Download The search of habitable Earth-like exoplanets

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
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Exoplanet status
IWF/ÖAW GRAZ
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The classical habitable zone definition
Has to be updated
Venus
Earth
Mars
Jupiter
Habitable zone
and habitats better
defined!
ƒ 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
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Terrestrial planet formation scenarios
Mpl ≤ 10 MEarth and Rpl ≤ 2 REarth
Raymond et al.: Icarus 168, 1, 2004]
Ice line
Volatile
poor area
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Volatile rich
area
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Terrestrial planet formation scenarios
Mpl ≤ 10 MEarth and Rpl ≤ 2 REarth
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[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]
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
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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 ?
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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
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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
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The upper atmosphere (Thermosphere,
exosphere)
exobase
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X-ray/EUV activity of low mass stars
Early
Venus, Earth, Mars,
Titan, gas giants,
comets
0.1 Gyr
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Exoplanets
0.3 Gyr
1.0 Gyr
[Scalo et al., Astrobiology, 7, 85, 2007]
3.16 Gyr
10 Gyr
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Thermospheric heating and cooling processes
ƒ 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]
ƒ
heating due to O2, N2, and O photoionization by solar XUV
radiation ( λ ≤ 1027 Å),
ƒ
heating due to O2 and O3 photodissociation by solar UV radiation
(1250 ≤ λ ≤ 3500 Å),
ƒ
chemical heating in exothermic binary and 3-body reactions,
ƒ
neutral gas molecular heat conduction,
ƒ
IR-cooling in the vibrational-rotational bands of CO2, NO, O3,
OH, NO+, 14N15N, CO, O2(1Δg), etc.
ƒ
heating and cooling due to contraction and expansion of the
thermosphere,
ƒ
turbulent energy dissipation and heat conduction.
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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]
ƒ
The blow-off temperature for atomic hydrogen of about 5000 K would
be exceded during the first Gyr
ƒ
For XUV fluxes more than 10 times the present flux (> 3.8 Gyr ago) one
would expect extremely high exospheric temperatures
ƒ
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
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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)
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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?)
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and cools the upper atmosphere so that expansion and loss rates should be reduced
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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]
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Early Earth ?
terrestrial
exoplanets
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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
ƒ Enhanced EUV and X-rays
ƒ Neutron fluxes
ƒ Coronal mass ejections
(CMEs)
ƒ Intense solar
proton/electron fluxes
(e.g., SPEs)
Solar – stellar analogy
ƒ Data from Sun + Stars
Space and ground-based data
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ƒ Correlated analysis of events
ƒ Establishing an extreme event data-base
(Venus, Earth, Mars, exoplanets)
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ƒ Input for models
Atmospheric ion loss processes
related to solar/stellar plasma
Venus
Titan
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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]
ƒ White light
ƒ Radio
[Vourlidas, et al., ESA SP-506, 1, 91, 2002]
[Gopalswamy and Kundu, Solar Phys., 143, 327, 1993]
ƒ UV
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[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;
-3
nsw=1000 cm (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;
-3
sw
n =1000 cm 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)
+
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)
could be expect
Class I habitats (Exo-Earth’s) may
evolve to class II habitat
types (Venus or Mars)
at M stars
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Where are they?
Star-types and expected preferred habitats
ƒ 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!
ƒ Class II, III and IV habitats should also populate G-type
and F, K, and M-type stars
ƒ 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
ƒ 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
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Space Missions which will study habitability
of planetary bodies besides Earth
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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
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