TOI-4860 b, a short-period giant planet transiting an M3.5 dwarf

TL;DR

This paper reports the discovery of TOI-4860 b, a massive giant planet transiting an M3.5 dwarf, with detailed characterization and implications for planet formation theories, also hinting at a potential additional long-period planet.

Contribution

First detailed characterization of a massive giant planet around a mid to late M dwarf, expanding understanding of planet formation in such systems.

Findings

TOI-4860 b has a radius of 0.77 R_J and a mass of 0.273 M_J.

The planet orbits its host star every 1.52 days.

An additional eccentric planet candidate with a 427-day period was identified.

Abstract

We report the discovery and characterisation of a giant transiting planet orbiting a nearby M3.5V dwarf (d = 80.4 pc, $G$ = 15.1 mag, $K$=11.2 mag, R$_\star$ = 0.358 $\pm$ 0.015 R$_\odot$, M$_\star$ = 0.340 $\pm$ 0.009 M$_\odot$). Using the photometric time series from TESS sectors 10, 36, 46, and 63 and near-infrared spectrophotometry from ExTrA, we measured a planetary radius of 0.77 $\pm$ 0.03 R$_J$ and an orbital period of 1.52 days. With high-resolution spectroscopy taken by the CFHT/SPIRou and ESO/ESPRESSO spectrographs, we refined the host star parameters ([Fe/H] = 0.27 $\pm$ 0.12) and measured the mass of the planet (0.273 $\pm$ 0.006 M$_J$). Based on these measurements, TOI-4860 b joins the small set of massive planets ($>$80 M$_E$) found around mid to late M dwarfs ($<$0.4 R$_\odot$), providing both an interesting challenge to planet formation theory and a favourable target for further atmospheric studies with transmission spectroscopy. We identified an additional signal in the radial velocity data that we attribute to an eccentric planet candidate ($e=0.66\pm0.09$) with an orbital period of $427\pm7$~days and a minimum mass of $1.66\pm 0.26$ M$_J$, but additional data would be needed to confirm this.