A remnant planetary core in the hot-Neptune desert

Abstract

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 M p =40.8 +2.4 −2.5 M ⊕ and density of 5.5 ± 0.8 gcm −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. Photoevaporation rates cannot provide the mass loss required to reduce a Jupiter-like gas giant, but can remove a few M ⊕ 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.

The TESS Objects of Interest Catalog from the TESS Prime Mission

“…TOI 849-A dense and unusually massive planet ( ), TOI 849 b is considered a planetary core of a hot Jupiter (Armstrong et al 2020). Of further interest is that TOI 849 b resides in the hot-Neptune "desert," a region in exoplanet mass-radius period parameter space that contains few planets.…”

Photometry of 10 Million Stars from the First Two Years of TESS Full Frame Images: Part I

“…The QLP has already been used to identify and alert planet candidates throughout the entire TESS Primary Mission, leading to the discovery of more than 1000 planetary candidates (Guerrero et al 2020, submitted to ApJS ), dozens of which have already been confirmed(e.g. Huang et al 2018b;Vanderspek et al 2019;Huang et al 2020;Armstrong et al 2020). Here, we describe how QLP produces light curves and evaluate its performance in terms of photometric precision.…”

Superstoichiometric Alloying of H and Close-Packed FeNi under High Pressures: Implications for Hydrogen Storage in Planetary Cores

“…Large exoplanets with unusually high densities such as Kepler-36b, NGTS-4b, and TOI-849b have been recently discovered and interpreted as the remnant cores of gas giants [39][40][41] . Several hypotheses have been proposed: (i) these planets once had a thicker atmosphere and suffered massive loss of their atmospheric H, or (ii) they were not able to accrete dense atmospheres either because of late formation or formation in a region of the protoplanetary disk devoid of gas 41 . Our study suggests the alternative possibility of H storage in the core.…”

“…TOI-849b and NGTS-4b require a thin atmosphere to explain their radii 41 . Although efficient packing would make FeH x much more dense than separate Fe and xH, the density of the H-bearing core would be less than that of a pure Fe core and the mass-radius relation approaches that of MgSiO 3 bridgmanite as the bulk H content of the planet increases (Fig.…”