Download 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
Pathways, Barcelona, 15 September 2009
“ 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
Pathways, Barcelona, 15 September 2009
(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
Pathways, Barcelona, 15 September 2009
(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
Pathways, Barcelona, 15 September 2009
(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?
Pathways, Barcelona, 15 September 2009
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)
Pathways, Barcelona, 15 September 2009
Greenhouse
gas!  HZ?
Good
bioindicator!
Continuous Flaring
UV Cet
M5.5
G1
300Myr
(Audard et al. 2003)
Pathways, Barcelona, 15 September 2009
(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
Pathways, Barcelona, 15 September 2009
(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)
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?
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)
Pathways, Barcelona, 15 September 2009
(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)
Pathways, Barcelona, 15 September 2009
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)
Pathways, Barcelona, 15 September 2009
M star HZ closer to star  planets may rotate synchronously (Grieβmeier et al. 2005)
smaller
distance
Pathways, Barcelona, 15 September 2009
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
Pathways, Barcelona, 15 September 2009
 “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