A remnant planetary core in the hot-Neptune desert (2003.10314v3)

Abstract: 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.

• The paper reveals TOI-849b as a remnant planetary core with high density, challenging typical envelope accretion models.
• Spectroscopic and transit data from TESS, HARPS, NGTS, and LCOGT precisely measured its 40 Earth-mass and 18.4-hour orbit.
• Its unique composition, with only a ~3.9% hydrogen/helium envelope, offers new insights into the evolution and stripping of gas giants.

A Remnant Planetary Core in the Hot Neptunian Desert

The paper presents the discovery of TOI-849b, a rare exoplanetary object characterized as a remnant planetary core residing within the hot Neptunian desert. This region of parameter space is typically sparse, as planets here are generally expected to be disrupted by photoevaporation or tidal forces. TOI-849b poses as an intriguing subject due to its unique characteristics: a radius smaller than Neptune but possessing an exceptionally high mass and density comparable to Earth.

Key Findings and Methodologies

TOI-849b was initially identified as a candidate transiting planet by the Transiting Exoplanet Survey Satellite (TESS). No accompanying stellar activity or additional planets were observed in the photometric data. The spectroscopic follow-up using the High Accuracy Radial velocity Planet Searcher (HARPS) confirmed the presence of TOI-849b, with measurements indicating a significant radial velocity signal. The exoplanet was further studied using data from ground-based facilities, including the Next Generation Transit Survey (NGTS) and the Las Cumbres Observatory (LCOGT), to refine its measured parameters.

TOI-849b has an orbital period of 18.4 hours around a G-type star, leading to a high equilibrium temperature of approximately 1800 K. Crucially, TOI-849b's mass (~40 Earth masses) exceeds the typical threshold for runaway gas accretion, suggesting that it might have initially formed as a larger gas giant whose envelope was subsequently stripped. Modelling efforts implied a limited hydrogen/helium envelope, constituting no more than about 3.9% of the planet's total mass, signifying its current solid core-dominant state.

Implications and Theoretical Insights

The findings challenge existing models of planet formation, particularly the core accretion model, as such cores are generally anticipated to accrete a substantial gaseous envelope when crossing certain mass thresholds. TOI-849b's present-day characteristics may be indicative of several formation pathways, including significant envelope loss due to tidal disruption or high-energy impacts, or incomplete initial accretion possibly due to late formation or gap opening in the protoplanetary disk.

The substantial density and composition of TOI-849b imply that it is composed primarily of heavy elements, with any remaining gaseous material potentially enriched by heavier volatiles like water from its interior. This suggests TOI-849b is a distinctive probe for planetary cores' formation and structural models that diverge from traditional assumptions.

Future Directions and Observational Opportunities

TOI-849b represents a singular specimen for astrophysical paper, offering a potential empirical look into the elusive original core composition of giant planets. Further observations to measure the atmospheric composition, potentially using spectroscopy from either Hubble or the upcoming James Webb Space Telescope (JWST), could provide deeper understanding about the processes that strip giant planets of their gaseous layers.

Continued exploration of such densely packed planetary remnants will refine theories of planetary evolution, especially in understanding the dynamics and frequency of envelope stripping events in various stellar environments. As observational techniques and equipment improve, the Neptunian desert may reveal further instances of similarly stripped giant planet cores, allowing for broader comparative analyses.

In sum, TOI-849b exemplifies a rare opportunity to paper a planetary core unaided by thick gaseous envelopes, thereby adding a pivotal piece to the complex puzzle of planetary formation and evolution.