Weighing the mass of LHS 3844 b

Abstract

Context: LHS 3844 b (TOI-136 b) is a ultra short-period, Earth-size exoplanet detected by TESS. It is one of the most favourable object for atmospheric characterisation and the study of its surface with the James Webb Space Telescope. However, the dynamical mass of this planet has not been measured yet. Aims: We aim to determine the mass of LHS 3844 b using high-precision radial velocity (RV) measurements and assess the robustness of the inferred signal across different noise and orbital modelling assumptions. Methods: We analyse 25 ESPRESSO RV observations within a fully Bayesian framework. We explore 15 competing RV models that differ in their treatment of correlated stellar variability (through different Gaussian Process kernels) and long-term drifts. Marginal likelihoods are computed for all models and used for Bayesian model comparison and evidence-weighted parameter estimation. Results: The RV planetary signal is robustly detected across all models, and the inferred semi-amplitude remains stable under all tested noise and drift prescriptions. From the evidence-weighted posterior samples we derive a planetary mass of $2.27 \pm 0.23$ M$_\oplus$ and a bulk density of $5.67 \pm 0.65$ gcm$^{-3}$, consistent with a predominantly rocky composition. Model comparison favours GP kernels including periodic or quasi-periodic components associated with stellar rotation and disfavors models with additional long-term drifts. Using interior-structure inference, we find that the core mass fraction is comparable to (or slightly smaller than) Earth's and only trace amounts of water are permitted, supporting a dry, terrestrial interior. We also investigate a tentative additional signal near $\sim 6.9$ days, but Bayesian model comparison does not provide conclusive support for its planetary interpretation.

Figures (13)

• Figure 1: GLS Periodograms zechmeisterkurster2009 for the ESPRESSO radial velocities, their residuals after removing the best fit model for a circular orbit, and several stellar activity indicators measured for LHS 3844. Vertical dashed lines mark the transit period $P = 0.46$ days (green) and the rotational period of the star $P_{\text{rot}} = 128$ days (red) vanderspek2019. The 10, 1 and 0.1 per cent False Alarm Probabilities (FAP) are shown as dashed horizontal lines.
• Figure 2: Posterior probability matrix for the 15 evaluated models, spanning all combinations of three secular acceleration models and five kernels. The numbers inside the cells indicate the normalized posterior probability of each model. The rightmost column and bottom row show the marginalized probabilities over drift and kernel types, respectively.
• Figure 3: Comparison of the inferred RV semi-amplitude ($K$) across different models, ordered by increasing marginal likelihood. Each violin plot represents the posterior distribution of $K$ for a given model, with the mean (solid line), median (dotted line), and 1st/3rd quartiles (green). The rightmost violin corresponds to the weighted average model. The overlaid bar plot (grey) shows the relative log-likelihood difference for each model. The numerical annotations indicate the mean, standard deviation, and relative uncertainty of $K$ for each model.
• Figure 4: Phase-folded RV curve of LHS 3844 , based on the best-fit model with the highest marginal likelihood: White + SE + QP Kernel with no drift ($d_0$). The data points correspond to ESPRESSO observations obtained before (green circles) and after (red squares) the instrumental upgrade. The data were corrected by subtracting the per-instrument velocity offsets but not the noise component of the GP model. Phase folding was performed using the median of the posterior distribution for the orbital period. Error bars represent the quadrature sum of the reported uncertainties and the empirically inferred per-instrument jitter. The black curve shows the median orbital model (circular orbit), while the gray shaded region represents the 68% credible interval derived from posterior samples.
• Figure 5: Mass-radius diagram for small exoplanets ($M_p < 5\,M_\oplus$) with precisely measured masses ($\sigma_M / M < 0.3$). Gray points represent the observed exoplanets with error bars, while the red point marks LHS 3844 b. The shaded background corresponds to a kernel density estimation (KDE) of the joint posterior distribution of mass and radius for LHS 3844 b. Solid lines indicate theoretical mass-radius relations for different compositions from Zeng2016. Known planets were sourced from the NASA Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu/) on 26 February 2026.