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)


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)


“ 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)


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)


The Young Sun was a Fainter Star....

(Sackmann & Boothroyd 2003)

30%

Deep freeze on young Earth and Mars?

Do other wavelength matter here?


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)


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


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


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


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


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


(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


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)


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

manuel güdel eth zürich switzerland with michael meyer & hans martin schmid

Documents

lowmass stars

young earth

low feh stars sousa

lower uv

uv cetm5

planetary mass

dwarf flares

uv domain segura