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An emerging field:Molecules in Extrasolar Planets

Jean Schneider - Paris Observatory

● Concepts and Methods

● First results

● Future perspectives

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Concepts and Methods (1/13)

● What is a « planet »?

● Interest of molecules for exoplanetology

● How to detect molecules on a planet

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Concepts and Methods (2/13)

● What is a « planet »?

1) A self-graviting object with no internal source of nuclear energy

==> M < 13 Jupiter mass (deuterium burning limit)● Substellar companion to a parent star● « Free floating » planet

2) An object formed in a circumstellar disc

Approaches 1) and 2) coïncide approximately

● Type of planets:● Giant, gazeous

● « Telluric », solid or liquid

Present findings: ~ 170 giant planetary companions

1 free floating giant planet

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Concepts and Methods (3/13)

● Interest for molecules

– Diagnostics of planet physical parameters

– Diversity in chemical composition

– Exobiology

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Concepts and Methods (4/13)

● Diagnostics of physical parameters● Mass● Radius <--> ● Temperature (<--> mass, age)● Albedo, opacities <--> Reflection and thermal spectra

g ~ M R2

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Concepts and Methods (5/13)

● An important difference with Solar System planets

Many planets are very closeto their parent star (up to 0.02 AU)

Consequence:

They are very strongly irradiatedby their parent star

==>

– Strong photochemistry

– High planet temperature (up to 2000 K)

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Concepts and Methods (6/13)

● Atmosphere models for giant planets.

Similar to stellar atmosphere models + illumination by parent star

Input parameters:

– Effective temperature

– Gravity (mass from age)

– Input luminosity L

– Abundances of chemical species

– Line transition rates (up to 700 million molecular bands)

– Complication: dust (SiO)

Output:

Planet spectra,

Teff

g GM R2

i ni nTot

Fpl refl F* <---> planet AlbedoFpl therm F*

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Concepts and Methods (7/13)

CO, H2O, and CH4 Opacities

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Concepts and Methods (8/13)

● Examples of spectra

KH20

H20

CH4H20 NH3

T = 1000 Klog g = 4.5Solar composition

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Concepts and Methods (9/13)

● Examples of spectra as a function of stellar illumination (no dust)

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Concepts and Methods (10/13)

● Interest for molecules

Diversity in chemical composition==> eq. of state:

– water planets

– carbon planets (< 10 Earth mass)

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Concepts and Methods (11/13)

● Interest for molecules

– Exobiology:● Molecules in the atmopshere: H20, oxygen, ozone, CH4

H2O: believed to be a pre-requisit for « life » - liquid ==> T~300K

Oxygen (ozone): believed to be only a by-product of some

photosynthesis:

CH4: by-product of fermentation4 h 2 H2O X > 4 H O2 X X complex organic mol.

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Concepts and Methods (12/13)

● Interest for molecules

– Exobiology:● Molecules at the planet surface:

chlorophyll (vegetation, plancton)

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Concepts and Methods (13/13)

● How to detect molecules on a planet:

– Transmission spectroscopy

– Direct detection:

Direct image Secondary eclipse

Planet X

Fro

m S

tar

To

Obs

erve

r

Parent star

Planet

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Very first results (1/4)

● GQ Lup

Depth and shape of CO lines

==> log g = 2.52

Planckian spectrum

==> R = 2 Rjup

==> M = 2 MJup

H2O CO

100 AU

g GM R2

L ~ R2 T4

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Very first results (2/4)

● HD 209458 b – transmission spectra ==> First detection of a molecule on an exoplanet (Charbonneau et al. 2000)

Na I

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Very first results (3/4)

● HD 209458 b – transmission spectra

Ly

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Very first results (4/4)

● Test of detectability of « chlorophyll » on an Earth-like exoplanet

Veget. Red edge

Arnold et al 2002

Globl spectrum of Earthseen as a single point

Rayleigh scat.

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Future perspectives

Suspendend to the imaging capabilities of future instruments

Problems:– Planets imbedded in the halo of their parent star due to:

● Atmospheric turbulence on the ground● Stellar diffraction peak

– Planet/star contrast very low:

Solutions:

– Suppress atmospheric turbulence with « Extreme » adaptive optics

– Suppress stellar diffraction peak by some coronagraphic mask

– Increase telescope size --> Interferometers

D

10 6 10 9

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Future perspectives

● Reflection spectra of hot giant planets:

– VLT-Planet Finder (coronagraphic camera at the VLT)– Pegase ? (1 km interferometric demonstrator in space – 2012 - CNES)

● Reflection spectra of cold giant planets– Space coronagraphic telescopes: TPF-C (3 m X 7 m – NASA 2015)– Ground-based Extremely Large Telescopes (>30 m)

● Thermal spectra of giant planets– VLT-Planet Finder– Pegase ?

● Search for molecules of life on terrestrial planets:– Oxygen, ozone, CH4, H2O: TPF-C , Darwin/TPF-I (200 m IR interferom. -

2017) – Chlorophyll: TPF-C

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Future perspectives

● Cartography of molecular distribution

– Imaging of transiting planets (baseline 10 km at 1µ)

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Future perspectives

● Cartography of molecular distribution

– Multi-pixel spectral cartography (multi-thousand km interferometers 2025+)

Everything about exoplanets: www.obspm.fr/planets