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2013 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2012 – AUG 2013

Exploring the Chemical Composition of Hot Exoplanets With the Hubble Space Telescope

Project Summary

We have used the new Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope (HST) to observe exoplanet transit and eclipse measurements for a number of highly irradiated, Jupiter-mass planets, with a focus on confirming which planets exhibit water absorption in transit and/or eclipse and measuring the characteristic brightness temperature at these wavelengths. Measurements of molecular absorption in the atmospheres of these planets offer the chance to explore several outstanding questions regarding the atmospheric structure and composition of hot Jupiters, including the possibility of bulk compositional variations between planets and the presence or absence of a stratospheric temperature inversion.

4 Institutions
3 Teams
2 Publications
1 Field Site
Field Sites

Project Progress

In this reporting period, we have focused on analyzing three exoplanets in primary transit (WASP-12b, WASP-17b, and WASP-19b) for signs of water vapor and other molecules in their atmospheres. We analyzed the data for a single primary transit of each planet with a new methodology that allows us to correct for channel– or wavelength–dependent instru-mental effects by utilizing the band-integrated time series and measurements of the drift of the spectra on the detector over time. We have developed a data reduction pipeline and analysis methodology that produces spectra with precision close to the theoretical noise floor for individual spectral bins. The final results are broadly consistent with the presence of absorption at 1.4 µm, most likely due to water, but the amplitude of the absorption was less than that expected based on previous observations with Spitzer – possibly due to hazes absorbing in the Near InfraRed (NIR) or to non-solar compositions.

Transit absorption depths at different wavelengths for two planets (WASP-17b and WASP-19b) examined by Mandell et al. 2013, with models based on the framework of Burrows et al. (left) and Madhusudhan et al. (right). Standard models with a broad H2O feature provide a good fit for WASP-17b, but a haze is required to match the flat spectrum at short wavelengths. For WASP-19b the water-rich models do not fit well beyond 1.45 μm, while a carbon-rich model provides a statistically better fit than the oxygen-rich model. This pioneering data set shows the power of HST to distinguish between different atmospheric compositions, provided sufficient S/N is achieved.

For WASP-17b the water signal was clear, but for the two other planets the degeneracy of models with different compositions and temperature structures combined with the low amplitude of any features in the data precluded our ability to place unambiguous constraints on the atmospheric composition. More sensitive observations with WFC3 and/or a comprehensive multi-wavelength analysis are planned. A paper detailing our efforts and results has been published (Mandell et al. 2013), and some of our data analysis strategies were also incorporated into a related publication (Deming et al. 2013).

    Avi Mandell
    Project Investigator

    Adam Burrows

    Drake Deming

    Korey Haynes

    Nikku Madhusudhan

    Evan Sinukoff

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 1.2
    Indirect and direct astronomical observations of extrasolar habitable planets.

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems