Spectroscopy of Water and Organics in
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Spectroscopy of Water and Organics in Exoplanet Atmospheres: First Detections and What the Future Holds. Avi M. Mandell NASA GSFC Collaborators :. Korey Haynes Evan Sinukoff Drake Deming Adam Burrows Nikku Madhusudhan Mark Clampin Don Lindler Natasha Batalha Heather Knutson

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Spectroscopy of Water and Organics in Exoplanet Atmospheres:

First Detections and What the Future Holds

Avi M. Mandell



Korey Haynes

Evan Sinukoff

Drake Deming

Adam Burrows


Mark Clampin

Don Lindler

Natasha Batalha

Heather Knutson

(others as well)

Henry et al. 1999

  • What is a

  • ExoplanetTransit?

How Do We Learn About the Atmospheresof Transiting Planets?

  • As starlight passes through the atmosphere of a planet, atoms and molecules absorb at different wavelengths

  • The more absorption, the deeper the depth of the transit… but absorption depends on abundance, temperature, optical depth

  • Exoplanet transits provide the opportunity to probe the absorption in a planet’s atmosphere

Planet Cross-Section

HST / WFC3 Grism Spectroscopy:

Resolving Molecular Absorption

  • Wavelength range (1.1 – 1.7 μm) samples water bands at 1.15 and 1.4 μm as well as several hydrocarbons and continuum regions on either side

  • Can answer major questions about temperature and chemistry

  • But at LOW resolution over a NARROW bandpass, degeneracies still remain!

Deming et al.Program (Cycles 18 & 19)

  • Large collaboration focused on hot giant exoplanets

    • Sample of 16 objects

    • A number of planets may have upper-atmosphere temperature inversions or high C/O ratios

  • We started with several interesting (and outlying) cases:

    • WASP-12: Very hot, first possible carbon-rich exoplanet, but results now in dispute

    • WASP-17:Ultra-low density, retrograde orbit

    • WASP-19: Shortest-period planet known (P ~ 19 hr) but no temperature inversion

    • WASP-33: Very hot and massive, orbiting an A-type star, possibly carbon-rich

List of Observed Planets

CoRoT-1 b

CoRoT-2 b

HAT-P-7 b

HAT-P-12 b

HAT-P-13 b

HD189733 b

HD209458 b

TrES-2 b

TrES-3 b

TrES-4 b

WASP-4 b

WASP-12 b

WASP-17 b

WASP-18 b

WASP-19 b

XO-1 b

WASP-33 b








Transit Spectra Analysis:

SystematicsRemoval Through Self-Calibration

We used the divide-ootmethod (Berta et al. 2012) to fit the band-integrated light curve

We subtracted the model from the raw light curve to obtain the residual systematic variation; additional trends due to spectral drift were also measured

We created a model for each wavelength bin, with a scaling parameter for each possible systematic trend in the data and an overall visit-long slope





  • PrimaryResult: Amplitude of water absorption band islower than expected (based on previous Spitzer obs.)

    • Due to either:

    • Anunexplained haze layer that increases the continuum opacity below a certain altitude

    • Less water due to non-solar abundances

(T ~ 2900K)

(T ~ 2000K)

(T ~ 2500K)

  • Primary Result: Amplitude of water absorption band is lower than expected (based on previous Spitzer obs.)

    • Due to either:

    • Anunexplained haze layer that increases the continuum opacity below a certain altitude

    • Less water due to non-solar abundances

(T ~ 2000K)

  • A

  • 2

  • Cooler planets seem to show well-defined spectral features, while hotter planets are ambiguous…


(T ~ 1800K)

(T ~ 1700K)

Eclipse Spectra Analysis:

  • We again used the divide-ootmethod to fit the band-integrated light curve,

    • WASP-33 presents additional complications due to Delta Scuti oscillations in the parent star

  • Band-integrated eclipse depths are much more uncertain than the transit measurements due to the low eclipse-to-noise ratio

    • WASP-4 is especially problematic due to very little temporal coverage during eclipse

  • WASP-4 PreliminaryResult: WFC3 data appear to match up with the thermal-inversionatmosphere model from Beerer et al. 2011; however, a blackbody seems to be an even better fit

(T ~ 2900K)

  • The spectrum seems to show a slight peak at 1.4 microns, indicative of a possible strong inversion

  • However, this model does not match the Spitzer data well

  • A blackbody with Tplan = 2200 K provides an excellent fit to all existing data

Tplan~ 2200K

(T ~ 2500K)

  • WASP-12 Preliminary Result: WFC3 data appear to support a carbon-rich model, showing no sign of the expected deep absorption band.

  • However, as noted in Crossfield et al. 2012, correcting the Spitzer data for the nearby companions leads to an isothermal interpretation

  • WASP-33 Preliminary Result: WFC3 data strongly support a model with no thermal inversion, and models that are carbon-rich fit better

  • Further modeling is required to determine whether we can break degeneracies between temperature and composition

The Future of Space-Based Characterization:

JWST (of course)

  • JWST will provide sensitivity gains of more than an order of magnitude

  • We are preparing to adapt our WFC3 analysis pipeline to JWST, based on current instrument models by M. Clampin & D. Lindler

  • For hot Jupiters, the real test lies in which instruments and filters to use in order to MOST EFFICIENTLY constrain the atmospheric parameters

JWST Wavelength Coverage & Resolution

JWST/NIRSPEC Simulations





Simulated Hot Super-Earth

(Teq ~ 500K) around an

M-star at 30 pc










(Teq ~ 300K)

around an

M-star at 20 pc








Deming et al. 2009


  • Begins with In-transit and Out-of-transit model

  • Maps onto pixel space

  • Convolves with PSF, multiplies by PRF

  • Add noise sources

    • Zodiacal and stray light

    • Flat field errors

    • Poisson and read noise

    • Spacecraft jitter and drift

14 pc, V = 15

4.5 pc

M-type host star

4 MEarth planet

25 transits

H2O & CH4

Images from Don Lindler, results from Batalha et al. (JWST White Paper)

Pre-JWST Characterization: Low-Cost NIR Spectroscopy from a Balloon?

  • Ultra-long duration (ULD) balloon platforms offer the potential for long-term, stable monitoring of transiting planets above almost all telluric contamination

    • As low as 1 - 2%of an equally-capable space mission

  • Test flight using the Wallops Arc Second Pointer (WASP) gondola system planned for September 2014

    • Use of existing and off-the-shelf parts will allow us to benchmark the current limits for stability and thermal control


  • The WFC3 instrument on HST has now been validated as a reliable platform for high-precision exoplanet transit observations

  • Observations of Hot Jupiters are revealing unexpected mysteries

    • Hazes and/or aerosols may be common, but vary with planet properties

    • Thermal emission measurements suggest blackbody emission at NIR wavelengths may be ubiquitous; unclear if this is due to thermal or compositional factors, and why it appears so uniform

  • Increased S/N and larger wavelength coverage (combining Spitzer, HST and ground) will be necessary to grapple with these questions

    • JWST will clearly change the landscape dramatically, but observing time will be precious, so we must pre-select targets for follow-up


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