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

TL;DR

Gliese 12 b is a Venus-sized temperate planet orbiting a nearby M dwarf. The study delivers the first precise mass measurement by combining nearly 200 high-precision RVs from HARPS-N, ESPRESSO, and CARMENES with an extensive photometric dataset (TESS, CHEOPS, MuSCAT2/3, and ground-based surveys). The joint analysis yields $M_p = 0.95^{+0.29}_{-0.30}\,M_\oplus$, $R_p = 0.93 \pm 0.06\,R_\oplus$, $K = 0.67 \pm 0.21$ m s$^{-1}$, and $P = 12.7614$ d, implying a density of $\rho_p = 6.4 \pm 2.4$ g cm$^{-3}$ and a predominantly rocky interior. Interior-structure modelling favours a Venus-like composition with a modest core fraction and negligible volatile envelope, while atmospheric-loss modelling indicates any primordial H/He would have been stripped long ago, making Lyman-$\alpha$ and JWST follow-up key to constraining possible atmospheres. The planet’s placement in a sparsely populated mass-radius-temperature region makes Gliese 12 b a prime target for atmospheric characterization and a valuable benchmark for rocky planets around M dwarfs.

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 $R_\mathrm{p} = 0.93\pm0.06 \,\mathrm{R_\oplus}$ and a mass of $M_\mathrm{p} = 0.95^{+0.29}_{-0.30} \,\mathrm{M_\oplus}$ (a $3.2σ$ measurement of the semi-amplitude $K=0.67\pm0.21\,\mathrm{m\,s^{-1}}$), and is on an orbit with a period of $12.761418^{+0.000060}_{-0.000055}\,\mathrm{d}$. 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 ($ρ_\mathrm{p}=6.4\pm2.4\,\mathrm{g\,cm^{-3}}$). 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--$T_\mathrm{eq}$ parameter space, making it a prime target for follow-up observations, including Lyman-$α$ studies.

Figures (14)

• Figure 1: Gliese 12 RVs with offsets removed; HARPS-N, ESPRESSO, and CARMENES data are plotted as blue circles, green squares, and purple triangles, respectively. Uncertainties on the ESPRESSO RVs are smaller than the marker points. Outliers are indicated by the black markers.
• Figure 2: BGLS periodogram of the ASAS-SN $V$ band (top) and $g'$ band (bottom) data of Gliese 12, of the full data and per season. The red vertical line indicates the planet period.
• Figure 3: Detrended and phase-folded transits of Gliese 12 b with the best-fitted model from juliet plotted in blue as well as the residuals on the bottom. Top left: TESS; top right: CHEOPS, middle: MuSCAT2; bottom: MuSCAT3.
• Figure 4: Phase-folded RVs from the best-fit informed RV model with GPs removed. Binned data shown by the large, purple points (10 bins). Phase-folded on the period of the planet (12.761421 d).
• Figure 5: A summary of the variety of models fitted to the RV data. The purple point highlights the adopted fit. Here, $\mathcal{U}$(wide) indicates wide uniform priors for the GP period (as in Table \ref{['tab:fit_priors_starinst']}), and $\mathcal{U}$(value) indicates uniform priors defined with 15 d either side of the specified value. The dashed, black line indicates the weighted mean of all the mass values.