Heather Knutson
Division of Geological and Planetary Sciences
California Institute of Technology
Artist’s impression of super-Earth Gl
667Cc (Image credit ESO/L. Calcada)

Everything* We Know About Exoplanet Atmospheres
Comes from Observations of Eclipsing Systems
The Kepler Telescope
(2009-2013) searched for new transiting planets in the constellation Cygnus
*Almost everything
– see talk by
Daniel Apai

Kepler Tells Us That Super-Earths are the
Most Common Kind of Planet
Petigura et al. (2013)
What are super-
Earths made of?

Rocky planets form in situ
Ice line
Water worlds form far out and migrate in.
Premise: small planets grow by accreting solids, so bulk composition reflects that of the solids in the disk at the formation location.

Figure courtesy Leslie Rogers
Super-Earths
From radial velocity or transit timing measurements

The presence of thick atmospheres makes it difficult to determine bulk compositions from mass and radius alone.
Rocky core
Dragomir et al. (2013)
Water-rich envelope
H/He envelope
Good targets for atmosphere studies
Stars too faint
Water world (migrated from beyond ice line) or rocky with a H/He envelope (in situ formation)?

Transmission Spectroscopy as a Probe of
Atmospheric Composition
Star
Planet
Atmosphere
Wavelength-dependent transit depth tells us about composition of planet’s atmosphere.
Requires a relative precision of 10-100 ppm over several hours
Time From Center of Transit [d]
Hubble STIS transits of HD 209458b from 290-1030 nm (Knutson et al. 2007)

Constraining Atmospheric Mean Molecular
Weight With Transmission Spectroscopy
Compositions: solar
30x solar
50x solar
H
2
0 (steam)
50/50 H
2
0, CO
2
CO
2
(Venus)
Star
Planet
Atmosphere
Scale Height
H
 kT g

Large scale height
Small scale height
Miller-Ricci & Fortney (2010)


Star
Planet
Atmosphere
WFC3 data
High altitude cloud layers can block absorption from the planet’s atmosphere.
Berta et al. (2012), Kreidberg et al. (2014)

Condensate clouds like the Earth?
Photochemical hazes like Titan?
Zinc sulfide or potassium chloride (Morley et al. 2013)
Photochemistry converts methane to “soot” (long hydrocarbon chains)

Solar metallicity model
Pure water model
Solar metallicity, 0.1 mbar clouds
HD 97658b, Knutson et al. (2014b)
New frontiers for HST: Precision of 20 ppm with two transits vs 30 ppm with 12 transits for GJ 1214b. Enabled by new scanning mode .

Absorption During
Transit (%):
10 R p
R
*
2
Habitable planets are small and cold

 kT p
 g


Signal is largest for
M dwarf primaries mean molecular weight planets transiting bright, nearby M dwarfs. K2
(now) and TESS (2017) will change this.

Kepler-138 (Kipping et al. 2014) :
Compact system of three super-Earths with periods of 10, 13, and 23 days
Simulated WFC3 Transmission
Spectrum of Kepler-138d
GO 13665, PI Benneke (6 transits)
Outer planet (d):
1.0 M
Earth
1.6 R
Earth
0.5 R
Sun
K=10
Rocky core
Hydrogen-rich or water-rich atmosphere?
Water-rich envelope
H/He envelope
Kepler-138d might be cool enough to host water clouds .

Simulated HST Transmission Spectrum
8 transits
EPIC 201912552b
(Montet et al. 2015) :
2.3 R
Earth super-
Earth with a 33 day orbital period
50% rock, 50% water core
0.4 R
Sun
K=9 primary
Cool enough to have methane and ammonia. Can measure C/N ratio of planet’s atmosphere using WFC3 alone.

K2 (now) and TESS (2017) will identify all of the planets transiting bright, nearby M dwarfs.
The James Webb Space Telescope
(2018) will characterize their atmospheres in unprecedented detail, including potentially habitable super-Earths.

Gas Giants
Super-Earths Implications for habitability?
Cloud compositions?
free planets to
?
study?
Terrestrial
Outgassed: melting of solids releases gas into atmosphere.
Primordial: hydrogenrich gas accreted directly from protoplanetary disk.
How does atmospheric composition vary with planet mass?
to scale.

Conclusion: What Can We Learn From
Atmosphere Studies of Small Planets?
Neptunes
Super-Earths
Cloud compositions?
Can we pick cloudfree planets to study?
Terrestrial
Habitable?
Primordial or outgassed atmosphere? Loss of light elements?
Breaking degeneracies in interior composition. Formation in situ or beyond ice line?
Planets drawn to scale.

Possible Condensate Cloud Species for GJ 1214b
50x solar models from
Morley et al. (2013)
Cloudy atmosphere model
Cloud-free atmosphere model

(Relatively) Clear Skies and Water
Vapor on Warm Neptune HAT-P-11b
Hydrogen rich (solar metallicity) model
Hydrogen rich + clouds
200x solar model
Pure water model
Data
1.2
1.3
1.4
Wavelength ( μm)
1.5
Detection of water absorption places upper limit on atmospheric metallicity (Fraine et al. 2014)
1.6

Metal-poor model (CH
4
/CO = 100)
Metal-rich model (CH
4
/CO < 0.001)
Can explain data if atmosphere has a metallicity of 300-2000x solar (Moses et al. 2013).
GJ 436b
HD 189733b is a
Models from Madhusudhan & with detectible features in its

Exoplanet Meteorology: Optical Wavelengths
Provide New Clues to Cloud Compositions
Transmission spectroscopy can constrain location and particle sizes for cloud layers.
Measurements of secondary eclipse can constrain cloud albedos.
Star
Planet
Atmosphere
Wavelength dependence of scattering scales with particle size.
Reflected light “color” constrains composition of cloud particles.
We don’t understand clouds well enough to predict in advance which super-Earths will have cloud-free atmospheres.

Optical Spectroscopy Can Break Degeneracy Between
Clouds and H-Poor Atmosphere for Super-Earths
HD 97658b H-rich + clouds
Cloud-free water
Simulated HST data
(GO 13665, PIs Benneke
& Crossfield)
Models from Benneke et al. (2012, 2013)

Proof of Concept for Albedo Measurements:
Reflected Light Spectrum for a Hot Jupiter
HD 189733b is blue!
Measure secondary eclipse at optical wavelengths for hot Jupiter
HD 189733 with HST/STIS.
Albedo is largest at short wavelengths (Evans et al. 2013).
This may be related to the presence of clouds (silicate?) detected via transmission spectroscopy.

Star
Planet
Atmosphere
A Hubble Space Telescope Transmission
Spectrum for Warm Neptune GJ 436b
Hydrogen-rich (solar) atmosphere model
Hydrogen-poor (1900x solar) model
Cloudy hydrogen rich (solar) atmosphere
Data
Knutson et al. (2014); also see Ehrenreich et al. (2014) for observations of GJ 3470b