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The search of habitable Earth-likeexoplanets

Helmut Lammer Austrian Academy of Sciences, Space Research InstituteSchmiedlstr. 6, A-8042 Graz, Austria (email: helmut.lammer@oeaw.ac.at)

Graz in Space 2008 / 4. – 5. September 2008

Exoplanet status

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The classical habitable zone definition

Jupiter

Habitable zone

Mars

Earth

Venus

Has to be updated

and habitats betterdefined!

The area around the Sun/star where the climate (CO2, CH4, etc.) and geophysical conditions allows H2O to be liquid on the surfaceof a terrestrial-type planet over geological time periods

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Raymond et al.: Icarus 168, 1, 2004]

Terrestrial planet formation scenariosMpl ≤ 10 MEarth and Rpl ≤ 2 REarth

Ice line

Volatile rich area

Volatile poor area

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Terrestrial planet formation scenariosMpl ≤ 10 MEarth and Rpl ≤ 2 REarth

[e.g., Raymond et al.: Astrobiology , 7, 66, 2007]

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A classification for habitats[Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008]

Habitats suitable for the evolution of higher lifeforms on the surface

Earth-like

Mars-like

Microbial life may have evolved and habitats in subsurface, ice/H2O, may have remained

Life forms may have evolvedand populate subsurfaceH2O oceans

Classical habitable zone

Inner and outer edge of the habitable zone or habitable zones of low mass stars

Class. I

Class. II

Water-rich bodies at the beginning

Evolutionary time line

Beyond the ice-line

Class. III

Migrating “super-Europa’s”,“hot ice giants”, “Ocean planets”Lower or/and higherlife may evolve but populate oceans ?

Ice-rich exoplanetswhich migrate insidea habitable zone orcloser to their host stars

Class. IV

Venus-like

V M

E

Icymoons

Europa-like

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Geophysical relevance of water: → Earth: Class I habitats

Dynamo Action

Atmosphere

Convecting mantle

Degassing

Regassing

Volcanism Subduction

Shielding

Efficient cooling

Crust

Magnetosphere Hydros- + Cryosphere Biosphere

Large amount of H2O in the mantle is important ! (Oceans)

⇒

From D. Breuer, DLR, Berlin

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One plate planets (present Venus and Mars): Class II habitats ?

Dynamo Action

Atmosphere

Convecting mantle

Degassing

Volcanism

Shielding

Inefficient cooling

Space

Erosion by solar/stellar plasma flow

Crust

Hydros- + CryosphereMagnetosphere Biosphere

Very hot (dry) or frozen planets(inner and outer boundary of the classical habitable zone)

From D. Breuer, DLR, Berlin

?

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The upper atmosphere (Thermosphere, exosphere)

exobase

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X-ray/EUV activity of low mass stars

[Scalo et al., Astrobiology, 7, 85, 2007]

Early Venus, Earth, Mars,Titan, gas giants,

comets Exoplanets

0.1 Gyr0.3 Gyr 1.0 Gyr

3.16 Gyr 10 Gyr

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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.

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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

Time evolution of the exobase temperature based on Earth's present atmospheric composition

[Kulikov et al., SpSciRev., 2007]

? Hydrostatic equilibrium is assumed →no hydrodynamic flow and adiabatic cooling

CO2? 5000 K (H atoms)

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Expected scenarios of atmosphere responsesduring the young Sun active star epochs

96 % CO2atmosphere

(Venus)

present Earth composition

(Earth)

[Kulikov et al., Planet. Space Sci., 54, 1425 – 1444, 2006]

[Lammer et al., Space Sci. Rev., in press, 2008; Tian et al., JGR,in press, 2008]

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Expected evolution of Earth’s atmosphere

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

SunG stars

Lower mass starsK, M stars

Atmosphere evolution of Earth-like planets will be different (low mass K and M stars)

Earth (G star Earth-like planets, F star Earth’s?)

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[Lammer et al., Astrobiology, 7, 185, 2007]

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Soft X-ray and EUV induced expansion of the upper atmospheres can lead to high non-thermal loss rates

present Earth

Early Earth ?terrestrialexoplanets

present Venus,Mars

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Its not so simple! →No analogy for habitable zones of lower mass stars (K and M-types)Atmospheric effects andhabitability of Earth-likeexoplanets 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 analogyData from Sun + Stars

Space and ground-based dataCorrelated analysis of eventsEstablishing an extreme event data-base(Venus, Earth, Mars, exoplanets)Input for models

Atmospheric ion loss processes related to solar/stellar plasma

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Venus

Titan

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Plasma environment within close-in habitable zones

nmin (d) = 4.88 d/d0 -2.31

nmax(d) = 7.10 d/d0 -2.99

v modCME = 450 km/s

[Khodachenko et al., Astrobiology, 7,167, 2007]

0.05 AU

0.1 AU

0.2 AU

1 AU

d0 = 1 AU

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 RSun: nCME ~106 cm –3

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Plasma environment within close-in habitable zones

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O+ loss rates of present Venus at 0.7 AU

Venus Express

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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

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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

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3D MHD simulation of a Venus-like planet under extreme stellar plasma conditions → 0.05 AU (100 XUV)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

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H2O inventories and atmospheres are strongly effected due to non-thermal loss processes

Class I habitats (Exo-Earth’s) mayevolve to class II habitattypes (Venus or Mars)

at M stars

Class I habitats (Exo-Earth’s)could be expect

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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-typeand 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 IV-type habitable planets may be common there due to the large size of stars of these spectral class

Where are they? Star-types and expected preferred habitats

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Space Missions which will study habitability of planetary bodies besides Earth

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Exoplanet missions

Kepler (NASA)

Super-Earth`s ≤ 0.5 AU Earth-size exoplanets ≤ 1 AU

Darwin (ESA) / TPF (NASA)

Atmospheric characterisation, biomarker, comparative planetology

Life Finder, Planet Imager, etc.

2006 2008 2010 2012 2015

CoRoT (CNES)

GAIA (ESA)

SIM (NASA)

Earth-massexoplanets

Thousands of Jupiters

> 2023

PLATO (ESA) ?

The search of habitable Earth-like�exoplanetsExoplanet status The classical habitable zone definition Terrestrial planet formation scenarios�Mpl 10 MEarth and Rpl 2 REarth Terrestrial planet formation scenarios�Mpl 10 MEarth and Rpl 2 REarth A classification for habitats�[Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008] Geophysical relevance of water: � → Earth: Class I habitats One plate planets (present Venus and Mars): Class II habitats ? The upper atmosphere (Thermosphere, �exosphere)X-ray/EUV activity of low mass stars Thermospheric heating and cooling processes Time evolution of the exobase temperature based �on Earth's present atmospheric composition Expected scenarios of atmosphere responses�during the young Sun active star epochsExpected evolution of Earth’s atmosphere Soft X-ray and EUV induced expansion of the upper �atmospheres can lead to high non-thermal loss rates Its not so simple! →No analogy for habitable zones of lower mass stars (K and M-types) Atmospheric ion loss processes related to solar/stellar plasma Plasma environment within close-in habitable zones Plasma environment within close-in habitable zones O+ loss rates of present Venus at 0.7 AU O+ loss rates of Venus 4.25 Gyr ago; 30 XUV; � nsw=1000 cm-3 (60 pr.) or M-star Exo-Venus O+ loss rates of Venus 4.5 Gyr ago 100 XUV; � nsw=1000 cm-3 or (active M-star) Exo-Venus at 0.3 A 3D MHD simulation of a Venus-like planet under � extreme stellar plasma conditions 0.05 AU (100 H2O inventories and atmospheres are strongly effected due to non-thermal loss processes Where are they? �Star-types and expected preferred habitats Space Missions which will study habitability �of planetary bodies besides EarthExoplanet missions