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

Abstract

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, ESPRESSO, and CARMENES radial velocities, along with new and archival \tess, \cheops, and MuSCAT2/3 photometry data. From fitting the available data, we find that the planet has a radius of $R_{\mathrm{p}}=0.93\pm 0.06{\mathrm{R}}_{\oplus }$ and a mass of $M_{\mathrm{p}}=0.95_{-0.30}^{+0.29}{\mathrm{M}}_{\oplus }$ (a $3.2\sigma$ measurement of the semi-amplitude $K=0.67\pm 0.21\mathrm{m}{\mathrm{s}}^{-1}$), and is on an orbit with a period of $12.761418_{-0.000055}^{+0.000060}\mathrm{d}$. A variety of techniques were utilised to attenuate stellar activity signals. Gliese 12 b has an equilibrium temperature of $T_{\mathrm{eq}}=317\pm 8\mathrm{K}$, assuming an albedo of zero, and a density consistent with that of Earth and Venus (${\rho }_{\mathrm{p}}=6.4\pm 2.4\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$). 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--$T_{\mathrm{eq}}$ parameter space, making it a prime target for follow-up observations, including Lyman-$\alpha$ studies.

25 pages, 14 figures; accepted for publication in MNRAS