The mass of TOI-654 b: A short-period sub-Neptune transiting a mid-M dwarf

TL;DR

This study precisely measures the mass and radius of the sub-Neptune TOI-654 b, a planet around a mid-M dwarf, providing insights into its composition and suitability for atmospheric characterization with upcoming telescopes.

Contribution

The paper presents the first precise mass measurement of TOI-654 b, combining multiple observational methods to characterize its bulk properties and atmospheric potential.

Findings

Mass of TOI-654 b is 8.71 ± 1.25 Earth masses.

Radius of TOI-654 b is 2.378 ± 0.089 Earth radii.

Planet lies at the radius valley and on the edge of the Neptune desert.

Abstract

Sub-Neptunes are small planets between the size of the Earth and Neptune. The orbital and bulk properties of transiting sub-Neptunes can provide clues for their formation and evolution of small planets. In this paper, we report on follow-up observations of a planetary system around the mid-M dwarf TOI-654, whose transiting sub-Neptune TOI-654 b ($P=1.53$ day) is validated as a suitable target for the atmospheric observation. We measure the planetary mass and stellar properties with the InfraRed Doppler instrument (IRD) mounted on the Subaru telescope and obtain the stellar and planetary properties from additional transit observations by the Transit Exoplanetary Survey Satellite (TESS) and a series of the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT). As a result, the planetary mass of TOI-654 b is determined to be $M_{{\rm p}} = 8.71 \pm 1.25 M_{\oplus}$, and the radius is updated to be $R_{\rm p} = 2.378 \pm 0.089 R_{\oplus}$. The bulk density suggests that the planet is composed of a rocky and volatile-rich core or a rocky core surrounded by a small amount of H/He envelope.TOI-654 b is one of unique planets located around the radius valley and and also on the outer edge of the Neptune desert. The precise mass determination enables us to constrain the atmospheric properties with future spectroscopic observations especially for the emission by the James Webb Space Telescope and Ariel.