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The search of habitable Earth-like exoplanets Helmut Lammer Austrian Academy of Sciences, Space Research Institute Schmiedlstr. 6, A-8042 Graz, Austria (email: [email protected]) IWF/ÖAW GRAZ Graz in Space 2008 / 4. – 5. September 2008 1 Exoplanet status IWF/ÖAW GRAZ 2 The classical habitable zone definition Has to be updated Venus Earth Mars Jupiter Habitable zone and habitats better defined! The area around the Sun/star where the climate (CO2, CH4, etc.) and geophysical conditions allows H2O to be liquid on the surface of a terrestrial-type planet over geological time periods IWF/ÖAW GRAZ 3 Terrestrial planet formation scenarios Mpl ≤ 10 MEarth and Rpl ≤ 2 REarth Raymond et al.: Icarus 168, 1, 2004] Ice line Volatile poor area IWF/ÖAW GRAZ Volatile rich area 4 Terrestrial planet formation scenarios Mpl ≤ 10 MEarth and Rpl ≤ 2 REarth IWF/ÖAW GRAZ [e.g., Raymond et al.: Astrobiology , 7, 66, 2007] 5 A classification for habitats [Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008] Water-rich bodies at the beginning Class. I E M V Evolutionary time line Venus -like Icy moons Classical habitable zone Class. II Inner and outer edge of the habitable zone or habitable zones of low mass stars Earth-like Mars -like Class. III Beyond the ice-line Class. IV IWF/ÖAW GRAZ Ice-rich exoplanets which migrate inside a habitable zone or closer to their host stars Habitats suitable for the evolution of higher life forms on the surface Microbial life may have evolved and habitats in subsurface, ice/H2O, may have remained Europalike Life forms may have evolved and populate subsurface H2O oceans Migrating “super-Europa’s”, “hot ice giants”, “Ocean planets” Lower or/and higher life may evolve but populate oceans ? 6 Geophysical relevance of water: → Earth: Class I habitats Convecting mantle Efficient cooling ⇒ Large amount of H2O in the mantle is important ! (Oceans) Regassing Volcanism Dynamo Action Subduction Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Biosphere Atmosphere From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 7 One plate planets (present Venus and Mars): Class II habitats ? Convecting mantle Inefficient cooling Volcanism Dynamo Action Very hot (dry) or frozen planets (inner and outer boundary of the classical habitable zone) Crust Degassing Magnetosphere Shielding ? Hydros- + Cryosphere Biosphere Atmosphere Erosion by solar/stellar plasma flow Space From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 8 The upper atmosphere (Thermosphere, exosphere) exobase IWF/ÖAW GRAZ 9 X-ray/EUV activity of low mass stars Early Venus, Earth, Mars, Titan, gas giants, comets 0.1 Gyr IWF/ÖAW GRAZ Exoplanets 0.3 Gyr 1.0 Gyr [Scalo et al., Astrobiology, 7, 85, 2007] 3.16 Gyr 10 Gyr 10 Thermospheric heating and cooling processes The most important heating and cooling processes in the upper atmosphere of Earth can be summarized as follows [e..g., Dickinson, 1972; Chandra and Sinha, 1974; Gordiets et al., 1978; Gordiets et al., 1981; Gordiets et al., 1982; Dickinson et al., 1987] heating due to O2, N2, and O photoionization by solar XUV radiation ( λ ≤ 1027 Å), heating due to O2 and O3 photodissociation by solar UV radiation (1250 ≤ λ ≤ 3500 Å), chemical heating in exothermic binary and 3-body reactions, neutral gas molecular heat conduction, IR-cooling in the vibrational-rotational bands of CO2, NO, O3, OH, NO+, 14N15N, CO, O2(1Δg), etc. heating and cooling due to contraction and expansion of the thermosphere, turbulent energy dissipation and heat conduction. IWF/ÖAW GRAZ 11 Time evolution of the exobase temperature based on Earth's present atmospheric composition ? Hydrostatic equilibrium is assumed →no hydrodynamic flow and adiabatic cooling CO2? 5000 K (H atoms) [Kulikov et al., SpSciRev., 2007] The blow-off temperature for atomic hydrogen of about 5000 K would be exceded during the first Gyr For XUV fluxes more than 10 times the present flux (> 3.8 Gyr ago) one would expect extremely high exospheric temperatures Therefore, the CO2 abundance in the Earth's atmosphere during the first 500 Myr should be much higher than ~ 3.5 Gyr ago to survive IWF/ÖAW GRAZ 12 Expected scenarios of atmosphere responses during the young Sun active star epochs present Earth composition (Earth) [Lammer et al., Space Sci. Rev., in press, 2008; Tian et al., JGR, in press, 2008] [Kulikov et al., Planet. Space Sci., 54, 1425 – 1444, 2006] 96 % CO2 atmosphere (Venus) IWF/ÖAW GRAZ 13 Expected evolution of Earth’s atmosphere Lower mass stars K, M stars Atmosphere evolution of Earth-like planets will be different (low mass K and M stars) Sun G stars Earth (G star Earth-like planets, F star Earth’s?) IWF/ÖAW GRAZ and cools the upper atmosphere so that expansion and loss rates should be reduced 14 Soft X-ray and EUV induced expansion of the upper atmospheres can lead to high non-thermal loss rates present Venus, Mars present Earth [Lammer et al., Astrobiology, 7, 185, 2007] IWF/ÖAW GRAZ Early Earth ? terrestrial exoplanets 15 Its not so simple! →No analogy for habitable zones of lower mass stars (K and M-types) Atmospheric effects and habitability of Earth-like exoplanets within close-in habitable zones Enhanced EUV and X-rays Neutron fluxes Coronal mass ejections (CMEs) Intense solar proton/electron fluxes (e.g., SPEs) Solar – stellar analogy Data from Sun + Stars Space and ground-based data IWF/ÖAW GRAZ Correlated analysis of events Establishing an extreme event data-base (Venus, Earth, Mars, exoplanets) 16 Input for models Atmospheric ion loss processes related to solar/stellar plasma Venus Titan IWF/ÖAW GRAZ 17 Plasma environment within close-in habitable zones 0.05 AU 0.1 AU 0.2 AU 1 AU nmin (d) = 4.88 d/d0 -2.31 nmax(d) = 7.10 d/d0 -2.99 v mod CME = 450 km/s d0 = 1 AU [Khodachenko et al., Astrobiology, 7,167, 2007] White light Radio [Vourlidas, et al., ESA SP-506, 1, 91, 2002] [Gopalswamy and Kundu, Solar Phys., 143, 327, 1993] UV IWF/ÖAW GRAZ [Ciaravella, et al., ApJ., 597, 1118, 2003] similar values at 3-5 RSun: nCME ~106 cm –3 18 Plasma environment within close-in habitable zones IWF/ÖAW GRAZ 19 + O loss rates of present Venus at 0.7 AU Venus Express 20 + O loss rates of Venus 4.25 Gyr ago; 30 XUV; -3 nsw=1000 cm (60 × pr.) or M-star Exo-Venus at 0.3 AU + Total O ion loss rate ~ 2 bar → 150 Myr 21 + O loss rates of Venus 4.5 Gyr ago 100 XUV; -3 sw n =1000 cm or (active M-star) Exo-Venus at 0.3 AU + Total O ion loss rate ~ 20 bar → 150 Myr 22 3D MHD simulation of a Venus-like planet under extreme stellar plasma conditions → 0.05 AU (100 XUV) + Total O ion loss rate ~ 500 bar → 150 Myr 23 H2O inventories and atmospheres are strongly effected due to non-thermal loss processes Class I habitats (Exo-Earth’s) could be expect Class I habitats (Exo-Earth’s) may evolve to class II habitat types (Venus or Mars) at M stars IWF/ÖAW GRAZ 24 Where are they? Star-types and expected preferred habitats Class I Earth-like habitable planets may preferably be found in orbits of Sun-like G-type and some K-type stars, F-type where the originally defined habitable zone definition is valid → see Earth! Class II, III and IV habitats should also populate G-type and F, K, and M-type stars Lower mass stars should have less class I habitable planets but class II, class III and class IV habitability-types may be common like on G-stars. Many planets which start in the habitability class I domain at its origin may evolve to class II-types Earth-like Class I habitable planets “MAY NOT” evolve around low mass active M-type stars. Most of them or even all of them may evolve from class I to class II during their lifetime. Class II, III and IVtype habitable planets may be common there due to the large size of stars of these spectral class IWF/ÖAW GRAZ 25 Space Missions which will study habitability of planetary bodies besides Earth IWF/ÖAW GRAZ 26 Exoplanet missions 2006 2010 2008 2015 2012 Kepler (NASA) GAIA (ESA) SIM (NASA) CoRoT (CNES) Super-Earth`s ≤ 0.5 AU Earth-size exoplanets ≤ 1 AU > 2023 Earth-mass exoplanets Thousands of Jupiters PLATO (ESA) ? Darwin (ESA) / TPF (NASA) Life Finder, Planet Imager, etc. Atmospheric characterisation, biomarker, comparative planetology IWF/ÖAW GRAZ 27