¹Kimberly Paragas, ¹Heather A. Knutson, ^2,3,4,5,6Renyu Hu, ^1,7,8,9Bethany L. Ehlmann, ¹⁰Aishwarya R. Iyer, ¹¹Sebastian Zieba, ¹²Laura Kreidberg
The Astrophysical Journal Letters, 1011, L8
Open Source Link to Article [DOI: 10.3847/2041-8213/aeb00b]

¹Division of Geological and Planetary Sciences, California Institute of Technology, USA
²Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
³Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁴Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802, USA
⁵Institute for Computational and Data Science, The Pennsylvania State University, University Park, PA 16802, USA
⁶Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁷Laboratory for Atmospheric & Space Physics, University of Colorado Boulder, CO 80309, USA
⁸Department of Earth Science, University of Colorado Boulder, CO 80309, USA
⁹Department of Astrophysical & Planetary Sciences, University of Colorado Boulder, CO 80309, USA
¹⁰NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA 02138, USA
¹²Max-Planck-Institut für Astronomie, D-69117 Heidelberg, Germany

The thermal emission spectra of terrestrial exoplanets provide a window into their surface compositions and
corresponding geological processes. Close-in rocky planets orbiting M dwarfs are ideal targets for these studies,
as observations have shown that most have little to no atmosphere. LHS 3844 b is an ultrashort-period super-Earth
orbiting a nearby M dwarf, and is the most favorable target that does not have a dayside magma ocean. We
present an updated JWST/MIRI Low Resolution Spectroscopy emission spectrum (5–12μm) of this planet,
derived from eight new and three archival eclipse observations. We combine these visits to obtain the highest
signal-to-noise ratio (SNR) emission spectrum for any rocky exoplanet thus far, with a median SNR of 29 across
12 wavelength bins. Our data are sensitive to the predicted Si–O stretching feature (the strongest mid-infrared
silicate feature) for a wide range of common materials, and the transparency feature, which is relatively
insensitive to composition and constrains the grain size and porosity of surface materials. We find that
LHS3844 b’s emission spectrum resembles a blackbody shortward of 10 μm, disfavoring the presence of a
transparency feature, and identify a tentative feature (1.9σ−3.5σ) between 10 and 12 μm. If this feature is
astrophysical and not instrumental in nature, it can be matched by a fayalite (the iron end-member of olivine)
surface, but might also be consistent with other candidate planet-forming materials like spinel or silicon carbide.
We discuss possible biases introduced by the wavelength-dependent instrumental ramp, and outline several
strategies to mitigate this effect in future observations of LHS 3844 b.