A remnant planetary core in the hot-Neptune desert

David J. Armstrong , Théo A. Lopez (1) , Vardan Adibekyan , Richard A. Booth , Edward M. Bryant , Karen A. Collins , Magali Deleuil (1) , Alexandre Emsenhuber , Chelsea X. Huang , George W. King , Jorge Lillo-Box , Jack J. Lissauer , Elisabeth Matthews , Olivier Mousis (1) , Louise D. Nielsen , Hugh Osborn (1) , Jon Otegi , Nuno C. Santos , Sérgio G. Sousa , Keivan G. Stassun , Dimitri Veras , Carl Ziegler , Jack S. Acton , Jose M. Almenara (2) , David R. Anderson , David Barrado , Susana C. C. Barros , Daniel Bayliss , Claudia Belardi , Francois Bouchy , César Briceño , 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 , Elisa Delgado Mena , Olivier D. S. Demangeon , Caroline Dorn , Xavier Dumusque , Philipp Eigmüller , Michael Fausnaugh , Pedro Figueira , Tianjun Gan , Siddharth Gandhi , Samuel Gill , Erica J. Gonzales , Michael R. Goad , Maximilian N. Günther , 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 (1) , Joshua E. Schlieder , Sara 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

1 LAM - Laboratoire d'Astrophysique de Marseille
2 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to large uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary processes provide a route to understanding planetary interiors. Planets found in and near the typically barren hot-Neptune `desert'^1,2 (a region in mass-radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b³, which is thought to have an unusually massive core, and recent discoveries such as LTT9779b⁴ and NGTS-4b⁵, on which photoevaporation has removed a substantial part of their outer atmospheres. Here we report observations of the planet TOI-849b, which has a radius smaller than Neptune's but an anomalously large mass of 39.1 _-2.6^+2.7? Earth masses and a density of 5.2 _-0.8^+0.7? grams per cubic centimetre, similar to Earth's. Interior-structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than 3.9_-0.9^+0.8? per cent of the total planetary mass. The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation⁶. Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet.

Keywords: Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Solar and Stellar Astrophysics

Domains: Sciences of the Universe [physics]

David J. Armstrong, Théo A. Lopez, Vardan Adibekyan, Richard A. Booth, Edward M. Bryant, et al.. A remnant planetary core in the hot-Neptune desert. Nature, 2020, 583, pp.39-42. ⟨10.1038/s41586-020-2421-7⟩. ⟨insu-03705194⟩