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Evidence for the volatile-rich composition of a 1.5-Earth-radius planet

Piaulet, Caroline and Benneke, Björn and Almenara, Jose M. and Dragomir, Diana and Knutson, Heather A. and Thorngren, Daniel and Peterson, Merrin S. and Crossfield, Ian J. M. and Kempton, Eliza M.-R. and Kubyshkina, Daria and Howard, Andrew W. and Angus, Ruth and Isaacson, Howard and Weiss, Lauren M. and Beichman, Charles A. and Fortney, Jonathan J. and Fossati, Luca and Lammer, Helmut and McCullough, P. R. and Morley, Caroline V. and Wong, Ian (2022) Evidence for the volatile-rich composition of a 1.5-Earth-radius planet. Nature Astronomy . ISSN 2397-3366. doi:10.1038/s41550-022-01835-4. (In Press)

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The population of planets smaller than approximately 1.7 Earth radii (R_⊕) is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial velocity mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate 1.51 ± 0.04 R_⊕ planet Kepler-138 d (T_(eq,AB) = 0.3 ≈ 350K) combined with new radial velocity measurements of its host star obtained with the Keck/High Resolution Echelle Spectrometer. We find evidence for a volatile-rich ‘water world’ nature of Kepler-138 d, with a large fraction of its mass M_d contained in a thick volatile layer. This finding is independently supported by transit timing variations and radial velocity observations (M_d = 2.1^(+0.6)_(−0.7) M_⊕), as well as the flat optical/infrared transmission spectrum. Quantitatively, we infer a composition of 11⁺³₋₄% volatiles by mass or ~51% by volume, with a 2,000-km-deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations (<0.003 M_⊕) would have swiftly been lost on a ~10 Myr timescale. The bulk composition of Kepler-138 d therefore resembles those of the icy moons, rather than the terrestrial planets, in the Solar System. We conclude that not all super-Earths are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations. Finally, our photodynamical analysis also reveals that Kepler-138 c (with a R_c = 1.51 ± 0.04 R_⊕ and a M_c = 2.3^(+0.6)_(−0.5) M_⊕) is a slightly warmer twin of Kepler-138 d (that is, another water world in the same system) and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.

Item Type:Article
Related URLs:
URLURL TypeDescription ReadCube access
Piaulet, Caroline0000-0002-2875-917X
Benneke, Björn0000-0001-5578-1498
Dragomir, Diana0000-0003-2313-467X
Knutson, Heather A.0000-0002-0822-3095
Thorngren, Daniel0000-0002-5113-8558
Peterson, Merrin S.0000-0002-0547-7671
Crossfield, Ian J. M.0000-0002-1835-1891
Kempton, Eliza M.-R.0000-0002-1337-9051
Kubyshkina, Daria0000-0001-9137-9818
Howard, Andrew W.0000-0001-8638-0320
Angus, Ruth0000-0003-4540-5661
Isaacson, Howard0000-0002-0531-1073
Weiss, Lauren M.0000-0002-3725-3058
Beichman, Charles A.0000-0002-5627-5471
Fortney, Jonathan J.0000-0002-9843-4354
Fossati, Luca0000-0003-4426-9530
Lammer, Helmut0000-0002-0547-7671
McCullough, P. R.0000-0001-9165-9799
Morley, Caroline V.0000-0002-4404-0456
Wong, Ian0000-0001-9665-8429
Additional Information:We gratefully acknowledge the open-source software that made this work possible: LDTK34, batman35, emcee36, TTVFast37, REBOUND38, WHFast39, nestle (; refs. 40,41,42,43), astropy126,127, numpy128, ipython129, matplotlib130, RadVel44, george45, smint14 and GNU parallel78. This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. We received support for the analysis by NASA through grants under the HST-GO-13665 programme (PI B.B). This work relies on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This paper includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. This study has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for DPAC has been provided by national institutions, particularly the institutions participating in the Gaia Multilateral Agreement. Data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) and the Spitzer Heritage Archive (SHA). This research has made use of NASA’s Astrophysics Data System and the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA within the Exoplanet Exploration Program. Parts of this analysis have been run on the Lesta cluster kindly provided by the Observatoire de Genève. C.P. acknowledges financial support from the Fonds de Recherche Québécois—Nature et Technologie (FRQNT; Quebec), the Technologies for Exo-Planetary Science (TEPS) Trainee Program and the Natural Sciences and Engineering Research Council (NSERC) Vanier Scholarship. D.D. acknowledges support from the TESS Guest Investigator Program grant number 80NSSC19K1727 and NASA Exoplanet Research Program grant number 18-2XRP18_2-0136. B.B. acknowledges financial support from the NSERC of Canada and the FRQNT. I.W. is supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. C.V.M. acknowledges HST funding through grant number HST-AR-15805.001-A from STScI.
Group:Astronomy Department, Division of Geological and Planetary Sciences, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Gaia Multilateral AgreementUNSPECIFIED
Fonds de recherche du Québec - Nature et technologies (FRQNT)UNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
NASA Postdoctoral ProgramUNSPECIFIED
Record Number:CaltechAUTHORS:20230110-58978000.9
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118764
Deposited By: Research Services Depository
Deposited On:06 Feb 2023 17:01
Last Modified:06 Feb 2023 17:01

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