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The Featureless Transmission Spectra of Two Super-puff Planets

Libby-Roberts, Jessica E. and Berta-Thompson, Zachory K. and Désert, Jean-Michel and Masuda, Kento and Morley, Caroline V. and Lopez, Eric D. and Deck, Katherine M. and Fabrycky, Daniel and Fortney, Jonathan J. and Line, Michael R. and Sanchis-Ojeda, Roberto and Winn, Joshua N. (2020) The Featureless Transmission Spectra of Two Super-puff Planets. Astronomical Journal, 159 (2). Art. No. 57. ISSN 1538-3881.

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The Kepler mission revealed a class of planets known as "super-puffs," with masses only a few times larger than Earth's but radii larger than Neptune, giving them very low mean densities. All three of the known planets orbiting the young solar-type star Kepler 51 are super-puffs. The Kepler 51 system thereby provides an opportunity for a comparative study of the structures and atmospheres of this mysterious class of planets, which may provide clues about their formation and evolution. We observed two transits each of Kepler 51b and 51d with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. Combining new WFC3 transit times with reanalyzed Kepler data and updated stellar parameters, we confirmed that all three planets have densities lower than 0.1 g cm⁻³. We measured the WFC3 transmission spectra to be featureless between 1.15 and 1.63 μm, ruling out any variations greater than 0.6 scale heights (assuming a H/He-dominated atmosphere), thus showing no significant water absorption features. We interpreted the flat spectra as the result of a high-altitude aerosol layer (pressure <3 mbar) on each planet. Adding this new result to the collection of flat spectra that have been observed for other sub-Neptune planets, we find support for one of the two hypotheses introduced by Crossfield & Kreidberg, that planets with cooler equilibrium temperatures have more high-altitude aerosols. We strongly disfavor their other hypothesis that the H/He mass fraction drives the appearance of large-amplitude transmission features.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Libby-Roberts, Jessica E.0000-0002-2990-7613
Berta-Thompson, Zachory K.0000-0002-3321-4924
Désert, Jean-Michel0000-0002-0875-8401
Masuda, Kento0000-0003-1298-9699
Morley, Caroline V.0000-0002-4404-0456
Fabrycky, Daniel0000-0003-3750-0183
Fortney, Jonathan J.0000-0002-9843-4354
Line, Michael R.0000-0002-2338-476X
Sanchis-Ojeda, Roberto0000-0002-6193-972X
Winn, Joshua N.0000-0002-4265-047X
Additional Information:© 2020. The American Astronomical Society. Received 2019 June 21; revised 2019 November 20; accepted 2019 November 21; published 2020 January 20. We are extremely grateful to Tricia Royle and the staff at the Space Telescope Science Institute for their heroic efforts in scheduling these HST observations. We thank Catherine Huitson discussing WFC3 instrumental systematics, Yifan Zhou and Daniel Apai for explaining their WFC3 charge trap model, Matteo Broggi and Eve Lee for their helpful Kepler 51 discussions, and Lile Wang and Fei Dai for sharing their dusty-outflow manuscript before publication. We acknowledge both the MIT Exoplanet Tea and the CU Boulder Exoplanet Coffee for helpful discussions that motivated and improved this work. We thank Sarah Millholland for sharing her manuscript on obliquity tides before publications. We acknowledge the anonymous reviewer whose suggestions and attention to detail both improved and strengthened this work. The work was based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Support for this work was provided by NASA through grant number HST-GO-14218.010-A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. J.M.D. acknowledges that the research leading to these results has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 679633; Exo-Atmos). J.M.D acknowledges support by the Amsterdam Academic Alliance (AAA) Program. This material is based upon work supported by the National Science Foundation (NSF) under grant No. AST-1413663. Support for this work was provided by NASA through an award issued by JPL/Caltech (P90092). Work by K.M. was performed under contract with the California Institute of Technology (Caltech)/Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. Facilities: HST (WFC3) - Hubble Space Telescope satellite, Kepler - The Kepler Mission. Software: astropy (Astropy Collaboration et al. 2013, 2018), photutils (Bradley et al. 2019), emcee (Foreman-Mackey et al. 2013), batman (Kreidberg 2015), isochrones (Morton 2015), ldtk (Parviainen & Aigrain 2015).
Funding AgencyGrant Number
NASANAS 5-26555
European Research Council (ERC)679633
Amsterdam Academic Alliance (AAA)UNSPECIFIED
Subject Keywords:Atmospheric composition ; Exoplanet atmospheres ; Exoplanet structure ; Exoplanet evolution ; Transit timing variation method ; Transits ; Hubble Space Telescope ; Exoplanets
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Atmospheric composition (2120); Exoplanet atmospheres (487); Exoplanet structure (495); Exoplanet evolution (491); Transit timing variation method (1710); Transits (1711); Hubble Space Telescope (761); Exoplanets (498
Record Number:CaltechAUTHORS:20200409-115739879
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Official Citation:Jessica E. Libby-Roberts et al 2020 AJ 159 57
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102458
Deposited By: George Porter
Deposited On:09 Apr 2020 20:11
Last Modified:09 Apr 2020 20:11

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