Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b

Abstract “Lava worlds”, Earth-sized planets hot enough ( T eq ≳ 1100 K) to melt their dayside silicate surfaces, have emerged as promising candidates for atmospheric detection and characterization. Thermal emission observations show an apparent dichotomy: The hottest lava worlds have colder daysides than the temperature of a maximally emitting bare rock, indicating the likely presence of thick and/or reflective atmospheres, while the coldest ones do not. However, where in instellation flux this potential bifurcation occurs is uncertain. We present a JWST/MIRI Low Resolution Spectrometer eclipse of the ultra-short-period (USP) lava world HD 3167 b ( T eq = 1786 K, R = 1.6 R ⊕ , P = 0.96 day) that helps bridge this gap. We measure the white-light eclipse depth to be 38 ± 11 ppm, more than 5 σ lower than the expected eclipse depth of a dark, maximally hot bare rock. We use this to derive a dayside brightness temperature that is best explained by the presence of an atmosphere that cools the dayside by reflecting incoming starlight and/or efficiently redistributing heat to the planet’s nightside. An atmosphere is further compatible with the planet’s slight underdensity compared to an Earth-like composition. The corresponding dayside emission spectrum is not precise enough to constrain atmospheric composition, motivating follow-up spectroscopic observations with JWST/NIRSpec. Lastly, we use our observation and existing data to refine key planetary parameters of the HD 3167 system. HD 3167 b is currently the least irradiated USP super-Earth with evidence for an atmosphere.