A Transmission Spectrum of the Sub-Earth Planet L98-59 b in 1.1–1.7 μm

Abstract

With the increasing number of planets discovered by the Transit Exoplanet Survey Satellite, the atmospheric characterization of small exoplanets is accelerating. L98-59 is an M-dwarf hosting a multiplanet system, and so far, four small planets have been confirmed. The innermost planet b is ∼15% smaller and ∼60% lighter than Earth, and should thus have a predominantly rocky composition. The Hubble Space Telescope observed five primary transits of L98-59 b in 1.1–1.7 μm, and here we report the data analysis and the resulting transmission spectrum of the planet. We measure the transit depths for each of the five transits and, by combination, we obtain a transmission spectrum with an overall precision of ∼20 ppm in for each of the 18 spectrophotometric channels. With this level of precision, the transmission spectrum does not show significant modulation, and is thus consistent with a planet without any atmosphere or a planet having an atmosphere and high-altitude clouds or haze. The scenarios involving an aerosol-free, H2-dominated atmosphere with H2O or CH4 are inconsistent with the data. The transmission spectrum also disfavors, but does not rule out, an H2O-dominated atmosphere without clouds. A spectral retrieval process suggests that an H2-dominated atmosphere with HCN and clouds or haze may be the preferred solution, but this indication is nonconclusive. Future James Webb Space Telescope observations may find out the nature of the planet among the remaining viable scenarios.