Exoplanet Atmospheres in the Super-Earth Era

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) Why Study Small Planet Atmospheres?

Records information about formation history.

Breaks degeneracies in interior compositions.

Figure courtesy Leslie Rogers How Do Planets Acquire Their Atmospheres? Gas Giants Super-Earths

Terrestrial ? Outgassed: melting of solids releases gas into What happens at atmosphere. Primordial: hydrogen- intermediate masses? rich gas accreted directly from protoplanetary disk. Depletion of lightPlanets elements drawn to scale. Breaking Degeneracies in Super-Earth Compositions 5 Caveat: the presence of thick atmospheres makes it difficult to 4 uniquely constrain bulk compositions from mass and radius alone. ] 3 Rocky core Earth GJ 1214b 100 % water HD 97658b 50% water

rock 55 Cnc e

Radius [R Radius 2 n maximum iron fractio Water-rich H/He envelope envelope 1 Good targets for atmosphere studies Water world (migrated from Stars too faint Dragomir et al. (2013) beyond ice line) or rocky 0 with a H/He envelope (in 5 10 15 situ formation)? Mass [MEarth] Transiting Planets as a Tool For Studying Planetary Atmospheres Hubble Space Telescope (Visible + near IR) Secondary Eclipse Temperature, composition of planet’s atmosphere

Transit Composition of Spitzer Space planet’s atmosphere Telescope (IR)

If the planet is eclipsing, we can study the detailed properties of its atmosphere. Transmission Spectroscopy as a Probe of Atmospheric Composition

Star

Planet

Atmosphere

Wavelength-dependent transit depth tells us about composition of planet’s atmosphere.

Hubble STIS transits of HD 209458b from 290-1030 nm (Knutson et al. 2007) Constraining Atmospheric Hydrogen Fraction With Transmission Spectroscopy

Compositions: solar 30x solar 50x solar

H20 (steam) 50/50 H20, CO2 CO2 (Venus)

Star Scale Height kT Planet H = Large scale Small scale gµ height height Atmosphere

Miller-Ricci & Fortney (2010) € A Hubble Space Telescope Transmission Spectrum for Warm Neptune GJ 436b Hydrogen-rich (solar) atmosphere model 7400 Hydrogen-poor (1900x solar) model Cloudy hydrogen rich (solar) atmosphere Data 7200

7000 Transit Depth (ppm) 6800 Star

Planet 1.0 1.2 1.4 1.6 1.8 Wavelength (micron) Atmosphere Knutson et al. (2014); also see Ehrenreich et al. (2014) for observations of GJ 3470b What Might Form Clouds on GJ 436b?

Condensate clouds like Photochemical hazes the Earth? like Titan?

Zinc sulfide or potassium Photochemistry converts methane chloride (Morley et al. 2013) to “soot” (long hydrocarbon chains) a 3700 3600 [ppm]

2 3500 ) s (Relatively) Clear Skies and Water 3400 /R p

R 3300 ( Vapor on Warm Neptune HAT-P-11b 3200 123 4 5

b 3650 Hydrogen rich (solar metallicity) model Transit ( Depth Hydrogen rich + clouds Data 3600

200x solar model 3550 Pure water model

[ppm] 3500 2 ) s 3450 /R p R ppm ( 3400

3350 )

3300

1.21. 21 1.3. 31 1.4. 41 . 51. 6 Wa velength [µm ] 1.5 1.6 Wavelength (µm)

Detection of water absorption places upper limit on atmospheric metallicity (Fraine et al. 2014) Next Up: Super-Earths

5 Caveat: the presence of thick atmospheres makes it difficult to 4 uniquely constrain bulk compositions from mass and radius alone. ] 3 Rocky core Earth GJ 1214b 100 % water HD 97658b 50% water

rock 55 Cnc e

Even metal-rich atmospheres would Star have been detectible, planet must have clouds regardless of composition. Could have same Planet composition as clouds on GJ 436b. Atmosphere Kreidberg et al. (2014), Berta et al. (2012) HST Observations of Super-Earth HD 97658b Solar metallicity model 1000 Pure water model Solar metallicity, 0.1 mbar clouds

950

900 Normalized Transit Depth (ppm) Knutson et al. (2014b)

1.0 1.2 1.4 1.6 1.8 Wavelength (micron) New frontiers for HST: Precision of 20 ppm with two transits vs 30 ppm with 12 transits for GJ 1214b. Exoplanet Meteorology: Optical Wavelengths Provide New Clues for Cloud Composition

Transmission spectroscopy can Measurements of secondary constrain location and particle eclipse can constrain cloud sizes for cloud layers. albedos.

Star

Planet

Atmosphere

Wavelength dependence of Reflected light “color” constrains scattering scales with particle size. composition of cloud particles. Optical Spectroscopy Can Break Degeneracy Between Clouds and H-Poor Atmosphere for Super-Earths 1150 HD 97658b H-rich + clouds Cloud-free water

million) 1100

− Simulated HST data (GO 13665, PIs Benneke

per 1050 & Crossfield) − 1000 950 900 850 Models from Benneke et al. (2012, 2013)

Transit Depth (parts Depth Transit 800 0.4 0.6 0.8 1 2 3 4 5 Wavelength [µm] Proof of Concept for Albedo Measurements: Reflected Light Spectrum for a Hot Jupiter HD 189733b is blue!

Measure secondary eclipse at Albedo is largest at short optical wavelengths for hot Jupiter wavelengths (Evans et al. 2013). HD 189733 with HST/STIS. This may be related to the presence of clouds (silicate?) detected via transmission spectroscopy. Can we use emission spectroscopy to see through clouds at infrared wavelengths?

Metal-poor model (CH /CO = 100) 4 GJ 436b Metal-rich model (CH4/CO < 0.001) Can explain data if atmosphere has a metallicity of 300-2000x solar (Moses et al. 2013).

Models from Madhusudhan & Seager (2011). Data from Stevenson et al. (2010), but also see Lanoe et al. (2014). Pushing Towards Atmosphere Studies of Terrestrial Planets

How can we make JWST into a better light- collecting bucket?

There are currently only a handful The James Webb Space Telescope of small planets transiting bright, (2018) will allow us to characterize nearby stars. K2 (now) and TESS even smaller and cooler planets (2017) will change this. (mainly around low-mass stars). Conclusion: What Can We Learn From Atmosphere Studies of Small Planets?

Neptunes Super-Earths

Cloud compositions? Terrestrial Can we pick cloud- free planets to study? Habitable?

Primordial or outgassed Breaking degeneracies in atmosphere? Loss of light interior composition. Formation elements? in situ or beyond ice line? Planets drawn to scale.