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This Article Appeared in a Journal Published by Elsevier. the Attached This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Available online at www.sciencedirect.com C. R. Palevol 8 (2009) 679–691 General palaeontology (Palaeobiochemistry) Characterizing habitable extrasolar planets using spectral fingerprints Lisa Kaltenegger a,∗, Franck Selsis b a Harvard Smithsonian Center for Astrophysics, 60, Garden Street, 02138 MA Cambridge, USA b Laboratoire d’astrophysique de Bordeaux (CNRS, Université Bordeaux-1), BP 89, 33271 Floirac cedex, France Received 9 January 2009; accepted after revision 3 July 2009 Available online 4 November 2009 Written on invitation of the Editorial Board Abstract The detection and characterization of an Earth-like planet is approaching rapidly thanks to radial velocity (RV) surveys (e.g. HARPS) and transit searches (Corot, Kepler). A rough characterization of these planets will be already achievable in 2014 with the James Webb Space Telescope, and more detailed spectral studies will be obtained by future large ground based telescopes (ELT, TNT, GMT), and dedicated space-based missions like Darwin, Terrestrial Planet Finder, New World Observer. In this article we discuss how we can read a planet’s spectrum to assess its habitability and search for the signatures of a biosphere. Identifying signs of life implies understanding how the observed atmosphere physically and chemically works, and thus gathering information on the planet in addition to observing its spectral fingerprint. To cite this article: L. Kaltenegger, F. Selsis, C. R. Palevol 8 (2009). © 2009 Published by Elsevier Masson SAS on behalf of l’Académie des sciences. Résumé Caractérisation spectrale de planètes extrasolaires habitables. Les planètes extrasolaires de type terrestre sont désormais à la portée de l’observation astronomique, via les mesures de vitesses radiales stellaires (HARPS) et la recherche de transits (Corot, Kepler). Les premières observations spectrales de ces objets seront réalisables dès 2014 avec le télescope spatial James Webb, tandis que des instruments dédiés à l’étude des planètes habitables sont d’ores et déjà en développement : télescopes au sol (ELT, TNT, GMT) ou spatiaux (Darwin, terrestrial planet finder ou new worlds observer). Dans cet article, nous discutons de l’interprétation du spectre de rayonnement des planètes telluriques qui seront observées, à la recherche de planètes habitables et de signatures d’activité biologique. Identifier des signes spectroscopiques de vie implique de pouvoir caractériser dans le détail les processus physico-chimiques à l’œuvre dans l’atmosphère observée. Pour citer cet article : L. Kaltenegger, F. Selsis, C. R. Palevol 8 (2009). © 2009 Publie´ par Elsevier Masson SAS pour l’Académie des sciences. Keywords: Habitable planets; Extrasolar planet search; Biomarkers; Planetary atmospheres Mots clés : Planètes habitables ; Recherche des planètes extrasolaires ; Biomarqueurs ; Atmosphère planétaire 1. Introduction The emerging field of extrasolar planet search has ∗ Corresponding author. E-mail addresses: [email protected] (L. Kaltenegger), shown an extraordinary ability to combine research by [email protected] (F. Selsis). astrophysics, chemistry, biology and geophysics into a 1631-0683/$ – see front matter © 2009 Published by Elsevier Masson SAS on behalf of l’Académie des sciences. doi:10.1016/j.crpv.2009.07.001 Author's personal copy 680 L. Kaltenegger, F. Selsis / C. R. Palevol 8 (2009) 679–691 new and exciting interdisciplinary approach to under- cal properties and composition of a broader diversity stand our place in the universe. Space missions like of planets, to understand the formation of planets and CoRoT (CNES, [45]) and Kepler (NASA, [4]) will give interpret potential biosignatures. On Earth, some atmo- us statistics on the number, size, period and orbital dis- spheric species exhibiting noticeable spectral features in tance of planets, extending to terrestrial planets on the the planet’s spectrum result directly or indirectly from lower mass range end as a first step, while future space biological activity: the main ones are O2,O3,CH4, and missions are designed to characterize their atmospheres. N2O. Sagan et al. [46] analyzed a spectrum of the Earth After a decade rich in giant exoplanet detection, indirect taken by the Galileo probe, searching for signatures of ground based observation techniques have now reached life and concluded that the large amount of O2 and the ability to find planets of less than 10 MEarth (so called the simultaneous presence of CH4 traces are strongly Super-Earths) around small stars that may potentially suggestive of biology. CO2 and H2O are, in addition, be habitable [35,64]. These planets can be characterized important as greenhouse gases in a planet’s atmosphere with future space missions. and potential sources for the high O2 concentration from The current status of exoplanet characterization photosynthesis. In this article we describe how to char- shows a surprisingly diverse set of planets. For a subset acterize a habitable planet in section 2, focus on low of these, some properties have been measured or inferred resolution biomarkers in the spectrum of an Earth-like using observations of the host-star, a background star, or planet in section 3, and discuss cryptic worlds, abiotic the combination of the star and planet photons (radial sources of biomarkers, spectral evolution of a habitable velocity [RV], microlensing, transits, and astrometry). planet and Earth’s spectra around different host stars in These observations have yielded measurements of plane- section 4. tary mass, orbital elements and (for transits) the planetary radius and during the last few years, physical and chem- 2. First steps to characterize a habitable planet ical characteristics of the upper atmosphere of some of the transiting planets. Specifically, observations of A planet is a very faint, small object close to a very transits, combined with RV information, have provided bright and large object, its parent star. In the visible part estimates of the mass and radius of the planet, plane- of the spectrum we observe starlight, reflected off the tary brightness temperature [6,12], planetary day-night planet; in the IR we detect the planet’s own emitted flux. temperature difference [20,30], and even absorption fea- The Earth-Sun intensity ratio is about 10−7 in the ther- tures of giant planetary upper-atmospheric constituents: mal infrared (∼10 ␮m), and about 10−10 in the visible sodium [7], hydrogen [66], water [58,60], methane [59], (∼0.5 ␮m). The spectrum of the planet can contain signa- carbon monoxide and dioxide [61]. Using the Earth tures of atmospheric species, which create its spectral fin- itself as a proxy shows the opportunities, limitations and gerprint. The tradeoff between contrast ratio and design importance of co-adding transits to detect biomarkers is not discussed here, but lead to several different con- on Earth-analog exoplanets [23]. The first imaged exo- figurations for a space-based mission concept. The inter- planet candidates or Brown Dwarfs around young stars ferometric systems suggested for Darwin and the TPF show the improvement in direct detection techniques Interferometer (TPF-I) mission operate in the mid-IR that are designed to resolve the planet and collect its (6–20 ␮m) and observe the thermal emission emanating photons. This can currently be achieved for widely sep- from the planet. The TPF Coronagraph (TPF-C) and the arated young objects, and has already detected exoplanet occulter of NWO look at the reflected light and operate candidates [2,8,18,21,31,34,43]. in the visible and near infrared (0.5–1 ␮m). The viewing Future space missions are designed to image and geometry results in a different flux contribution of the characterize smaller planets, down to Earth-size (e.g. overall detected signal from the bright and dark side, for James Webb Space Telescope [JWST], Darwin, Terres- the reflected light, and the planet’s hot and cold regions trial Planet Finder [TPF], New World Observer [NWO]), for the emitted flux. Both spectral regions contain the sig- and to measure the color and spectra of terrestrial plan- nature of atmospheric gases that may indicate habitable ets, giant planets, and zodiacal dust disks around nearby conditions and, possibly, the presence of a biosphere: stars [3,16,21–23,32]. These missions have the explicit CO2,H2O, O3,CH4, and N2O in the thermal infrared, purpose of detecting other Earth-like worlds, analyz- and H2O, O3,O2,CH4 and CO2 in the visible to near- ing their characteristics, determining the composition of infrared. The presence or absence of these spectral fea- their atmospheres, investigating their capability to sus- tures (detected individually or collectively) will indicate tain life as we know it, and searching for signs of life. similarities or differences with the atmospheres of ter- They also have the capacity to investigate the physi- restrial planets, and the astrobiological potential (Fig. 1). Author's personal copy L. Kaltenegger, F. Selsis / C. R. Palevol 8 (2009) 679–691 681 Fig. 1. (Left) Synthetic reflection and emission spectra [23] and (right) transmission spectra [23] of the Earth from UV to IR shown. The intensity is given as a fraction of solar intensity. The atmospheric features are indicated. Fig. 1. Spectres synthétiques de la Terre de l’ultraviolet à l’infrarouge, en réflexion et émission [23] (gauche) et en transmission [23] (droite). L’intensité est donnée comme une fraction de l’intensité solaire.
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