The mass of the exo-Venus Gliese 12b, as revealed by HARPS-N, ESPRESSO, and CARMENES

Small temperate planets are prime targets for exoplanet studies due to their possible similarities with the rocky planets in the Solar system. M dwarfs are promising hosts since the planetary signals are within our current detection capabilities. Gliese 12 b is a Venus-sized temperate planet orbiting a quiet M dwarf. We present here the first precise mass measurement of this small exoplanet. We performed a detailed analysis using HARPS-N (High Accuracy Radial velocity Planet Searcher for the Northern Hemisphere), ESPRESSO (Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations), and CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs) radial velocities, along with new and archival TESS (Transiting Exoplanet Survey Satellite), CHEOPS (CHaracterising ExOPlanet Satellite), and MuSCAT2/3 (Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets) photometry data. From fitting the available data, we find that the planet has a radius of Rp = 0.93 ± 0.06 R⊕ and a mass of Mp = 0.95+0.29-0.30, M⊕ (a 3.2σ measurement of the semi-amplitude K = 0.67 ± 0.21 ms-1), and is on an orbit with a period of 12.761418+0.000060-0.000055, d. A variety of techniques were utilized to attenuate stellar activity signals. Gliese 12b has an equilibrium temperature of Teq = 317 ± 8 K, assuming an albedo of zero, and a density consistent with that of Earth and Venus (ϱp = 6.4 ± 2.4 g cm-3). We find that Gliese 12 b has a predominantly rocky interior and simulations indicate that it is unlikely to have retained any of its primordial gaseous envelope. The bulk properties of Gliese 12 b place it in an extremely sparsely populated region of both mass–radius and density–Teq parameter space, making it a prime target for follow-up observations, including Lyman-α studies.