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Characterizing exoplanets’ atmospheres and surfaces

Characterizing exoplanets’ atmospheres and surfaces. Thérèse Encrenaz LESIA, Observatoire de Paris Pathways Toward Habitable Planets Barcelona, 14-18 September 2009. Outline. The planetary zoo Rocky Exoplanets (warm) Spectral variations with spectral type, R H , abundances

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Characterizing exoplanets’ atmospheres and surfaces

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  1. Characterizing exoplanets’ atmospheres and surfaces Thérèse Encrenaz LESIA, Observatoire de Paris Pathways Toward Habitable Planets Barcelona, 14-18 September 2009

  2. Outline • The planetary zoo • Rocky Exoplanets (warm) • Spectral variations with spectral type, RH, abundances • Atmosphere: constraints on resolving power • Surface: mineralogy, Red Vegetation Edge • Icy Exoplanets (cold) • Atmosphere: constraint on R • Surface: ices • Giant Exoplanets (from hot to very cold) • Atmosphere: importance of thermal profile, constraint on R • Conclusions

  3. Spectroscopy of an exoplanet • Reflected starlight component (UV, visible, near-IR) • Albedo is about 0.3 for most of solar-system planets • Absorption lines or bands in front of stellar blackbody • Thermal component (IR, submm & mm) • Mostly depends upon the temperature of the emitting region • Emission lines in the stratosphere, absorption lines in the troposphere (function of T(P)) • Fluorescence emission (UV, visible, near-IR) • Emission lines in the upper atmospheres (H, H2, N2, radicals) • The IR range is best suited for probing exoplanets’ neutral atmospheres

  4. The Solar System: A planetary zoo • Planets with an atmosphere • Rocky planets (warm) • Mars-type (CO2, N2 + H2O) No stratosphere • Earth-type (N2, O2 + H2O) Stratosphere (O3)   • Icy planets (cold) • Titan-type (N2, CH4 + CO) Stratosphere (hydrocarbons, nitriles) • Giant planets (cold to very cold) • Jupiter-type (H2, CH4, NH3 +H2O) Stratosphere (hydrocarbons) • Neptune-type (H2, CH4) «  • Bare planets • Mercury/asteroid-type (refractories) • TNO-type (ices)

  5. What kind of exoplanet can we expect? [F*/D2](1-a) = 4  Te4 Te (K) 1200 850 460 220 120 50 Stellar distance (AU) 0.05 0.1 0.3 1.5 5.0 20.0 (solar-type star) Small Exoplanet < ROCKY PLANETS > <ICY PLANETS > (WARM) (COLD) (0.01 - 10 ME) No atmosphere Atmosphere Atmosphere (Mercury-type) N2, CO2, CO, H2 N2, CH4(+CO) (Mars-Venus type) hydrocarbons, nitriles if O2 -> O3 (Earth-type) (Titan-type) STRATOSPHERE STRATOSPHERE Giant Exoplanet<PEGASIDES>< GASEOUS GIANTS > <ICY GIANTS> (HOT) (WARM) (COLD) (10 - 1000 ME) Atmosphere Atmosphere Atmosphere H2,CO,N2,H2O H2,CH4,NH3,H2O H2,CH4 hydrocarbons hydrocarbons (Jupiter-type) (Neptune-type) STRATOSPHERE STRATOSPHERE

  6. Variations of asterocentric distances with the stellar type Te (K) 1200 850 460 273 220 120 50 Stellar type A 0.15 0.3 0.9 3.0 4.5 15.0 60.0 (T=10000 K) F 0.08 0.16 0.5 1.6 2.4 8.0 32.0 (T=7000 K) G 0.05 0.1 0.3 1.0 1.5 5.0 20.0 (T=5700 K) K 0.04 0.12 0.4 0.6 2.0 8.0 (T=4200 K) M 0.04 0.14 0.2 0.4 1.4 (T=3200 K) HZ

  7. However, this is not so simple! Why? • Other parameters are involved: • Albedo -> effect on Te • Rotation period -> effect on Te • Phase-locked planets -> strong day/night contrasts • Possible greenhouse effect -> may increase Ts vs Te • Earth: 15 K; Venus: over 200 K • Obliquity • Atmospheric dynamics -> may change day/night contrasts • Magnetic field -> may prevent atmospheric escape • Migration is possible!

  8. Rocky Planets The IR spectrum of Mars (ISO-SWS) Ps = 6 mb CO2 CO2 H2O CO Hydrated silicates CO2 CO2 Spectral signatures: CO2, H2O, CO (+ traces H2O2, CH4) Lellouch et al., 2000

  9. Variation of a Mars-type spectrum as a function of the stellar type (D = 1 UA) Te (K) 476 346 273 174 Stellar Type A (10000 K) F (7000 K) G (5700 K) K (4200 K)

  10. Variation of a Mars-type spectrum as a function of the asterocentric distance D (solar-type star) D = 0.07, 0.1, 0.3, 1.0 UA Te = 1000, 863, 496, 273 K NB: For small D, the reflected component dominates -> Atmospheric signatures mostly in absorption

  11. Variation with atmospheric composition: H2O-dominated (Earth-like) spectrum (above clouds) H2O H2O H2O ice CO2 H2O CO2 Pcl = 10 mb

  12. The infrared spectrum of the Earth as seen by the NIMS instrument aboard Galileo (Earth flyby, December 1990) Drossart et al., 1993

  13. The thermal spectrum of telluric planets Venus Earth Mars Hanel et al., 1992

  14. Thermal spectra of rocky planets Earth R=70,10,5 Earth Mars Venus Resolving power required : CO2= 3 mm R = 3 O3 = 1 mm R = 10 CH4 = 0.15mm R = 50

  15. Solid signatures in rocky planets Reflected spectrum:H2O ice 1.25, 1.5, 2.0 m Silicates 1.0, 2.0 m (broad) Ferric oxides 1.0 m Carbonates 2.35, 2.5 m Hydrated silicates 3.0-3.5 mm (broad) Thermal spectrum: Mid-latitudes Tsurf > Tatm silicates Polar cap Tsurf < Tatm H2O ice Silicates: 1000 - 1200 cm-1(broad) Water ice: 700 - 900 cm-1 (broad) Hanel et al., 1992

  16. The Red Vegetation Edge (Earth spectrum) Seager et al. 2005

  17. RVE : Earthshine observations Problems: -partial coverage of the vegetation -clouds (20-30% of the disk) Seager et al. 2005

  18. The reflected spectrum of a CH4-dominated planet (icy or giant) Larson, 1980

  19. The atmosphere of an icy planet: The thermal component Titan - SWS: CH4, hydrocarbons, nitriles C2H6 HCN, C2H2 CH4 Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

  20. Solid signatures on icy planets H2O ice H2O, CH4, CO, N2 (Ganymede) (Pluto)

  21. The atmosphere of two gaseous giants: The thermal component Jupiter & Saturn - ISO-SWS Jupiter Saturn CH3D, PH3 NH3 C2H6 PH3 CH4 NB: Jupiter and Saturn are VERY different!

  22. Jupiter -SWS The 6-12-mm range: CH4, CH3D, C2H6, NH3, PH3 Resolving power required: - for NH3 detection: R > 100 - for CH4 detection: R > 150 -for C2H6 detection: R > 20

  23. The atmosphere of an icy giant Neptune - SWS The 2-18-m range: CH4, CH3D, C2H2, C2H6 CH4 C2H6 C2H2 Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

  24. In summary… • The diversity in solar-system bodies opens the same possibilities for exoplanets • A resolving power higher than 10 is required for the identification of major gaseous and solid signatures • In the thermal range, hydrocarbons (C2H2, C2H6) are easier to detect than methane • Knowing the thermal structure is essential for interpreting thermal spectra • No stratosphere expected for Rocky Exoplanets (N2, CO2, H2O) except if O2 is present • A stratosphere is expected for Icy Exoplanets (N2, CH4) and Giant Exoplanets (H2, CH4,…)

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