Reanalysis of the Eclipses of LHS 1140 c: No Evidence of an Atmosphere and Implications for the Internal Structure of the Planet

Abstract We present a reanalysis of three 15 μ m JWST/MIRI secondary eclipses of LHS 1140 c, a warm super-Earth ( R p = 1.272 R ⊕ ) in a 3.78 day orbit around an M4.5 dwarf. We present a novel method for data reduction that leverages spatial derivatives of the point-spread function and compare it to widely used aperture photometry. Both methods yield an eclipse depth consistent within 1 σ of the values reported in the literature. We measure an eclipse depth of 271 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ppm corresponding to a brightness temperature of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>59</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>34</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>33</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K, consistent with a bare rock. The secondary eclipse occurs 4.1 ± 0.8 minutes before the circular-orbit-predicted time. We explore the implications of our results on the internal structure of LHS 1140 c, the orbital architecture of the system, and the possibility of future observations with JWST. We find a core-mass fraction (CMF) informed by the stellar abundances of refractory elements of 0.34 ± 0.11, inflated compared to the CMF from radius and mass measurements, suggesting the possible presence of bulk volatiles in the interior.