An M dwarf accompanied by a close-in giant orbiter with SPECULOOS

TL;DR

This paper confirms the existence of a Jupiter-sized planet orbiting an M4.5 star, using multi-instrument observations, and discusses its implications for planetary formation theories around low-mass stars.

Contribution

It reports the discovery and detailed characterization of TOI-4860 b, a close-in giant planet around an M dwarf, validated through multiple observational methods.

Findings

Confirmed planetary nature with multicolor photometry and spectroscopy.

Measured planet radius of 0.76 R_Jup and mass of 0.67 M_Jup.

No secondary eclipse detected, constraining planetary brightness.

Abstract

In the last decade, a dozen close-in giant planets have been discovered orbiting stars with spectral types ranging from M0 to M4, a mystery since known formation pathways do not predict the existence of such systems. Here, we confirm TOI-4860 b, a Jupiter-sized planet orbiting an M4.5 host, a star at the transition between fully and partially convective interiors. First identified with TESS data, we validate the transiting companion's planetary nature through multicolour photometry from the TRAPPIST-South/North, SPECULOOS, and MuSCAT3 facilities. Our analysis yields a radius of $0.76 \pm 0.02~ \rm R_{Jup}$ for the planet, a mass of $0.34~\rm M_\odot$ for the star, and an orbital period of 1.52 d. Using the newly commissioned SPIRIT InGaAs camera at the SPECULOOS-South Observatory, we collect infrared photometry in zYJ that spans the time of secondary eclipse. These observations do not detect a secondary eclipse, placing an upper limit on the brightness of the companion. The planetary nature of the companion is further confirmed through high-resolution spectroscopy obtained with the IRD spectrograph at Subaru Telescope, from which we measure a mass of $0.67 \pm 0.14~\rm M_{Jup}$ . Based on its overall density, TOI-4860 b appears to be rich in heavy elements, like its host star.