPS combination (0.5–1 m s–1) and may observatory (Ukraine). Within this project known improve it by a factor of 10. The main problem is the non transiting exoplanets will be observed by means of activity of the target stars which demand more or less the polarimetric method, looking for their tangential continuous coverage of the whole phase of the orbital transits, and using the experimental and theoretical revolution of a planetary candidate and, in the case of models for inferring more data from these objects low mass exoplanets like super Earth’s, many periods. (Ksanfomality, 2007). However, specially developed telescopes and instru The polarimetric search method has been tested ments will carry the present day RV studies into a new experimentally for this purpose; the necessary astro domain of low exoplanet masses. nomical observations and their processing have been The ESPRESSO and VLT may detect Earth mass performed. The results obtained allow one to assert exoplanets within the habitable zone of low mass M with caution that the suggested method yields positive and K stars with a velocity of about 10 cm s–1. Future results and can be of use both in searching and charac ground based RV programs such as CODEX and the terizing exoplanets and refining their masses. The high ELT may reach the detection of Earth mass exoplan correlation between the polarization variations and ets within the habitable zones over spectral types M, the revolution period of an exoplanet would suggests K, G and F around 2020. If space observatories like that the angle between the orbital plane of the planet TESS or PLATO which are dedicated to detect such and the observer’s direction is small (sini ≈ 1). This low mass exoplanets close to our solar system are allows refining the exoplanet mass, since the parame approved, the ground based RV follow up with these ter Msin i turns into М if the effect is actually related to instruments will deliver the masses of the discovered a tangential transit of the exoplanet’s comet like tail. transiting rocky exoplanets with a high accuracy. Single aperture imaging is another technique where Besides the RV method, projects liked the recently instruments with or without coronagraphs related to started MEarth project in the USA, which is a photo direct detection surveys on the VLT, Gemini, Keck, metric ground based survey of about 2000 nearest M and HST are planned. Future planet finder instru dwarfs are also established. The MEarth project with ments on 8 m class telescopes on the VLT and Gemini an accuracy of <5 × 10–3 mag, is optimized for to S./Subaru, such as SPHERE, GPI, HiCIAO are search for transiting super Earth’s in the habitable planned to be in operation in the near future (2010– zone of close M dwarfs. The project consist of a suite 2011). The main observations with these instruments of eight amateur sized telescopes with 40 cm diameter are dedicated for spectral characterization of self mirrors, each with a sensitive CCD light detector that luminous exosolar gas giants. In the visible, the measures the near infrared brightness in wavelength SPHERE instrument is capable to detect irradiated range between 700–900 nm of a star. In case the gas giants with R > 1 RJup, at about 1AU around nearby brightness dims for about 60 minutes and repeats its bright stars. dimming periodically over days and weeks, an Ground based astrometry projects on large tele exoplanet candidate is discovered, which can be con scopes like with FORS2 instrument on the VLT can firmed after the mass determination via a RV follow reach typically 100 arcs over a few years necessary to up. The MEarth project discovered recently a sub detect exosolar gas giants around late M type stars and Uranus/super Earth type planet with the size of about Neptune mass objects at brown dwarfs. One can 2.7 Earth radii with an orbital period of 1.6 days expect that future astrometric facilities are based on
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Fig. 11. The Russian Terskol telescope in the northern Caucasus mountains is planned to test the characterization of known mon transiting exoplanets by a polarimetric method. the use of adaptive optics and imaging cameras at large 10 mag detected with PLATO would also be detectable ground based telescopes. with EPICS. With regard to young, forming exoplan Several ground based surveys related to micro ets, it will be possible to study the early evolution lensing observation are currently on going and have which will lead to a better understanding of the initial been successful in the detection of the first super mass function of such objects (see Fig. 12). Earth’s in orbits of very distant stars. The micro lens Besides CODEX and EPICS two other instru ing method appears to have significant capability as ments, namely SIMPLE and OPTIMOS EVE are the technology evolves. also studied for the E ELT. SIMPLE will have the Several large facilities are being planned for the capability to observe atmospheres of exoplanets in the near future. Based on adaptive optics and segmented wavelength range of 0.8–2.5 μm and OPTIMOS EVE telescopes, examples are the Thirty Meter Telescope will be dedicated for RV surveys of extragalatic (TMT), with an 30 m effective aperture, in the USA exoplanets in the optical and IR channels. and in Europe the 42 m European Extremely Large Telescope (E ELT). Operating from wavelengths ranging from UV to IR, science addressed by these CHARACTERISATION OF TARGET STARS large instruments include besides star and planet for AND EXOPLANET ENVIRONMENTS mation, cosmological and extra galactic research also the observation of exoplanets. There are currently four Our knowledge regarding the discovered exoplan E ELT instruments for the characterization of ets is directly related by our knowledge of the parent exoplanets under study: star. The stellar X ray and EUV (XUV) radiation and CODEX: The accuracy of the COsmic Dynamics the stellar plasma outflow constitute a permanent and Exo earth experiment, CODEX’s radial velocity forcing of the upper atmosphere of the exposed messurement accuracy will be about 2 cm/s, which exoplanets and therefore can affect the atmospheric will allow to detect Earth mass exoplanets at 1 AU if evolution and even habitability. The main effect of they orbit on Sun like stars with low activity; these forcing factors is to ionize heat, chemically modify, expand and slowly erode the upper atmo EPICS: The Exoplanet Imaging Camera and sphere throughout the lifetime of an exoplanet. Spectrograph, EPICS, is an optical/NIR planet imager and spectrograph with extreme adaptive optics The host star and its related activity can influence which will detect transiting “super Earth” planets the ability to detect and measure exoplanet parame around K and M stars which are brighter than about ters. This can result in systematic errors:
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Field Plate in reconfiguration position
Robot positioner
Moving platform
Fixed raised platform
Fig. 12. Illustration of the baseline design of the Exoplanet Imaging Camera and Spectrograph (EPICS) on the left and OPTI MOS EVE on the right (ESO). Key: 1. Field Plate in reconfiguration position; 2. Robot positioner; 3. Moving platform; 4. Fixed raised platform
—in indirect methods, all we see is the starlight Short term variations up to years can be studied by (activity, binarity/rotation); establishing time series with photometry. For longer —in direct methods, there can be challenges to timescales observations of stellar proxies or stars of the detectability (zodiacal light, binarity); same spectral class with different ages are the best —calculated planetary properties (mass, radius) options. are relative to those of the star (in a linear manner). By ultimately determining the intrinsic properties Stellar Radiation and Plasma Environment of exoplanets, their host star is the overwhelmingly as Function of Age larger source of energy. The stellar radiations and plasma environment (winds and CMEs) critically In general, the boundaries of the habitable zone affect the composition, thermal properties, atmo change throughout the star’s lifetime as the stellar sphere evolution and even the mere existence of a luminosity and activity evolve (Lammer et al., 2009). planetary atmosphere, or its water inventory. Because the evolution is different for different star types, and furthermore depends on their age and the The physical and fundamental properties for target location of the habitable zone around the host star one stars are, the stars mass, radius, chemical composition can expect that the atmospheric evolution of Earth and age. These properties can be determined by the like exoplanets is strongly coupled with the evolution following methods: of the host star. This fact makes the type and the age of —calibrations (from binaries, clusters, etc); exoplanet host stars very important. —stellar evolution models; From that point of view the PLATO mission con —asteroseismology; cept is an interesting one, because the resulting high —age: kinematics, asteroseismology, activity, quality light curves will be used for measuring the chemical abundances (Li, Be), theoretical models. characteristics, and on the other hand to provide a —the radiative properties of the targets can be seismic analysis of the host stars of the detected plan studied if one obtains a detailed spectral energy distri ets, from which precise measurements of their radii, bution from X rays to IR, and radio. Other important masses, and ages will be derived. stellar parameters are related to the time variability of Activity in late type stars (i.e., spectral types G, K, the stellar emissions. Such processes can vary from: M) has been the subject of intense studies for many —minutes to hours: micro variability, flares, mass years by various satellites such as: ASCA, ROSAT, ejections; EUVE, FUSE and IUE. The most important physical phenomena of stellar activity include modulations of —days: spot modulations, spot evolution; years: the stellar photospheric light due to stellar spots, inter spot cycles; mittent and energetic flares, coronal mass ejections —centuries: Maunder like minima; (CMEs), stellar cosmic rays, enhanced coronal X —to billions of years: rotational spin down. rays, and enhanced chromospheric UV emission. For
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 306 LAMMER et al. solar type stars this has been studied with high accu ZAMS one will never fully understand its role in the racy within the “Sun in Time” project, where a sample evolution of planetary atmospheres and even habit of single nearby G0–V main sequence stars with ability. known rotation periods and well determined physical There are also other wavelength ranges which properties, luminosities and ages which cover 100 include phenomena which are known signatures of Myr–8.5 Gyr has been analyzed. From these data one CMEs on the Sun. An analysis of tens of solar CMEs can conclude that the solar integrated flux between 0.1 and temporal correlated decameter type II bursts indi 120 nm, 3.5 Gyr ago, was higher by a factor of 6, and cates, that every solar decameter type II burst of this up to 100 times more intense than that of todays Sun study is correlated with a CME, but not every CME is during the first 100 Myr after its arrival at the zero age correlated with a decametric type II burst. We are main sequence (ZAMS) (Ribas et al., 2005). using this context to search for stellar analogues of This activity–age relation indicates that lower mass these decametric type II bursts on active late type stars spend more time in this highly active but satu main sequence stars, especially on M stars, due to the rated phase before beginning their activity decay. In before mentioned effect on exoplanetary atmospheres. particular, solar type stars stay at saturated emission Due to this analysis a project was initiated to levels until they age to ~100 Myr, and then their X ray, observe such decametric type II bursts on active M SXR luminosities rapidly decrease as a function of age dwarfs with the present time World’s largest decameter following a power law relationship (Ribas et al., 2005). array, the UTR 2 (Ukrainian T shaped Radiotele But low mass M dwarfs, on the other hand, have scope 2nd modification) shown in Fig. 13 and hosted saturated emission periods up to 0.5–1 Gyr and possi by the Institute of Radio Astronomy in Kharkov of the bly longer for late type M stars (Scalo et al., 2007). Ukrainian Academy of Sciences. The huge effective After that the luminosity decreases in a way similar to area of the UTR 2 is more than 150000 m2 large and solar type stars. Therefore, one may expect that the ensures a sensitivity of about 1 Jy or less depending on atmospheres and their evolution will be stronger— the observing conditions. The instrument has a multi longer—affected at lower mass stars compared to beam capability of five simultaneously operating more massive solar like G type stars of F type stars. beams. Further a selection of five digital back ends is Stellar winds and CME plasma outflows have also available from 1024 up to 16384 frequency channels. important implications for the stellar and exoplane All of them perform FFT (Fast Fourier Transforma tary environments. The outflowing stellar plasma can tion) in realtime. lead to non thermal losses of planetary atmospheres First results were obtained during an observational exposed to it, especially non or weakly magnetized campaign which took place in February of 2007 at the ones. Due to tidal locking and slow rotation the mag UTR 2. The target for this campaign, AD Leo, was netic dynamos of rocky exoplanets within orbits of selected due to its young age of about 200 Myr, a lgLX lower mass stars may be weaker compared to fast rotat value of 28.8, the close distance of 4.7 pc, and of ing planets like the Earth. Additionally close in course suitable coordinates. Two digital back ends exoplanets are exposed by dense plasma flows close to were installed on beam 3 and 5 (beam 3 is pointing at their host stars. the target and beam 5 is pointing one degree apart for During the last years several indirect methods for the investigation of the background). The integration studying tenous stellar winds of Sun like stars have time was set to 100 ms in a bandwidth of 18–30 MHz. been evaluated. The method which yielded the largest number of mass loss estimates so far was the search for During more than 60 hours of ADLeo observations at astrospheric absorption in the region where the stellar the UTR 2, many structures could be detected in the wind meets the local interstellar medium and neutral dynamic spectra. After careful analysis only a handful hydrogen is formed via charge exchange. This gives shows a high probability of being of stellar origin, accord rise to an absorbtion of the blue side of the stellar ing to their only appearance in spectra of beam 3 and fur Lyman α line. The amount of absorption is propor ther only appearance in the main lobe of the antenna pat tional to the stellar wind ram pressure and therefore tern. So far structures which show a slow frequency drift (type II) could not be detected, instead structures with a yields estimates of the mass loss rate (Wood et al., –1 2002, 2005). By applying this method with the Hubble high frequency drift (1–2 MHz s ) similar to solar deca STIS spectrograph the mass loss rates of about a dozen meter type III bursts were detected. solar like stars and cooler stars have been derived. A possible interpretation of the non detection of Depending on the age of the observed star, the mass stellar analogues of solar decameter type II bursts loss rates are in the order of <0.2–100 times the might arise from the difference of M and G stars present solar rate. atmospheres (temperature and spatial scales) and due One needs clearly more observations in the future to the difference of convective layers. with HST, WSO UV or IXO, etc. Although stellar Besides investigations of stellar plasma events, the wind observations are difficult to obtain, but if we detection of radio emission in certain wavelengths don’t understand how the stellar plasma outflow would convey information on the rotation of exoplan evolves with the age of the star after its arrival at the ets, and the existence and intensity of a planetary mag
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 307
Fig. 13. The World’s largest decameter array operated by the Institute of Radio Astronomy in Kharkov of the Ukrainian Academy of Sciences is used also for the search of radio signals originated in exoplanet magnetospheres and decametric type II bursts which are related to stellar CMEs. netic field. The latter factors are important ingredients The limit of the confusion effect sensitivity of UTR 2 for studies of planetary habitability. Using known for the continuum point radio source (25 MHz) is planetary characteristics (mass, radius, age, orbit, dis about 1 Jy. However, the sensitivity without the confu tance, etc) Grießmeier et al. (2007) estimated the max sion effect is about 10 mJy. Horizontal line at 10 mJy: imum emission frequency and the radio flux for all upgraded UTR 2, sloped lines: low band and high exoplanets known as of 13.1.2007. Figure 14 gives a band of LOFAR. The instruments' sensitivities are brief update, including all exoplanets that were discov defined by the radio sky background. For a given ered since. The results are visualized in Fig. 14. For each exoplanet individually, the expected maximum emis sion frequency and the estimated maximum radio flux 101 (maximum of the three models: magnetic energy model, CME model, kinetic energy model) is given. The predicted planetary radio emission is denoted by open triangles (two for potentially locked exoplanets, ] 10–1 otherwise one per exoplanet). The typical uncertain –1 ties (a factor of 2–3 for the maximum emission fre Hz quency, and approx one order of magnitude for the –2 10–3 flux) are indicated by the arrows in the upper right cor Wm
ner. Due to the relatively faint signal, large antenna –26 arrays are required. For this reason, this kind of obser vation can only be performed by ground based radio 10–5
telescopes as the UTR 2 shown in Fig. 13 or the future [Jy = 10
Low Frequency Array (LOFAR). As the terrestrial Φ ionosphere is reflective for low frequencies, frequen cies below 10 MHz are not observable from the 10–7 ground, which gives the lower limit for the frequency range of interest. Figure 14 shows that the upper limit of that frequency range appears to be around ~200 1 MHz 10 MHz 100 MHz 1 GHz MHz. f
The expected sensitivity of new and future detec Fig. 14. Estimated the maximum emission frequency and tors (for 1 h integration and 4 MHz bandwidth, or any the radio flux for all exoplanets currently known (updated equivalent combination) is shown for comparison. from Grieβmeier et al., 2007).
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 308 LAMMER et al. instrument, a planet is observable if it is located either instrument on board of STERO, particle, near Earth above the instrument’s symbol or above and to its right. proton fluxes can be used from GOES satellites, mag Up to ~30 exoplanets should be observable using netic field and electromagnetic data from the Phobos either of these instruments. It can also be seen that the 2 mission to Mars and its Moons, plasma data from maximum emission frequency of a considerable num ASPERA 3 (Mars Express/MEX) and ASPERA ber of exoplanets lie below the ionospheric cut off fre 4/magnetic MAG data (Venus Express/VEX), CLUS quency 10 MHz, making ground based observation of TER and Double Star, ENA data from MEX, VEX, these planets impossible. The figure also shows that and NUADU (Double Star). the relatively high frequencies of the planned LOFAR There are many atmospheric data available for CO2 high band are probably not very well suited for the rich atmospheres beginning with the Venera missions, search for exoplanetary radio emission. These instru MEX, VEX, and other missions. Density and tempera ments could, however, be used to search for radio ture changes in the Earth’s thermosphere during extreme emission generated by unipolar interaction between solar events could be analyzed by using low perigee satel exoplanets and strongly magnetized stars. lite data (e.g., CHAMP, GOCE). Short/medium dura tion ionospheric disturbances can be monitored by GPS USING VENUS, EARTH AND MARS and GLONASS transmissions revealing large variations AS EXOPLANET PROXIES WHEN THEY in the northern hemisphere total electron content EXPERIENCING EXTREME SOLAR EVENTS (TEC). Europe’s GOMOS/Envisat instruments provide observations of both short term (days) and long term Exoplanet science has a strong link to Solar System (months) atmospheric effects caused by extreme solar science where many important data and expertise have events such as SPEs. been collected during the past decades. By establishing a coordinated study of the behaviour of the upper GOMOS which was developed at the FMI mea atmospheres, ionospheres, magnetospheric environ sures e.g. ozone, NO2, and NO3 in the stratosphere and ments and thermal and non thermal atmospheric loss mesosphere. Data obtained by the Millimetre wave processes of Venus, Earth, and Mars during extreme Airborne Receivers for Spectroscopic CHaracterisa solar events important knowledge could be established tion in Atmospheric Limb Sounding (MARSCHALS) related to exoplanet atmospheres orbiting within project will be utilized to analyze the chemistry and extreme steller environments. composition in the Earth’s thermosphere. Solar and stellar data needed for the reconstruction of the his Such studies can also serve as a proxy for the influ tory of G star (solar) radiation and particle emissions ence of the active young Sun (G star) with implica are available as discussed before from observations tions for the evolution of planetary atmospheres (solar obtained aboard the SOHO, STEREO, ASCA, system and exoplanets), water inventories and habit ROSAT, EUVE, FUSE, and IUE a satellites. New ability. Extreme events involve enhanced solar EUV specific observations will be obtained by XMM/New and X ray radiation, neutron fluxes, coronal mass ton, Chandra and Suzaku. Feasibility studies to ejections (CMEs) and related intense solar pro observe the stellar hard non thermal emission with ton/electron fluxes (e.g., SPEs), auroral phenomena, Symbol X will be performed. magnetic storms, etc. Responses of planetary atmo spheres include: thermospheric and ionospheric den Especially studies which set the Earth in context sity variations, changes in atmospheric composition with Earth like exoplanets under extreme stellar con including ozone depletion leading to changes in heat ditions could also use ground based observations, ing/cooling, temperature and wind perturbations, which may include radio tomographic monitoring of photochemistry, collisional excitation, deactivation the TEC (Total Electron Content). This can allow and cooling due to IR , optical and UV emissions, reconstruct the 2 D structure of the terrestrial iono magnetospheric compression, enhancement of sec sphere/thermosphere during extreme solar events. ondary particles, etc. Studies of the evolution of the Parameters of the ionosphere can be determined by 1 spectral irradiances (X ray, EUV, UV) of solar type using the EISCAT radar network and magnetometer stars of different ages will be used as a proxy for recon chain in northern Europe. structing the history of the Sun’s radiation output. There are also many theoretical codes available for Complementary information concerning solar mass the analysis and study of exoplanet proxy events at loss (solar wind) will be derived from studies of the Venus, Earth and Mars. Test particle/Monte Carlo, energy distributions of flare related CMEs during low MHD, hybrid, diffusive photochemical, thermal bal and high solar activity by estimating CME related ance, and global circulation models are available for: solar mass loss to the early stage of the solar system investigating ionized and neutral atmospheres, solar using solar proxies. wind/planetary interactions, and particle energy dep There are many space based data available. Ener osition in the upper atmospheres. The 1 D Sodankyla getic particle data (40 keV, several 10s MeV) recorded Ion and Neutral Chemistry Model will be used to at L1 over eleven years by the LION instrument on study the ionospheric atmosphere interaction pro SOHO, data which will be obtained by the WAVE cesses, caused by e.g. proton/electron precipitation or
SOLAR SYSTEM RESEARCH Vol. 44 No. 4 2010 EXOPLANET STATUS REPORT: OBSERVATION, CHARACTERIZATION 309 solar X ray flares, which lead to changes in atmo Charbonneau, D., Berta, Z.K., Irwin, J., et al., A Super– spheric composition. Earth Transiting a Nearby Low–Mass Star, Nature, 2009, vol. 462, pp. 891–894. Casertano, S., Lattanzi, M.G., Sozzetti, A., et al., Double– ESTABLISHING A WELL COORDINATED Blind Test Program for Astrometric Planet Detection EXOPLANET COMMUNITY with Gaia 2008, Astron. Astrophys., 2008, vol. 482, pp. 699–729. At the present, scientists do an excellent job in the Dvorak, R., Schneider, J., Lammer, H., and the CoRoT field of exoplanet discoveries and research. Many sci Team, CoRoTs First Seven Planets: An Overview, Astro entific studies are carried out in various institutes, phys. J., 2010. research groups and single researchers. In the next Ekenbäck, A., Holmström, M., Wurz, P., et al., Energetic step, exoplanet research needs to be more seen within Neutral Atoms around HD 209458b: Estimations of the scientific community and the public. We think that Magnetospheric Properties, Astrophys. J., 2010, institutions such as the ESF together with existing vol. 709, pp. 670–679. exoplanet related science teams like existing ISSI Glassman, T., Newhart, L., Barber, G., Turnbull, M., and teams, or networking research projects such as the NOW Study Team, Planning an Efficient Search for German based Helmholtz Alliance project Planetary Extra–Solar Terrestrial Planets: How to Find Exo– Evolution and Life and working groups within the EU Earths with New World Observer, American Astron. Soc. FP7 funded Europlanet project (especially Na1 and (AAS), 2009. Na2) can be very useful supporters to build up a well Greiβmeier, J. M., Zaraka, P., Spreeuw, H., et al., Predict coordinated and structured exoplanet research infra ing Radio Fluxes of Known Extrasolar Planets, Astron. structure and community in Europe. Astrophys., 2007, vol. 475, pp. 359–368. Holmström, M., Ekenbäck, A., Selsis, F., et al., Energetic Neutral Atoms as the Explanation for the High–Veloc ACKNOWLEDGMENTS ity Hydrogen around HD 209458b, Nature, 2008, vol. 451, pp. 970–972. The authors acknowledge the support by tire ESF, Koechlin, L., Serre, D., Debra, P., et al., The Fresnel Inter and the International Space Science Institute (ISSI, ferometric Imager, Exp. Astron., 2009, vol. 23, pp. 379– Bern, Switzerland) and the ISSI teams Evolution of 402. Habitable Planets and Evolution of Exoplanet Atmo Ksanformality, L.V., Transits of Extrasolar Planets, Solar spheres and Their Characterization. The authors also Syst. Res., 2007, vol. 41, pp. 463–482. acknowledge also support from the Europlanet Net working activity Na1 (Observational Infrastructure Lammer, H., Kasting, J.F., Chassfière, E., et al., Atmo spheric Escape and Evolution of Terrestrial Planets and Networking), and Science Networking Na2 working Satellites, Space Sci. Rev., 2008, vol. 139, pp. 399–436. group (WG4 and WG59 activities. Furthermore, we acknowledge support by the Austrian Academy of Sci Lammer, H., Bredehöft, J.H., and Coustenis, A., What Makes a Planet Habitable?, Astron. Astrophys., 2009, ences, Verwaltungsstelle für Auslandsbeziehungen, vol. 17, pp. 181–249. and by the Russian Academy of Sciences (RAS), Rus sian Federation. We acknowledge the Austrian FWF Lammer, H., Odert, P., Leitzinger, M., et al., Determining the Mass Loss Limit for Close–in Exoplanets: What (Wissenschaftsfond) and the Russian Fund for Basic Can We Learn from Transit Observations?, Astron. Research (RFBR) for support via the projects P20145 Astrophys., 2009, vol. 506, pp. 399–410. N16, FWF I 199 N16 and RFBR grant no. 09 02 Léger, A., Rouan, D., Schneider, J., et al., Transiting 91002. V. Eybel and S. Eggl also acknowledge support Exoplanets from the CoRoT Space Mission VIII. from the FWF under projects P18930 N16 and CoRoT–7b: The First Super–Earth with Measured P20216. We acknowledge also support from the Aero Radius, Astron. Astrophys., 2009, vol. 506, pp. 287–302. nautics and Space Agency, Austrian Research Promo Lawson, P., Lay, O., Martin, S., et al., Terrestrial Planet tion Agency (FFG). Furthermore, we acknowledge Finder Interferometer: 2007–2008 Progress and Plans, the Helmholtz Association through the research alli Proc. SPIE, 2008, vol. 7013, pp. 70132N–70132N–15 ance Planetary Evolution and Life. Finally we Lillie, C., TRW TPF Architecture. 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