A remnant planetary core in the hot-Neptune desert
Creators: name: Armstrong, David J.
name: Lopez, Théo A.
name: Adibekyan, Vardan
name: Booth, Richard A.
name: Bryant, Edward M.
name: Collins, Karen A.
name: Emsenhuber, Alexandre
name: Huang, Chelsea X.
name: King, George W.
name: Lillo-box, Jorge
name: Lissauer, Jack J.
name: Matthews, Elisabeth C.
name: Mousis, Olivier
name: Nielsen, Louise D.
name: Osborn, Hugh
name: Otegi, Jon
name: Santos, Nuno C.
name: Sousa, Sérgio G.
name: Stassun, Keivan G.
name: Veras, Dimitri
name: Ziegler, Carl
name: Acton, Jack S.
name: Almenara, Jose M.
name: Anderson, David R.
name: Barrado, David
name: Barros, Susana C. C.
name: Bayliss, Daniel
name: Belardi, Claudia
name: Bouchy, Francois
name: Briceno, César
name: Brogi, Matteo
name: Brown, David J. A.
name: Burleigh, Matthew R.
name: Casewell, Sarah L.
name: Chaushev, Alexander
name: Ciardi, David R.
name: Collins, Kevin I.
name: Colón, Knicole D.
name: Cooke, Benjamin F.
name: Crossfield, Ian J. M.
name: Díaz, Rodrigo F.
name: Deleuil, Magali
name: Mena, Elisa Delgado
name: Demangeon, Olivier D. S.
name: Dorn, Caroline
name: Dumusque, Xavier
name: Eigmuller, Philipp
name: Fausnaugh, Michael
name: Figueira, Pedro
name: Gan, Tianjun
name: Gandhi, Siddharth
name: Gill, Samuel
name: Goad, Michael R.
name: Guenther, Maximilian N.
name: Helled, Ravit
name: Hojjatpanah, Saeed
name: Howell, Steve B.
name: Jackman, James
name: Jenkins, James S.
name: Jenkins, Jon M.
name: Jensen, Eric L. N.
name: Kennedy, Grant M.
name: Latham, David W.
name: Law, Nicholas
name: Lendl, Monika
name: Lozovsky, Michael
name: Mann, Andrew W.
name: Moyano, Maximiliano
name: McCormac, James
name: Meru, Farzana
name: Mordasini, Christoph
name: Osborn, Ares
name: Pollacco, Don
name: Queloz, Didier
name: Raynard, Liam
name: Ricker, George R.
name: Rowden, Pamela
name: Santerne, Alexandre
name: Schlieder, Joshua E.
name: Seager, S.
name: Sha, Lizhou
name: Tan, Thiam-Guan
name: Tilbrook, Rosanna H.
name: Ting, Eric
name: Udry, Stéphane
name: Vanderspek, Roland
name: Watson, Christopher A.
name: West, Richard G.
name: Wilson, Paul A.
name: Winn, Joshua N.
name: Wheatley, Peter
name: Villasenor, Jesus Noel
name: Vines, Jose I.
name: Zhan, Zhuchang
Publisher: name: arXiv
Year: 2020
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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' (a region in mass-radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b, which is thought to have an unusually massive core, and recent discoveries such as LTT9779b and NGTS-4b, 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 $39.1^{+2.7}_{-2.6}$ Earth masses and a density of $5.2^{+0.7}_{-0.8}$ 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 $3.9^{+0.8}_{-0.9}$ 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 formation. 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.
description: Published in Nature. This is a preprint of the article, before minor changes made during the refereeing and editing process. The published PDF is at https://www.nature.com/articles/s41586-020-2421-7 and can be accessed for free by following this link: https://rdcu.be/b5miB . Abstract updated to match published version