Planetary Habitability

Transcription

1 Planetary Habitability

2 New Planets This Week K2-18 M2.5V, d=34 pc, P rot =38 days K2-18b: (transigng, P=32.9d, in HZ) M= 8.0 ± 1.9M <ρ>: 3.3 ± 1.2 g/cm 3 K2-18c: (RV, P~9 d) M c sin i c = 7.5 ± 1.3 M SeparaGon: 23 mutual R H ClouGer, et al., 2017, A&A, 608, A35

3 New Planets This Week WASP 18b Hot Jupiter (2400K, 10.3 M J ) CO atmosphere No H 2 O, VO, TiO C/O ~1 C/H ~ µm emission, 1.6µm absorpgon Sheppard, et al., 2017, ApJL, 850, L32

4 New Planets This Week WASP 18b: Hot stratosphere Sheppard, et al., 2017, ApJL, 850, L32

5 The Habitable Zone Need liquid H 2 O * A surface An atmosphere 273 < <T(K)> < 373 Need a source of energy Starlight Internal heat * for life as we know it

6 Why liquid water? Amino acids and other organic molecules react through collisions. Transport via a fluid increases mobility/ number of reacgons

7 Proteins and PepGdes Amino Acids form chains by a chemical process called hydrolysis. The OH from the carboxyl group combines with the H from the amino group to make H 2 O. The C and N bond directly. This is a pep2de bond.

8 Water Water requires heat and pressure to remain stable as a liquid

9 Stellar Effects Primary Energy Secondary GravitaGonal effects Spectral energy distribugons Variable SEDs

10 Energy Balance (1- a) πr 2 (L / 4π d2) 4πR 2σT 4

11 The Terrestrial Greenhouse mean temperature: 287 K equilibrium temperature (a=0): 280 K equilibrium temperature (a=0.39): 247 K Greenhouse effect: 40K

12 Terrestrial Energy Balance Kaltenegger 2017 ARAA, 55, 433

13 The Habitable Zone Exact locagon depends on assumpgons about atmospheric composigon and albedo Inner edge: AU Outer edge: AU AU KasGng, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993)

14 EvoluGon of the Habitable Zone Kaltenegger 2017, ARAA, 55, 433

15 Faint Young Sun Problem

16 The ConGnuously Habitable Zone The faint young Sun problem: Stars evolve - stars brighten with Gme 4.5 Gya, the Sun was 70% of its current luminosity In 5 Gyr, the Sun will brighten by a factor of 2 T = (( [1- a] L)/ (σπd 2 )) ¼ Temperature increases as L ¼

17 The ConGnuously Habitable Zone

18 The ConGnuously Habitable Zone Details depend on assumed planetary atmosphere, and its evolugon Inner edge at 0.9 x 0.7 ¼ = 0.8 AU Width esgmated to be AU Earth exits CHZ by 7 Gyr

19 Planetary Effects Lessons from the Solar System InsolaGon Atmospheres Greenhouse gases Non- equilibrium atmospheres MagneGc fields

20 Levels of the Atmosphere Troposphere: temperature falls with height Heated from below: Unstable to convecgon Stratosphere: temperature rises with height Heated in- situ by solar UV Exosphere: essengally the vacuum of space Heated by X- rays Includes the ionosphere

21 Greenhouse Effect The blackbody is the most efficient radiator possible The Earth is not exactly a blackbody It must heat up to compensate Greenhouse gases include carbon dioxide (CO 2 ) Methane (CH 4 ) water vapor (H 2 O) nitrous oxides (NO x ) chlorofluorocarbons, These all absorb infrared light.

22 Molecular Spectroscopy Molecules have more degrees of freedom: VibraGons: ω~ (k/m eff ) k: spring constant, m eff : reduced mass E = (ν+½)hω/2π typical λ 2-20 µm (IR) RotaGon: E = (h/2π) 2 /2I J(J+1) I: moment of inerga, J: 0,1,2, Typical wavelengths: sum- mm to mm 1 ev = 1.6 x erg; λν=c; E = hν

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25 EvoluGon of the Terrestrial Amosphere Oceans had formed and stabilized Atmosphere had stabilized

26 Evidence for Biota Non- equilibrium chemistry as biosignatures O 2 highly reacgve O 3 Photolysis (UV dissociagon of O 2 ) H 2 O OH + O; O+O 2 - > O 3 CH 4 (terrestrial planets) CH 4 + OH CH 3 + H 2 O τ ~ 10 years at Earth; 300 at Mars NO x τ ~ 100 years

27 Plates Tectonics and Habitability SubducGon removes carbonates (XCO 3 ) into mantle Otherwise greenhouse CO 2 accumulates Vulcanism ejects gas, including H 2 O ReconsGtutes atmosphere Vulcanism provides new land and raises mountains Counters weathering/erosion

28 CO 2 Cycle

29 Oxygen Cycle Kaltenegger 2017, ARAA, 55,433

30 Temperature Cycle

31 What Planets are Tectonically AcGve? All planets have internal heat. ConvecGon depends on the heat gradient (T core ) The heat content is proporgonal to the volume (R 3 ) Heat loss is proporgonal to the surface area (R 2 ) Planets stay warm for a Gme τ R 3 /R 2, so τ R Big planets (like Earth) are acgve

32 Another Advantage of a Tectonically AcGve Planet Molten iron cores are convecgve, and generate a magne2c field through dynamo acgon (much like the Sun). MagneGc Fields divert charged pargcles.

33 . When Solar EnergeGc ParGcles Reach Earth

34 Venus Distance from Sun: 0.72 AU Insolation: 1.9 SC Orbital Period: 224 days Eccentricity: Rotation Period: 243 days Radius: 0.95 R Mass: 0.82 M Density: 5.3 gm/cm 3 Albedo: 0.75 Moons: 0

35 Venus A twin of the Earth? 0.81 M 0.95 R ρ = 5.25 gm/cm 3 Equilibrium temperature: 290K Surface temperature: 740K Surface pressure: 90 bars CO 2 atmosphere with H 2 SO 4 clouds No congnental plates

36 Venus Slow RotaGon Tectonics: More volcanos than Earth 167 volcanos over 100 km wide Less subduction Ongoing vulcanism: Variable SO 2 concentrations Lightning. Associated with volcanic ash?

37 Venus: What Went Wrong? Venus and Earth have similar masses Venus and Earth probably outgassed similar amounts of CO 2 Venus has no magnegc field Venus is closer to the Sun, and has twice the insolagon (offset by higher albedo) A runaway greenhouse: H 2 O evaporates; re- enforces greenhouse H 2 O dissociates from UV radiagon; removed by solar wind CO 2 can t dissolve in oceans carbonate rocks do not form H 2 O evaporates from crust plate tectonics slows carbon is not sequestered in mantle

38 Mars Distance from Sun: 1.52 AU Insolation: 0.43 SC Orbital Period: 1.88 years Eccentricity: 0.09 Rotation Period: 1.03 days Radius: 0.53 R Mass: 0.11 M Density: 3.9 gm/cm 3 Albedo: 0.16 Moons: 2

39 Mars Old, cold, and dead RotaGon period 23 h 37 m as Earth Radius ½ Earth Cooling Gmescale ~ R, so Mars cooled faster Mars does have a liquid interior, but possibly not a solid inner core (Science, 316,1323). It may not be convecgve.

40 Maven at Mars 65% of Ar has been lost Current atmospheric loss rate: 100 gm/s Loss due to UV ionizagon & entrainment in the solar wind

41 Mars vs. Earth

42 Other Stars Stellar Luminosity On main sequence, Luminosity ~ M 3 On lower main sequence, L~M 4.5 T = (( [1- a] L)/ (σπd 2 )) ¼ (T increases as L ¼ ) Stellar LifeGme τ ~ M/L τ~ M - 2 (upper MS); τ~m (lower MS)

43 Tidal Locking

44 Planets of M stars Tidal locking affects atmospheric dynamics Thick atmosphere è uniform T (like Venus) Atmospheric collapse? Tidal locking è Loss of magnegc fields Stellar wind stripping of atmosphere Enhanced ionizing flux for long periods Habitable? maybe Earth- like? No

45 Planet of M7 Star: Atmospheric Temperature Profile Güdel et al. PP VI, Fig 7

46 Tidally- Locked Planet For a transparent atmosphere: Substellar point: hot, high pressure Terminator: gradient (pressure- driven) winds AnG- stellar point: cold. PrecipitaGon?

47 Flares from Stars Solar flares: Biggest: total luminosity ~ erg (~0.1L ) dn/de = E-2.3+/- 0.3

48 Flares from Stars Stellar flares: Biggest: up to ~10 37 erg Energy distribution ~ solar Normal M star flares outshine continuum Chang et al. 2015, ApJ, 814, 35

49 Stellar Winds Dong et al., 2017, ApJL, 837 Compute atmospheric escape losses for Proxima Centauri b ProxCen b: a: 0.05 au M sini = 1.27 M R > 1.1 R (Anglada- Escudé et al. 2016, Nature, 536, 437) Conclusions : unmagnegzed: escape rates 100 x terrestrial magnegzed: rates sgll exceed terrestrial

50 Effects of Evolving Stellar Luminosity EffecGve edges of habitable zone move out from star on main sequence.

51 EvoluGon of the Habitable Zone Kaltenegger 2017, ARAA, 55, 433

52 Changing SED Güdel et al. PP VI, Fig 5

53 Changing SED Güdel et al. PP VI, Fig 6

54 Kaltenegger 2017, ARAA, 55,433, Fig 12

55 Atmospheric Feedback As planet with water heats up EvaporaGon increases Cloud cover increases Albedo increases Greenhouse increases Balance unknown

56 The GalacGc Habitable Zone Reference: Lineweaver, C.H., Fenner, Y. & Gibson, B.K Science, 303, 59 (2004)

57 I: Metals

58 Metals

59 II: Danger (nearby SNe)

60 The Habitable Zone

61 The Habitable Zone for Complex Life

62 Spectra of Planets: Biosignatures

63 Exoplanetary Biosignatures

64 Transit Spectra

65 Other Habitats Exomoons Subsurface habitats Europa Enceladus

66 SETI