Hot Rocks Survey IV: Emission from LTT 3780 b is consistent with a bare rock
/ Authors
N. H. Allen, N. Espinoza, H. Diamond-Lowe, J. Mendoncca, B. Demory, A. Gressier, Jegug Ih, M. Fortune, P. C. August, M. Holmberg
and 12 more authors
E. M. Vald'es, Merlin Zgraggen, Lars A. Buchhave, A. Burgasser, C. Fisher, N. P. Gibson, K. Heng, J. Hoeijmakers, D. Kitzmann, B. Prinoth, A. Rathcke, Brett M. Morris
/ Abstract
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 (15$\mu$m) eclipses of LTT 3780 b, an ultra-short period super-Earth ($P=0.768$ d, $R=1.325 \,R_\oplus$, $M = 2.46\,M_\oplus$) that receives 111x Earth's instellation, the highest in the survey. We find a combined eclipse depth of $312\pm38$ 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, $98\pm9$ % of the maximum temperature predicted for a zero albedo, zero heat redistribution blackbody. We are able to confidently rule out CO$_2$-based atmospheres down to 0.01 bar surface pressure to greater than 3$\sigma$ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H$_2$O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O$_2$ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO$_2$-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.