A transiting giant planet in orbit around a 0.2-solar-mass host star (2506.07931v2)

Abstract: Planet formation models suggest that the formation of giant planets is significantly harder around low-mass stars, due to the scaling of protoplanetary disc masses with stellar mass. The discovery of giant planets orbiting such low-mass stars thus imposes strong constraints on giant planet formation processes. Here, we report the discovery of a transiting giant planet orbiting a $0.207 \pm 0.011 M_{\odot}$ star. The planet, TOI-6894 b, has a mass and radius of $M_P = 0.168 \pm 0.022 M_J (53.4 \pm 7.1 M_{\oplus})$ and $R_P = 0.855 \pm 0.022 R_J$, and likely includes $12 \pm 2 M_{\oplus}$ of metals. The discovery of TOI-6894 b highlights the need for a better understanding of giant planet formation mechanisms and the protoplanetary disc environments in which they occur. The extremely deep transits (17% depth) make TOI-6894 b one of the most accessible exoplanetary giants for atmospheric characterisation observations, which will be key for fully interpreting the formation history of this remarkable system and for the study of atmospheric methane chemistry.

Overview of a Giant Planet Transiting a 0.2 Solar Mass Host Star

The paper presents the discovery of a giant planet, TOI-6894b, transiting a very low-mass star (0.207 solar masses), challenging established models of planet formation around low-mass stars. This discovery raises intriguing questions regarding the processes underpinning giant planet formation and the environments of protoplanetary discs in such contexts.

Results

The star TOI-6894, harboring the planet, was observed using TESS over several campaigns, confirming the transit signal. Ground-based observations and further spectroscopic analysis ruled out alternate scenarios such as background eclipsing binaries, cementing the planetary nature of TOI-6894b. The radial velocity data yielded an estimated planet mass of 0.168 Jupiter masses and a radius of 0.855 Jupiter radii, delineating TOI-6894b as a low-density sub-Saturn giant.

Analysis and Implications

The analysis posits TOI-6894b as an exoplanet in a Mundane, near-circular orbit with low eccentricity. It exhibits substantial enrichment in metals relative to its host star, pointing to peculiar formation characteristics markedly distinct from typical expectations for low-mass stellar systems. The presence of such a planet around a 0.2 solar mass star adds constraints to core-accretion models and suggests an alternative scenario that involves either inhibited gas accretion or novel pathways of early planet formation possibly borrowed from gravitational instability frameworks.

Discussion

The findings open a discourse on the core-accretion mechanism, widely accepted as integral in giant planet formation, yet here is confronted with limits around low-mass stars. TOI-6894 exemplifies a case where theoretical adjustments may include considering higher formation efficiency, undetected disc mass, or earlier formation phases distinct from typical observations. Alternatively, gravitational instability could provide a mechanism for the formation, though this would require conditions conducive to fragment accretion absent in traditional models.

Future Directions

Studying TOI-6894b's atmosphere could provide insights into its retention of metals and subsequent accretion history—a factor indispensable to modeling its developmental chronology. Spectroscopic observations are expected to clarify atmospheric composition and cloud presence, aiding in probing the methane chemistry of its atmosphere. Given access to high transmission metrics, TOI-6894b stands as an optimal candidate for exo-atmospheric studies, potentially shifting paradigms on planetary compositions and formation routes in low-mass star systems.

Overall, TOI-6894 leads us to reevaluate prevailing assumptions around planet formation within less massive environments, necessitating innovative theoretical frameworks and observational strategies to grasp their complexity and diversity fully.