Heap_ habitability_PSF.doc.doc - The Direct Approach to Finding Earth-Like Planets Science White Paper Submitted to The Science Frontier Panel on

Heap_ habitability_PSF.doc.doc - The Direct Approach to...

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The Direct Approach to Finding Earth-Like Planets Science White Paper Submitted to The Science Frontier Panel on Planetary Systems and Star Formation Sara R. Heap & Marc Kuchner [email protected] [email protected] 301-286-5359 301-286-5165 Laboratory for Exoplanets and Stellar Astrophysics NASA’s Goddard Space Flight Center, Greenbelt MD 20771 15 February 2009 1
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1. Introduction to Habitable Planets and Earth-Like Planets There is no universally accepted definition of habitable planets or Earth-like planets, so for clarity, we define these terms as follows. Habitable planets 1 are planets that can sustain life: they provide long-term, continuous access to liquid water, and they have access to a source of energy sufficient to drive metabolism. Liquid water is a basic requirement for habitability for several reasons: it is the primary constituent of living cells; it is an essential ingredient of important biochemical reactions such as photosynthesis; it is the “universal solvent”, where simple molecules can interact to form the complex molecules necessary for life; and it works to promote plate tectonics, which is needed for a stable climate. Earth-like planets are habitable planets that have carbon-based life and that use starlight or chemical energy for metabolism. We are interested in detectable habitable planets and planets with life. This means that liquid water needs to be at the surface where it can modify the planetary atmosphere and surface in a way that is detectable remotely. How do we find habitable planets? Enhanced Doppler surveys 2 or astrometric missions like the Space Interferometry Mission 3 (SIM) cannot determine whether a planet is habitable, because their detections are indirect: they only infer the presence of a planet from the periodic motions of its central star. Parameters derived from indirect observations (planetary mass and orbital parameters) are only marginally helpful for predicting whether a planet is habitable, since (1) the presence of surface liquid water depends on albedo and greenhouse warming as well as on distance from the central star, and (2) long-term climate stability depends on mantle convection via plate tectonics, which in turn depends on planet mass and the radioactive heat content of the planetary interior. All of these factors are unknown. There are still other factors determining habitability; see ref. 1. To establish the habitability of a planet or the presence of surface life requires direct observation of the planet itself. We need to detect atmospheric water vapor in the planet’s spectrum. We need to search the spectrum for methane produced by methanogens 4 , and for oxygen produced by photosynthesizing organisms 5 . The rest of this paper develops these themes in more detail. In Section 2, we argue that for each potentially habitable planet, there is a habitable era, mass range, and chemical composition, and that the factors determining its habitability are unknowable. In Section
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