Helmut Lammer Austrian Academy of Sciences, Space Research Institute Schmiedlstr. 6, A-8042 Graz, Austria (

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1 The search of habitable Earth-like exoplanets Helmut Lammer Austrian Academy of Sciences, Space Research Institute Schmiedlstr. 6, A-8042 Graz, Austria ( Graz in Space 2008 / September

2 Exoplanet status 2

3 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 (CO 2, CH 4, etc.) and geophysical conditions allows H 2 O to be liquid on the surface of a terrestrial-type planet over geological time periods 3

4 Terrestrial planet formation scenarios M pl 10 M Earth and R pl 2 R Earth Raymond et al.: Icarus 168, 1, 2004] Ice line Volatile poor area Volatile rich area 4

5 Terrestrial planet formation scenarios M pl 10 M Earth and R pl 2 R Earth [e.g., Raymond et al.: Astrobiology, 7, 66, 2007] 5

6 A classification for habitats [Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008] Water-rich bodies at the beginning Evolutionary time line V E M Venus -like Class. I Classical habitable zone Class. II Inner and outer edge of the habitable zone or habitable zones of low mass stars Earth-like Mars -like Habitats suitable for the evolution of higher life forms on the surface Microbial life may have evolved and habitats in subsurface, ice/h 2 O, may have remained Icy moons Class. III Beyond the ice-line Europalike Life forms may have evolved and populate subsurface H 2 O oceans Class. IV Ice-rich exoplanets which migrate inside a habitable zone or closer to their host stars Migrating super-europa s, hot ice giants, Ocean planets Lower or/and higher life may evolve but populate oceans? 6

7 Geophysical relevance of water: Earth: Class I habitats Efficient cooling Convecting mantle Large amount of H 2 O in the mantle is important! (Oceans) Regassing Dynamo Action Volcanism Subduction Degassing Crust Magnetosphere Hydros- + Cryosphere Biosphere Shielding Atmosphere From D. Breuer, DLR, Berlin 7

8 One plate planets (present Venus and Mars): Class II habitats? Inefficient cooling Dynamo Action Convecting mantle Volcanism Very hot (dry) or frozen planets (inner and outer boundary of the classical habitable zone) Degassing Crust? Magnetosphere Hydros- + Cryosphere Biosphere Shielding Atmosphere Erosion by solar/stellar plasma flow Space From D. Breuer, DLR, Berlin 8

9 The upper atmosphere (Thermosphere, exosphere) exobase 9

10 X-ray/EUV activity of low mass stars Early Venus, Earth, Mars, Titan, gas giants, comets Exoplanets 0.1 Gyr 0.3 Gyr 1.0 Gyr 3.16 Gyr 10 Gyr [Scalo et al., Astrobiology, 7, 85, 2007] 10

11 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 O 2, N 2, and O photoionization by solar XUV radiation ( λ 1027 Å), heating due to O 2 and O 3 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 CO 2, NO, O 3, OH, NO +, 14 N 15 N, CO, O 2 (1Δg), etc. heating and cooling due to contraction and expansion of the thermosphere, turbulent energy dissipation and heat conduction. 11

12 Time evolution of the exobase temperature based on Earth's present atmospheric composition? Hydrostatic equilibrium is assumed no hydrodynamic flow and adiabatic cooling CO 2? 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 CO 2 abundance in the Earth's atmosphere during the first 500 Myr should be much higher than ~ 3.5 Gyr ago to survive 12

13 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, , 2006] 96 % CO 2 atmosphere (Venus) 13

14 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?) and cools the upper atmosphere so that expansion and loss rates should be reduced 14

15 Soft X-ray and EUV induced expansion of the upper atmospheres can lead to high non-thermal loss rates present Earth present Venus, Mars [Lammer et al., Astrobiology, 7, 185, 2007] Early Earth? terrestrial exoplanets 15

16 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 Correlated analysis of events Establishing an extreme event data-base (Venus, Earth, Mars, exoplanets) 16 Input for models

17 Atmospheric ion loss processes related to solar/stellar plasma Venus Titan 17

18 Plasma environment within close-in habitable zones 0.05 AU 0.1 AU 0.2 AU n min (d) = 4.88 d/d n max (d) = 7.10 d/d AU v mod CME = 450 km/s d 0 = 1 AU [Khodachenko et al., Astrobiology, 7,167, 2007] White light [Vourlidas, et al., ESA SP-506, 1, 91, 2002] Radio [Gopalswamy and Kundu, Solar Phys., 143, 327, 1993] UV [Ciaravella, et al., ApJ., 597, 1118, 2003] similar values at 3-5 R Sun : n CME ~10 6 cm 3 18

19 Plasma environment within close-in habitable zones 19

20 O + loss rates of present Venus at 0.7 AU Venus Express 20

21 O + loss rates of Venus 4.25 Gyr ago; 30 XUV; nsw=1000 cm -3 (60 pr.) or M-star Exo-Venus at 0.3 AU Total O + ion loss rate ~ 2 bar 150 Myr 21

22 O + loss rates of Venus 4.5 Gyr ago 100 XUV; nsw=1000 cm -3 or (active M-star) Exo-Venus at 0.3 AU Total O + ion loss rate ~ 20 bar 150 Myr 22

23 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

24 H 2 O 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 24

25 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 25

26 Space Missions which will study habitability of planetary bodies besides Earth 26

27 Exoplanet missions 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 27