A remnant planetary core in the hot Neptunian desert

David J. Armstrong, Théo A. Lopez, Vardan Adibekyan, Richard A. Booth, Edward M. Bryant, Karen A. Collins, Alexandre Emsenhuber, Chelsea X. Huang, George W. King, Jorge Lillo-box, Jack J. Lissauer, Elisabeth C. Matthews, Olivier Mousis, Louise D. Nielsen, Hugh Osborn, Jon Otegi, Nuno C. Santos, Sérgio G. Sousa, Keivan G. Stassun, Dimitri VerasCarl Ziegler, Jack S. Acton, Jose M. Almenara, David R. Anderson, David Barrado, Susana C. C. Barros, Daniel Bayliss, Claudia Belardi, Francois Bouchy, César Briceno, Matteo Brogi, David J. A. Brown, Matthew R. Burleigh, Sarah L. Casewell, Alexander Chaushev, David R. Ciardi, Kevin I. Collins, Knicole D. Colón, Benjamin F. Cooke, Ian J. M. Crossfield, Rodrigo F. Díaz, Magali Deleuil, Elisa Delgado Mena, Olivier D. S. Demangeon, Caroline Dorn, Xavier Dumusque, Philipp Eigmuller, Michael Fausnaugh, Pedro Figueira, Tianjun Gan, Siddharth Gandhi, Samuel Gill, Michael R. Goad, Maximilian N. Guenther, Ravit Helled, Saeed Hojjatpanah, Steve B. Howell, James Jackman, James S. Jenkins, Jon M. Jenkins, Eric L. N. Jensen, Grant M. Kennedy, David W. Latham, Nicholas Law, Monika Lendl, Michael Lozovsky, Andrew W. Mann, Maximiliano Moyano, James McCormac, Farzana Meru, Christoph Mordasini, Ares Osborn, Don Pollacco, Didier Queloz, Liam Raynard, George R. Ricker, Pamela Rowden, Alexandre Santerne, Joshua E. Schlieder, S. Seager, Lizhou Sha, Thiam-Guan Tan, Rosanna H. Tilbrook, Eric Ting, Stéphane Udry, Roland Vanderspek, Christopher A. Watson, Richard G. West, Paul A. Wilson, Joshua N. Winn, Peter Wheatley, Jesus Noel Villasenor, Jose I. Vines, Zhuchang Zhan

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The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to major uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary pathways provide a new route to understanding planetary interiors. We present the discovery of TOI-849b, the remnant core of a giant planet, with a radius smaller than Neptune but an anomalously high mass Mp=40.8^{+2.4}_{-2.5} Mearth and density of 5.5 ± 0.8 g cm^{-3}, similar to the Earth. Interior structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than 3.9^{+0.8}_{-0.9}% of the total mass of the planet. TOI-849b transits a late G type star (T_{mag}=11.5) with an orbital period of 18.4 hours, leading to an equilibrium temperature of 1800K. The planet's mass is larger than the theoretical threshold mass for runaway gas accretion. As such, the planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it avoided substantial gas accretion, perhaps through gap opening or late formation. Photo-evaporation rates cannot provide the mass loss required to reduce a Jupiter-like gas giant, but can remove a few Earth mass hydrogen and helium envelope on timescales of several Gyr, implying that any remaining atmosphere is likely to be enriched by water or other volatiles from the planetary interior. TOI-849b represents a unique case where material from the primordial core is left over from formation and available to study.
Original languageEnglish
Pages (from-to)32
Issue number2020
Early online date01 Jul 2020
Publication statusEarly online date - 01 Jul 2020


  • Astrophysics - Earth and Planetary Astrophysics
  • Astrophysics - Solar and Stellar Astrophysics


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