Hot Rocks Survey. IV. Emission from LTT 3780 b Is Consistent with a Bare Rock

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 nine rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15 μ m) eclipses of LTT 3780 b, an ultrashort-period super-Earth ( P = 0.768 days, R = 1.325 R ⊕ , M = 2.46 M ⊕ ) that receives 111× 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>day</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>114</mml:mn> <mml:msubsup> <mml:mn>3</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>99</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>104</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K, 98% ± 9% 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 a 0.01 bar surface pressure to greater than 3 σ (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.