Transcript Targets and their Environments - Pathways Towards Habitable Planets

Habitable Planets:
Targets and their
Environments
Manuel Güdel
ETH Zürich
Switzerland
http://motls.blogspot.com/2007/04/gliese-581-has-habitable-planet.html
Pathways, Barcelona, 15 September 2009
With Michael Meyer & Hans Martin Schmid
Outline
THE STARS:
What role for planetary habitability?
(luminosity, age, metallicity, high-energy radiation and particles)
Not discussed here:
• Star and planet formation, disks & gaps/migration/zodi light: see M. Meyer‘s talk
• Galactic population statistics
• Geophysical issues
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“ classical definition of HZ“
Spec.
Type
A0
G0
K0
M0
M5
Luminosity
(L)
HZ
(Unsöld & Baschek)
(Kasting & Catling 03)
54
F0
1.5
0.43
0.077
0.011
(Scalo et al. 2007)
radius (AU)
G, K
≈4
6.5
2.5
1.5
0.9
0.3
0.1
M

Luminosity
log m
Toward smaller HZ:
less perturbation by Jupiters & companions
and:
low-mass stars have fewer Jupiters
(Endl et al. 03, Butler et al. 07)
 stable orbits & conditions
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(Kasting & Catling 03)
Metallicity
(Fischer & Valenti 2005)
High-[Fe/H] stars more likely to
host Jupiter-like planets
Not true for Neptunes/Super-Earths
(more easily found around low [Fe/H] stars;
Sousa et al. 2008, Mayor et al. 2009)
However: Earth-like planetary mass in solar
system ≈ 2ME  [Fe/H] ≥ -0.3 (Turnbull 08)
 requirement: stars in young disk population
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(Sousa
et al.
2008)
Neptunes
Age
Age can be estimated from position in HRD, from rotation period, or
from magnetic activity.
Spec.
Type
Mass
(M)
main sequence
lifetime (Gyr) (Unsöld & Baschek)
A0
F0
G0
K0
M0
3
1.5
1.1
0.8
0.5
0.39
1.8
5.1
14
48
too short for biology
still short…
(>30% evolutionary change in Lbol)
very slow evolution  stable HZ
Con-M: Evolution toward MS very slow as well:
on MS with stable HZ only after 1 Gyr for 0.1M
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(Burrows et al. 2001)
The Young Sun was a Fainter Star....
30%
(Sackmann & Boothroyd 2003)
Deep freeze on young Earth and Mars?
Do other wavelength matter here?
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Wavelength-Dependent
Evolution Emission
The "Young
Active Sun": Non-Flaring
soft X
EUV
UV
soft X
age
EUV
UV
optical
(Guinan & Ribas 2002)
(Ribas, Guinan, Guedel 2005)
Luminosity decay more rapid
over much larger scale in X-rays
than in UV (while optical radiation
is increasing)
Pathways, Barcelona, 15 September 2009
Irradiance Normalized to HZ
M dwarf
chromosphere
M dwarf
photospheres
LU,V =
3x10-7-0.02
(Segura et al. 2005,
Scalo et al. 2007)
Even active M dwarfs show lower UV in their HZ outside flares
Different photochemistry: Less molecule formation (OH) or destruction (CH4, N2O)
(Segura et al. 2005)
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Greenhouse
gas!  HZ?
Good
bioindicator!
Continuous Flaring
UV Cet
M5.5
G1
300Myr
(Audard et al. 2003)
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(Telleschi, Guedel et al. 2005)
EUV flare rate (above 1032 erg)  LX
(Audard, Guedel, et al. 2000)
Flares:
LUV  LX
for biologically relevant UV
(Mitra-Kraev, Harra, Guedel et al. 2005)
Slope 1.17±0.05
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(2450-3200 Å)
mass
10
G
3
K
M
M
N (>E) per day
0.6
age
G
0.2
0.01
XUV flare rate above a given threshold
decreases with
- decreasing mass
- increasing age
as does the overall emission
(Audard, Guedel, et al. 2000)
E (0.01-10 keV)
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G and M dwarf flares physically/spectrally similar, related to LX But:
larger relative modulation in UV domain (Segura et al. 2005, Scalo et al. 2007):
consequence for (non-equilibrium) atmospheric photochemistry or life?
Dependent on amplitudes?
50-70%
Hα active
M stars stay at a
„relatively“ high
(X-ray) activity
level for a longer
time
M Dwarfs
normalized
LX
Sun
(Scalo et al. 2007)
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(West et al. 04,
see also
Silvestri et al. 05,
Feigelson et al. 04)
EUV
Evaporation of Planetary Atmospheres
 < 1700 Å heats “thermosphere”(by photoioniz./dissociation)
mv2/2 > GMm/R: particle escapes: up to several bars!
Exosphere: mean free path > local scale height
500km 210km
Exosphere
Texo
__
Mars
Thermosphere
90km 90km
(Kulikov et al. 2007) blow-off
dissociation H2O  2H + O (+ further reactions)
Loss of large amounts of water
Earth Mars
(eg, Watson 1981, Kasting & Pollack 1983, Chassefiere & Leblanc 2004,
Kulikov et al. 2007, Tian et al. 2008)
Semi-Empirical Mass-Loss Estimates for the Young Sun
(Wood et al. 2005)
Wind mass loss decreases with age:
dM/dt  t-2.3
old
young
young
old
age
Further, Coronal Mass Ejections in active stars act like continuous wind
(500 km/s, 103 cm-3)
(Khodachenko et al. 2007, Lammer et al. 2007)
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Nonthermal Escape
Dissociative recombination
Molecule ionization, recombination  fast neutrals
Wind
CME
UV
Sputtering
Ions reimpact atmosphere  eject molecules
atmospheric
loss
Ion pickup
Impact ionization + charge exchange, E and B fields




Interaction atmosphere – environment (solar wind)
http://www.irf.se/~rickard/Rickard_research_interest.html
(see, e.g., Lammer et al. 2003, Lundin & Barabas 2004, Lundin et al. 2007)
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M star HZ closer to star  planets may rotate synchronously (Grieβmeier et al. 2005)
smaller
distance
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synchronous rotation

weaker magnetospheric
shielding
Tidal Locking and Magnetospheres
& high activity & flares
 „continuous“ CMEs
 EUV heating  atmospheric expansion
 small magnetospheric standoff distance
atmospheric erosion for M dwarf planets,
10s to 100s of bars (Khodachenko et al. 2007, Lammer et al. 2007)
& denser stellar wind
 weaker magnetic shielding
 stronger cosmic ray flux
 more NOx production
 ozone destruction
 biological damage?
or evolutionary driver?
M dwarf planet
Earth
(Grieβmeier et al. 2005)
To make a planet habitable....
Watch out for the host stars!
optical spectrum and luminosity
metallicity
age/evolutionary scales
XUV activity
XUV variability
winds, CMEs, particles
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 “traditional” HZ
 planetary rotation (locked?)
 magnetic moment of planet
 formation of terrestrial planets
 usefulness of HZ for life
 heating/ionizing upper atmosphere
 atmosph. photochemistry
 atmospheric erosion
 non-equilibrium atmospheres?
 ionisation, erosion
END