Manuel Güdel ETH Zürich Switzerland With Michael Meyer & Hans Martin Schmid

1. Habitable Planets: Targets and their Environments Manuel Güdel ETH Zürich Switzerland With Michael Meyer & Hans Martin Schmid http://motls.blogspot.com/2007/04/gliese-581-has-habitable-planet.html

2. 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

3. “ classical definition of HZ“ Luminosity (Scalo et al. 2007) Spec. Luminosity HZ Type (L) radius (AU) (Unsöld & Baschek) (Kasting & Catling 03) A0 54 ≈ 4 F0 6.5 2.5 G0 1.5 1.5 K0 0.43 0.9 M0 0.077 0.3 M5 0.011 0.1 G, K M 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 (Kasting & Catling 03)

4. (Fischer & Valenti 2005) 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 (Sousa et al. 2008) Neptunes

5. Age Age can be estimated from position in HRD, from rotation period, or from magnetic activity. Spec. Mass main sequence Type (M) lifetime (Gyr) A0 30.39too short for biology F0 1.51.8still short… G0 1.1 5.1 (>30% evolutionary change in Lbol) K0 0.8 14 M0 0.548very 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)

6. The Young Sun was a Fainter Star.... 30% (Sackmann & Boothroyd 2003) Deep freeze on young Earth and Mars? Do other wavelength matter here?

7. Wavelength-Dependent Evolution The "Young Active Sun": Non-Flaring Emission 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)

8. 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) Good bioindicator! Greenhouse gas!  HZ?

9. Continuous Flaring G1 M5.5 UV Cet 300Myr (Audard et al. 2003) (Telleschi, Guedel et al. 2005)

10. 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)

11. mass 10 G K M M 3 0.6 N (>E) per day age 0.01 • XUV flare rate above a given threshold • decreases with • decreasing mass • increasing age • as does the overall emission G 0.2 (Audard, Guedel, et al. 2000) E (0.01-10 keV)

12. 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 (West et al. 04, see also Silvestri et al. 05, Feigelson et al. 04) M stars stay at a „relatively“ high (X-ray) activity level for a longer time M Dwarfs normalized LX Sun (Scalo et al. 2007)

13. 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 dissociation H2O  2H + O (+ further reactions) Loss of large amounts of water Exosphere Texo 500km 210km Mars __ Thermosphere 90km 90km blow-off (Kulikov et al. 2007) Earth Mars (eg, Watson 1981, Kasting & Pollack 1983, Chassefiere & Leblanc 2004, Kulikov et al. 2007, Tian et al. 2008)

14. Semi-Empirical Mass-Loss Estimates for the Young Sun Wind mass loss decreases with age: (Wood et al. 2005) 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)

15. Nonthermal Escape Dissociative recombination Molecule ionization, recombination  fast neutrals Sputtering Ions reimpact atmosphere  eject molecules Ion pickup Impact ionization + charge exchange, E and B fields Wind CME UV atmospheric loss     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)

16. M star HZ closer to star  planets may rotate synchronously (Grieβmeier et al. 2005) smaller distance synchronous rotation  weaker magnetospheric shielding

17. 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 M dwarf planet Earth • biological damage? • or evolutionary driver? (Grieβmeier et al. 2005)

18. To make a planet habitable.... Watch out for the host stars! optical spectrum and luminosity  “traditional” HZ  planetary rotation (locked?)  magnetic moment of planet metallicity  formation of terrestrial planets age/evolutionary scales  usefulness of HZ for life XUV activity  heating/ionizing upper atmosphere  atmosph. photochemistry  atmospheric erosion XUV variability  non-equilibrium atmospheres? winds, CMEs, particles  ionisation, erosion

19. END