Hot Rocks Survey IV: Emission From LTT 3780 b Is Consistent With A Bare Rock

It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing 9 rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres.

In this paper, we present two MIRI F1500W (15μm) eclipses of LTT 3780 b, an ultra-short period super-Earth (P=0.768 d, R=1.325R_⊕, M=2.46M_⊕) that receives 111x Earth’s instellation, the highest in the survey. We find a combined eclipse depth of 312±38 ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection.

This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of T_d=1143+104−99 K, 98±9 % of the maximum temperature predicted for a zero albedo, zero heat redistribution blackbody. We are able to confidently rule out CO₂-based atmospheres down to 0.01 bar surface pressure to greater than 3σ (ruling out an approximately Mars-like atmosphere).

We are unable to rule out a pure H₂O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O₂ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO₂-mixture atmospheres.

As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.

Natalie H. Allen, Néstor Espinoza, Hannah Diamond-Lowe, João M. Mendonça, Brice-Olivier Demory, Amélie Gressier, Jegug Ih, Mark Fortune, Prune C. August, Måns Holmberg, Erik Meier Valdés, Merlin Zgraggen, Lars A. Buchhave, Adam J. Burgasser, Chloe Fisher, Neale P. Gibson, Kevin Heng, Jens Hoeijmakers, Daniel Kitzmann, Bibiana Prinoth, Alexander D. Rathcke, Brett M. Morris

Comments: 16 pages, 5 figures, accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.14210 [astro-ph.EP] (or arXiv:2508.14210v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.14210
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Submission history
From: Natalie Allen
[v1] Tue, 19 Aug 2025 19:01:37 UTC (5,855 KB)
https://arxiv.org/abs/2508.14210

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