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 ± 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 atmospheres and surfaces of rocky exoplanets.
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BibTeX
@article{hacker2026weighing,
title = {{Weighing the mass of LHS 3844 b}},
author = {A. Hacker and N. Astudillo-Defru and R. Díaz and C. Dorn and Xavier Bonfils and J. Almenara and P. Cortés-Zuleta and X. Delfosse and T. Forveille and S. Udry},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202558030},
eprint = {2603.28237},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05843481v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Weighing the mass of LHS 3844 b
AU - A. Hacker
AU - N. Astudillo-Defru
AU - R. Díaz
AU - C. Dorn
AU - Xavier Bonfils
AU - J. Almenara
AU - P. Cortés-Zuleta
AU - X. Delfosse
AU - T. Forveille
AU - S. Udry
PY - 2026
DA - 2026/10/29
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202558030
UR - https://hal.science/hal-05843481v1
AB - 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 ± 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 atmospheres and surfaces of rocky exoplanets.
SN - 0004-6361
SN - 1432-0746
ER -