Hot Rocks Survey

II. The thermal emission of TOI-1468 b reveals a bare hot rock^★

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

^★★ Corresponding author.

Received: 14 December 2024
Accepted: 24 March 2025

Abstract

Context. Terrestrial exoplanets orbiting nearby small cool stars, known as M dwarfs, are well suited for an atmospheric characterisation. Because the intense X-ray and UV (XUV) irradiation from M dwarf host stars is strong, orbiting exoplanets are thought to be unable to retain primordial hydrogen- or helium-dominated atmospheres. However, it is currently unknown whether heavier secondary atmospheres can survive.

Aims. The aim of the Hot Rocks Survey programme is to determine whether exoplanets can retain secondary atmospheres in the presence of M dwarf hosts. In the sample of nine exoplanets in the programme, we aim to determine whether TOI-1468 b has a substantial atmosphere or is consistent with a low-albedo bare rock.

Methods. The James Webb Space Telescope provides an opportunity to characterise the thermal emission with MIRI at 15 μm. The occultation of TOI-1468 b was observed three times. We compared our observations to atmospheric models that include varying amounts of CO₂ and H₂O.

Results. The observed occultation depths for the individual visits are 239±52 ppm, 341±53 ppm, and 357±52 ppm. A joint fit yields an occultation depth of 311±31 ppm. The thermal emission is mostly consistent with no atmosphere and a zero Bond albedo at a confidence level of 1.65σ, or a blackbody at a brightness temperature of 1024 ± 78 K. A pure CO₂ or H₂O atmosphere with a surface pressure above 1 bar is ruled out at higher than 3σ.

Conclusions. Surprisingly, the surface of TOI-1468 b is marginally hotter than expected. This indicates an additional source of energy on the planet. This source might originate from a temperature inversion or induction heating, or it might be an instrumental artefact. The results within the Hot Rocks Survey build on the legacy of studying the atmospheres of exoplanets around M dwarfs. The outcome of this survey will prove useful to the large-scale survey of M dwarfs that was recently approved by the STScI.