Hints of a sulfur-rich atmosphere around the 1.6 R${}_{\oplus }$ Super-Earth L98-59 d from JWST NIRSpec G395H transmission spectroscopy

Abstract

Detecting atmospheres around planets with a radius below 1.6 R${}_{\oplus }$, commonly referred to as rocky planets (Rogers_2015, Rogers_2021), has proven to be challenging. However, rocky planets orbiting M-dwarfs are ideal candidates due to their favorable planet-to-star radius ratio. Here, we present one transit observation of the Super-Earth L98-59d (1.58 R${}_{\oplus }$, 2.31 M${}_{\oplus }$), at the limit of rocky/gas-rich, using the JWST NIRSpec G395H mode covering the 2.8 to 5.1 microns wavelength range. The extracted transit spectrum from a single transit observation deviates from a flat line by 2.6 to 5.6$\sigma$, depending on the data reduction and retrieval setup. The hints of an atmospheric detection are driven by a large absorption feature between 3.3 to 4.8 microns. A stellar contamination retrieval analysis rejected the source of this feature as being due to stellar inhomogeneities, making the best fit an atmospheric model including sulfur-bearing species, suggesting that the atmosphere of L98-59d may not be at equilibrium. This result will need to be confirmed by the analysis of the second NIRSpec G395H visit in addition to the NIRISS SOSS transit observation.

Accepted for publication in the Astrophysical Journal Letters (ApJL), August 25, 2024

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