Hot Rocks Survey I: A possible shallow eclipse for LHS 1478 b

¹ Department of Space Research and Technology, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁴ School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK
⁵ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ School of Physics, Trinity College Dublin, University of Dublin, Dublin 2, Ireland
⁷ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁸ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁰ Ludwig Maximilian University, Faculty of Physics, Scheinerstr. 1, Munich 81679, Germany
¹¹ ARTORG Center for Biomedical Engineering Research, University of Bern, Murtenstrasse 50, 3008 Bern, Switzerland
¹² University College London, Department of Physics & Astronomy, Gower St, London, WC1E 6BT, UK
¹³ University of Warwick, Department of Physics, Astronomy & Astrophysics Group, Coventry CV4 7AL, UK
¹⁴ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
¹⁵ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
¹⁶ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁷ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
¹⁸ Center for Astrophysics|Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
¹⁹ Department of Astronomy, University of Washington, Seattle, WA 98105, USA

^★ Corresponding author; prua@space.dtu.dk

Received: 14 October 2024
Accepted: 14 February 2025

Abstract

Context. M-dwarf systems offer an opportunity to study terrestrial exoplanetary atmospheres due to their small size and cool temperatures. However, the extreme conditions imposed by these host stars raise a question about whether their close-in rocky planets are able to retain any atmosphere at all.

Aims. The Hot Rocks Survey aims to answer this question by targeting nine different M-dwarf rocky planets spanning a range of planetary and stellar properties. Of these, LHS 1478 b orbits an M3-type star, has an equilibrium temperature of T_eq = 585 K, and receives 21 times Earth’s instellation.

Methods. We observed two secondary eclipses of LHS 1478 b using photometric imaging at 15 µm using the Mid-Infrared Instrument on the James Webb Space Telescope (JWST MIRI) to measure thermal emission from the dayside of the planet. We compared these values to atmospheric models to evaluate potential heat transport and CO₂ absorption signatures.

Results. We find that a secondary eclipse depth of 138 ± 53 ppm at the expected time for a circular orbit is preferred over a null model at 2.8σ, a moderate detection, though dynamical models do favour a non-eccentric orbit for this planet. The second observation results in a non-detection due to significantly larger unexplained systematics. Based on the first observation alone, we can reject the null hypothesis of the dark (zero Bond albedo) no atmosphere bare rock model with a confidence level of 3.3σ, though for A_B = 0.2 the significance decreases to 2.1σ. The tentative secondary eclipse depth is consistent with the majority of the atmospheric scenarios we considered, spanning CO₂-rich atmospheres with surface pressures from 0.1 to 10 bar. However, we stress that the two observations from our programme do not yield consistent results, and more observations are needed to verify our findings. The Hot Rocks Survey serves as a relevant primer for future endeavours such as the Director’s Discretionary Time (DDT) Rocky Worlds programme.