Surviving in the Hot Neptune Desert: The Discovery of the Ultra-Hot Neptune TOI-3261b

TL;DR

This paper reports the discovery and characterization of TOI-3261b, an ultra-hot Neptune in the Hot Neptune Desert, providing insights into its mass, radius, potential atmospheric composition, and formation mechanisms.

Contribution

It presents the discovery of TOI-3261b, a unique ultra-hot Neptune with detailed mass, radius, and atmospheric potential analysis, advancing understanding of planet formation in extreme environments.

Findings

TOI-3261b has a radius of approximately 3.82 Earth radii.

The planet's mass is about 30.3 Earth masses, more than twice that of typical Neptune-sized planets.

It likely retains a volatile-enriched envelope constituting around 5% of its total mass.

Abstract

The recent discoveries of Neptune-sized ultra-short period planets (USPs) challenge existing planet formation theories. It is unclear whether these residents of the Hot Neptune Desert have similar origins to smaller, rocky USPs, or if this discrete population is evidence of a different formation pathway altogether. We report the discovery of TOI-3261b, an ultra-hot Neptune with an orbital period $P$ = 0.88 days. The host star is a $V = 13.2$ magnitude, slightly super-solar metallicity ([Fe/H] $\simeq$ 0.15), inactive K1.5 main sequence star at $d = 300$ pc. Using data from the Transiting Exoplanet Survey Satellite and the Las Cumbres Observatory Global Telescope, we find that TOI-3261b has a radius of $3.82_{-0.35}^{+0.42}$ $R_{\oplus}$. Moreover, radial velocities from ESPRESSO and HARPS reveal a mass of $30.3_{-2.4}^{+2.2}$ $M_{\oplus}$, more than twice the median mass of Neptune-sized planets on longer orbits. We investigate multiple mechanisms of mass loss that can reproduce the current-day properties of TOI-3261b, simulating the evolution of the planet via tidal stripping and photoevaporation. Thermal evolution models suggest that TOI-3261b should retain an envelope potentially enriched with volatiles constituting $\sim$5% of its total mass. This is the second highest envelope mass fraction among ultra-hot Neptunes discovered to date, making TOI-3261b an ideal candidate for atmospheric follow-up observations.