Hot Rocks Survey

III. A deep eclipse for LHS 1140c and a new Gaussian process method to account for correlated noise in individual pixels

¹ School of Physics, Trinity College Dublin, University of Dublin, Dublin 2, Ireland
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, 21218, MD, USA
³ Department of Space Research and Space Technology, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
⁴ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁵ School of Ocean and Earth Science, University of Southampton, Southampton, SO14 3ZH, UK
⁶ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁷ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁸ Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
⁹ Centre for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁰ ARTORG Center for Biomedical Engineering Research, University of Bern, Murtenstrasse 50, 3008, Bern, Switzerland
¹¹ Ludwig Maximilian University, Faculty of Physics, Scheinerstr. 1, Munich 81679, Germany
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

^★ Corresponding author: fortunma@tcd.ie

Received: 20 February 2025
Accepted: 27 May 2025

Abstract

Time-series photometry at mid-infrared wavelengths is becoming a common technique to search for atmospheres around rocky exoplanets. This method constrains the brightness temperature of the planet to determine whether heat redistribution is taking place, which would be indicative of the presence of an atmosphere, or whether the heat is reradiated from a low-albedo bare rock. By observing at 15μm, we are also highly sensitive to CO₂ absorption. We observed three eclipses of the rocky super-Earth LHS 1140c, using MIRI/Imaging with the F1500W filter. We found a significant variation in the initial settling ramp for these observations and identified a potential trend between the detector settling and the previous filter used by MIRI. We analysed our data using aperture photometry, however, we also developed a novel approach, which performs a joint fit of the pixel light curves using a shared eclipse model and a flexible multi-dimensional Gaussian process which can model changes in the PSF over time. Using simulated data, we demonstrate that our method has the ability to weight away from particular pixels that exhibit increased systematics, allowing for the recovery of eclipse depths in a more robust and precise way. Both methods, as well as an independent analysis, have detected the eclipse at >5σ, while recovering an eclipse depth consistent with a low-albedo bare rock. We measured a dayside brightness temperature of T_day = 561 ± 44 K, close to the theoretical maximum of T_{day; max} = 537 ± 9 K. We rule out a wide range of atmospheric forward models to >3σ, including pure CO₂ atmospheres with surface pressure ≥10 mbar and pure H₂O atmospheres with surface pressure ≥1 bar. Our strict constraints on potential atmospheric composition, in combination with future observations of the exciting outer planet LHS 1140b, could provide a powerful benchmark for understanding atmospheric escape around M dwarfs.