An Unusual Transmission Spectrum for the Sub-Saturn KELT-11b Suggestive of a Subsolar Water Abundance

Knicole D. Colón, Laura Kreidberg, Luis Welbanks, Michael R. Line, Nikku Madhusudhan, Thomas Beatty, Patrick Tamburo, Kevin B. Stevenson, Avi Mandell, Joseph E. Rodriguez, Thomas Barclay, Eric D. Lopez, Keivan G. Stassun, Daniel Angerhausen, Jonathan J. Fortney, David J. James, Joshua Pepper, John P. Ahlers, Peter Plavchan, Supachai AwiphanCliff Kotnik, Kim K. Mcleod, Gabriel Murawski, Heena Chotani, Danny Lebrun, William Matzko, David Rea, Monica Vidaurri, Scott Webster, James K. Williams, Leafia Sheraden Cox, Nicole Tan, Emily A. Gilbert

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21 Scopus citations


We present an optical-to-infrared transmission spectrum of the inflated sub-Saturn KELT-11b measured with the Transiting Exoplanet Survey Satellite (TESS), the Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectroscopic grism, and the Spitzer Space Telescope (Spitzer) at 3.6 μm, in addition to a Spitzer 4.5 μm secondary eclipse. The precise HST transmission spectrum notably reveals a low-amplitude water feature with an unusual shape. Based on free-retrieval analyses with varying molecular abundances, we find strong evidence for water absorption. Depending on model assumptions, we also find tentative evidence for other absorbers (HCN, TiO, and AlO). The retrieved water abundance is generally ≲0.1 solar (0.001-0.7 solar over a range of model assumptions), several orders of magnitude lower than expected from planet formation models based on the solar system metallicity trend. We also consider chemical-equilibrium and self-consistent 1D radiative-convective equilibrium model fits and find that they, too, prefer low metallicities ([M/H] ≲ -2, consistent with the free-retrieval results). However, all of the retrievals should be interpreted with some caution because they either require additional absorbers that are far out of chemical equilibrium to explain the shape of the spectrum or are simply poor fits to the data. Finally, we find that the Spitzer secondary eclipse is indicative of full heat redistribution from KELT-11b's dayside to nightside, assuming a clear dayside. These potentially unusual results for KELT-11b's composition are suggestive of new challenges on the horizon for atmosphere and formation models in the face of increasingly precise measurements of exoplanet spectra.

Original languageEnglish (US)
Article number280
JournalAstronomical Journal
Issue number6
StatePublished - Dec 2020

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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