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

Characterizing exoplanets’ atmospheres and surfaces

Thérèse EncrenazLESIA, Observatoire de Paris

Pathways Toward Habitable Planets

Barcelona, 14-18 September 2009

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

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

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)

Te (K) 1200 850 460 220 120 50Stellar 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, nitrilesif 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

What kind of exoplanet can we expect? [F*/D2](1-a) = 4 Te4

Te (K) 1200 850 460 273 220 120 50

Stellar typeA 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

Variations of asterocentric distances with the stellar type

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!

Rocky PlanetsThe IR spectrum of Mars (ISO-SWS)

Spectral signatures: CO2, H2O, CO (+ traces H2O2, CH4)

H2O

CO2 CO2

COCO2

Lellouch et al., 2000

Hydrated silicates

CO2

Ps = 6 mb

Variation of a Mars-type spectrum as a function of the stellar type (D = 1 UA)

StellarTypeA(10000 K)

F(7000 K)

G(5700 K)K(4200 K)

Te (K)

476

346

273

174

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 UATe = 1000, 863, 496, 273 K

NB: For small D, the reflected component dominates-> Atmospheric signatures mostly in absorption

Variation with atmospheric composition: H2O-dominated (Earth-like) spectrum (above clouds)

H2O H2O

H2O CO2

CO2

H2Oice

Pcl = 10 mb

The infrared spectrum of the Earth as seen by the NIMS instrument aboard Galileo (Earth flyby, December 1990)

Drossart et al., 1993

The thermal spectrum of telluric planets

Venus

Earth

Mars

Hanel et al., 1992

Thermal spectra of rocky planets

Resolving power required :CO2 = 3 m R = 3O3 = 1 m R = 10CH4 = 0.15m R = 50

EarthMarsVenus

EarthR=70,10,5

Hanel et al., 1992

Solid signatures in rocky planets

Mid-latitudesTsurf > Tatm

Polar capTsurf < Tatm

Silicates: 1000 - 1200 cm-1(broad)Water ice: 700 - 900 cm-1 (broad)

Reflected spectrum: H2O ice 1.25, 1.5, 2.0 mSilicates 1.0, 2.0 m (broad)Ferric oxides 1.0 m Carbonates 2.35, 2.5 mHydrated silicates 3.0-3.5 m (broad)

Thermal spectrum:

H2O ice

silicates

The Red Vegetation Edge (Earth spectrum)

Seager et al. 2005

RVE : Earthshine observations

Seager et al. 2005

Problems:-partial coverage of the vegetation-clouds (20-30% ofthe disk)

The reflected spectrumof a CH4-dominatedplanet(icy or giant)

Larson, 1980

The atmosphere of an icy planet:The thermal component

Titan - SWS: CH4, hydrocarbons, nitriles

Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

CH4

C2H6

HCN, C2H2

Solid signatures on icy planets

H2O ice H2O, CH4, CO, N2

(Ganymede) (Pluto)

The atmosphere of two gaseous giants: The thermal component Jupiter & Saturn - ISO-SWS

CH4

CH3D, PH3 NH3

C2H6

Jupiter

Saturn

NB: Jupiter and Saturn are VERY different!

PH3

Jupiter -SWS The 6-12-m range: CH4, CH3D, C2H6, NH3, PH3

Resolving power required:- for NH3 detection: R > 100- for CH4 detection: R > 150-for C2H6 detection: R > 20

The atmosphere of an icy giant Neptune - SWS The 2-18- range: CH4, CH3D, C2H2, C2H6

Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

CH4 C2H6 C2H2

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,…)