Work

Weighing the mass of LHS 3844 b

Year: 2026

Type: article

Abstract: Context . LHS 3844 b (TOI-136b) is an ultra short-period, Earth-size exoplanet detected by TESS. It is one of the most favourable objects for atmospheric characterisation and study of its surface with the James Webb Space Telescope. However, the dynamical mass of this planet has not yet been measured. 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 (GP) 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 ± 0.23 M ⊕ and a bulk density of 5.67 ± 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 disfavours 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 ~6.9 days, but Bayesian model comparison does not provide conclusive support for its planetary interpretation. Conclusions . We report the first dynamical mass measurement of LHS 3844 b, confirming it as a dense, terrestrial ultra-short-period planet. With its exceptionally well-constrained bulk properties and extensive JWST programme, LHS 3844 b remains a benchmark target for studies of the atmospheres and surfaces of rocky exoplanets.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202558030