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Signatures of Life Sara Seager Carnegie Institution of Washington Signatures of Life Spectra are required … … with observations from beyond Earth’s atmosphere Habitable exoplanets may already exist atmosphere habitability L. Cook interior surface Limits to Life on Earth HIGH TEMPERATURE Life in the lab Evidence for intact life LOW TEMPERATURE Life in the lab Microbial activity Enzyme activity measured in methanol, ethylene glycol, water T (K) T (C) 393 473 120 200 253 253 173 -20 -20 -100 Life in the Universe All Life Chemical Carbon Energy -based Liquid water What is life? What does life do? Life Metabolizes Redox Reactions • All Earth life uses chemical energy generated from redox reactions • Life takes advantage of these spontaneous reactions that are kinetically inhibited Diversity of metabolisms rivals diversity of exoplanets Lane, Nature May 2006 Chemical Energy Generation Rosing 2005 A huge amount of chemical energy is generated by photosynthesis from “light energy” Earth Through Time Kasting Sci. Am. 2004 See Kaltenegger et al. 2006 Earth from the Moon Seager Signatures of Life Spectra are required … … from beyond Earth’s atmosphere Habitable exoplanets may already exist Transiting Planet Science 10-3 10-4 10-2 Courtesy Lori Allen Transit Exoplanet Atmosphere Detection Scorecard Technique Ground Space Primary eclipse 0 10+ 21 Secondary eclipse 0 2 31 Phase curves 0 5 10 Signatures of Life Spectra are required … … from beyond Earth’s atmosphere Habitable may already exist Hot Super-Massive Earths • Ten exoplanets with Msini < 21 M – Short period – High chance to transit • What is their basic nature – Mini gas giants? – Ice giants? – Or rock giants? Exoplanet Bulk Composition • Transiting planets: densities will give exoplanet bulk composition – – – – Large gas envelopes? Ocean planets? Silicate planets Carbon planets … dM(r) 4 (r) 2 r 2 dr dP(r) GM(r) (r) dr r2 P(r) F[(r)] Seager, Kuchner, Hier-Majumder, Militzer Exoplanet Bulk Composition Current surveys MOST Corot/present RV Kepler/future RV Seager, Kuchner, Hier-Majumder, Militzer Exoplanet Bulk Composition Seager, Kuchner, Hier-Majumder, Militzer Generic Mass-Radius Relation Solid planets have a similar M-R relation EOSs of planetary materials are similar and approximately follow = b+cPn. n 3 2 4 3 2 M s Rs 1 1 n Rs 3 5 3 Seager, Kuchner, Hier-Majumder, Militzer Hot Super-Massive Earths • Ten exoplanets with Msini < 21 M – Short period – High chance to transit – Four orbit M stars • Tidally-locked – Day/night side -> hot/cold side? Atmospheric Circulation Venus – 1 day is almost the same as 1 year! (243 vs 225 days) – No day/night temperature gradient – Wind speed a few m/s up to 100s m/s Atmospheric Circulation Upsilon Andromedae b – Large thermal contrast at 24 m – We infer a global day-night temperature – and that the stellar energy is deposited high in the atmosphere Harrington, Hansen, et al. 2006 Spitzer Space Telescope R. Hurt/NASA/Caltech-JPL Habitable Zones on Tidally-Locked Exoplanets Planet diversity: • We expect the full range of planet day and night side temperatures! • Planets with hot/cold sides • Uniformly heated planets could be habitable 450-700K Liquid Water at High Pressure http://www.lsbu.ac.uk/water/phase.html Challenges to Life on Hot Exoplanets • Not necessarily the lack of liquid water • Kinetics may be too fast Signatures of Life Spectra are required … Metabolic byproducts cause chemical disequilibrium … from beyond Earth’s atmosphere Transit planet signal is smaller than changes in atmosphere Theoretical limit to adaptive optics: 108 contrast Telluric lines are a problem Life on hot rocky exoplanets? A range of T on tidally-locked super Earths Liquid water can exist at high T and high P atmosphere habitability L. Cook interior surface Transit Planet Follow-up • Transit [Rp/R*]2 ~ 10-2 – Transit radius -> density Lynnette Cook • Emission spectra Tp/T*(Rp/R*)2 ~10-3 – Emitting atmosphere ~2/3 – Thermal phase curve – Temperature and T • Transmission spectra atm/R*2 ~10-4 – Upper atmosphere – Exosphere (0.05-0.15) • Reflection spectra p[Rp/a]2~10-5 – – – – Albedo Reflected light phase curve Polarization Scattering atmosphere Enabled by a differential measurement Seager et al. 2005 NASA/JPL-Caltech R. Hurt (SSC)
