2025AJ....170...49L

We report JWST NIRSpec/G395H observations of TOI-1685 b, a hot rocky super-Earth orbiting an M2.5V star, during a full orbit. We obtain transmission and emission spectra of the planet and characterize the properties of the phase curve, including its amplitude and offset. The transmission spectrum rules out clear H₂-dominated atmospheres, while secondary atmospheres (made of water, methane, sulfur dioxide, or carbon dioxide) cannot be statistically distinguished from a flat line. The emission spectrum is featureless and consistent with a dark surface, ruling out carbon dioxide-, sulfur dioxide-, or methane-dominated atmospheres thicker than 100 mbar. Collecting all evidence, the properties of TOI-1685 b are consistent with a blackbody with no heat redistribution and a low albedo, with a dayside brightness temperature 0.99 ± 0.07 times that of a perfect blackbody in the NIRSpec NRS2 wavelength range (3.823-5.172 μm). Our results add to the growing number of seemingly airless M-star rocky planets, thus constraining the location of the “Cosmic Shoreline.” Three independent data reductions have been carried out, all showing a high-amplitude correlated noise component in the white and spectroscopic light curves. The correlated noise properties are different between the NRS1 and NRS2 detectors-importantly, the timescales of the strongest components (4.5 hr and 2.5 hr, respectively)-suggesting the noise is from instrumental rather than astrophysical origins. We encourage the community to look into the systematics of NIRSpec for long time series observations.