Discovery and mass measurement of the hot, transiting, Earth-sized planet GJ 3929 b

TL;DR

This paper reports the discovery and mass measurement of GJ 3929 b, a hot Earth-sized exoplanet orbiting a nearby M dwarf, highlighting its potential for atmospheric studies and evidence for an additional planet candidate.

Contribution

The study provides the first combined photometric and spectroscopic characterization of GJ 3929 b, including its mass, radius, density, and potential additional planets, using multi-instrument data.

Findings

GJ 3929 b has a radius of 1.15 R_earth and a mass of 1.21 M_earth.

The planet's density is compatible with Earth's, suggesting a rocky composition.

Evidence for a second planet candidate with a 14.3-day orbit was found.

Abstract

We report the discovery of GJ 3929 b, a hot Earth-sized planet orbiting the nearby M3.5 V dwarf star, GJ 3929 (G 180--18, TOI-2013). Joint modelling of photometric observations from TESS sectors 24 and 25 together with 73 spectroscopic observations from CARMENES and follow-up transit observations from SAINT-EX, LCOGT, and OSN yields a planet radius of $R_b = 1.150 +/- 0.040$ R$_{earth}$, a mass of $M_b = 1.21 +/- 0.42$ M$_{earth}$, and an orbital period of $P_b = 2.6162745 +/- 0.0000030$ d. The resulting density of $\rho_b= 4.4 +/- 1.6$ g/cm$^{-3}$ is compatible with the Earth's mean density of about 5.5 g/cm$^{-3}$. Due to the apparent brightness of the host star (J=8.7 mag) and its small size, GJ 3929 b is a promising target for atmospheric characterisation with the JWST. Additionally, the radial velocity data show evidence for another planet candidate with $P_{[c]} = 14.303 +/- 0.035$ d, which is likely unrelated to the stellar rotation period, $P_{rot} = 122+/-13$ d, which we determined from archival HATNet and ASAS-SN photometry combined with newly obtained TJO data.