Manuel Güdel ETH Zürich Switzerland With Michael Meyer & Hans Martin Schmid
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Transcript of Manuel Güdel ETH Zürich Switzerland With Michael Meyer & Hans Martin Schmid
Manuel Güdel ETH Zürich Switzerland
With Michael Meyer & Hans Martin Schmid
Habitable Planets:Targets and theirEnvironments
Pathways, Barcelona, 15 September 2009
http://motls.blogspot.com/2007/04/gliese-581-has-habitable-planet.html
Not discussed here:
• Star and planet formation, disks & gaps/migration/zodi light: see M. Meyer‘s talk • Galactic population statistics• Geophysical issues
Outline
THE STARS:
What role for planetary habitability? (luminosity, age, metallicity, high-energy radiation and particles)
Pathways, Barcelona, 15 September 2009
Luminosity
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
Spec. Luminosity HZ Type (L) radius (AU)
(Unsöld & Baschek) (Kasting & Catling 03)
A0 54 ≈ 4F0 6.5 2.5
G0 1.5 1.5K0 0.43 0.9M0 0.077 0.3M5 0.011 0.1
M
G, K
(Scalo et al. 2007)
(Kasting & Catling 03)
Pathways, Barcelona, 15 September 2009
“ classical definition of HZ“
log m
Metallicity
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
(Fischer & Valenti 2005)
Neptunes
(Sousa et al. 2008)
Pathways, Barcelona, 15 September 2009
Age
Age can be estimated from position in HRD, from rotation period, orfrom magnetic activity.
Spec. Mass main sequenceType (M) lifetime (Gyr)
A0 3 0.39 too short for biologyF0 1.5 1.8 still short…G0 1.1 5.1 (>30% evolutionary change in Lbol)
K0 0.8 14M0 0.5 48 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 (Burrows et al. 2001)
(Unsöld & Baschek)
Pathways, Barcelona, 15 September 2009
The Young Sun was a Fainter Star....
(Sackmann & Boothroyd 2003)
30%
Deep freeze on young Earth and Mars?
Do other wavelength matter here?
Pathways, Barcelona, 15 September 2009
The "Young Active Sun": Non-Flaring EmissionWavelength-Dependent Evolution
(Guinan & Ribas 2002) (Ribas, Guinan, Guedel 2005)
age
soft X EUV UVsoft X
EUV
UV
optical
Luminosity decay more rapidover much larger scale in X-raysthan in UV (while optical radiationis increasing)
Pathways, Barcelona, 15 September 2009
M dwarf photospheresLU,V =
3x10-7-0.02
M dwarf chromosphere
Irradiance Normalized to HZ
Even active M dwarfs show lower UV in their HZ outside flares
(Segura et al. 2005, Scalo et al. 2007)
Different photochemistry: Less molecule formation (OH) or destruction (CH4, N2O)
Good bioindicator!
(Segura et al. 2005)Greenhousegas! HZ?Pathways, Barcelona, 15 September
2009
Continuous Flaring
300Myr
(Audard et al. 2003) (Telleschi, Guedel et al. 2005)
UV Cet M5.5 G1
Pathways, Barcelona, 15 September 2009
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
(2450-3200 Å)Pathways, Barcelona, 15 September 2009
XUV flare rate above a given thresholddecreases with - decreasing mass- increasing ageas does the overall emission
E (0.01-10 keV)
103
0.6
0.01
0.2
N (
>E
) pe
r da
y
(Audard, Guedel, et al. 2000)
age
mass
G K M M
G
Pathways, Barcelona, 15 September 2009
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?
Sun
M Dwarfs
(Scalo et al. 2007)
50-70%Hα active(West et al. 04,see alsoSilvestri et al. 05,Feigelson et al. 04)
normalized LX
M stars stay at a „relatively“ high (X-ray) activitylevel for a longer time
Pathways, Barcelona, 15 September 2009
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
dissociation H2O 2H + O (+ further reactions)Loss of large amounts of water
EUV
Thermosphere
Exosphere Texo
__
Earth Mars
500km 210km
90km 90km
blow-off(Kulikov et al. 2007)
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
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)
(Wood et al. 2005)
old young young old
age
Wind mass loss decreases with age:
dM/dt t-2.3
Pathways, Barcelona, 15 September 2009
WindCMEUV
Dissociative recombinationMolecule ionization, recombination fast neutrals
SputteringIons reimpact atmosphere eject molecules
Ion pickupImpact ionization + charge exchange, E and B fields
atmospheric loss
Interaction atmosphere – environment (solar wind)
Nonthermal Escape
Pathways, Barcelona, 15 September 2009
(see, e.g., Lammer et al. 2003, Lundin & Barabas 2004, Lundin et al. 2007)
http://www.irf.se/~rickard/Rickard_research_interest.html
M star HZ closer to star planets may rotate synchronously (Grieβmeier et al. 2005)
synchronous rotation weaker magnetosphericshielding
smaller
distance
Pathways, Barcelona, 15 September 2009
Tidal Locking and Magnetospheres
& denser stellar wind
weaker magnetic shielding stronger cosmic ray flux more NOx production ozone destruction biological damage?
or evolutionary driver?(Grieβmeier et al. 2005)
Earth
M dwarf planet
& 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)
Pathways, Barcelona, 15 September 2009
To make a planet habitable....
Watch out for the host stars!
optical spectrum and luminosity “traditional” HZ planetary rotation (locked?)
magnetic moment of planetmetallicity formation of terrestrial planetsage/evolutionary scales usefulness of HZ for lifeXUV activity heating/ionizing upper atmosphere
atmosph. photochemistry atmospheric erosion
XUV variability non-equilibrium atmospheres?winds, CMEs, particles ionisation, erosion
END