A transmission spectrum of the sub-Earth planet L98-59~b in 1.1-1.7 $\mu$m

TL;DR

This study presents the transmission spectrum of the sub-Earth exoplanet L98-59b, obtained with Hubble, which constrains its atmospheric composition and suggests it may lack an atmosphere or have high-altitude clouds, with future JWST observations needed.

Contribution

First transmission spectrum of L98-59b with high precision, providing constraints on its atmospheric composition and ruling out certain atmospheric scenarios.

Findings

Spectrum consistent with no atmosphere or high-altitude clouds/haze

Inconsistent with aerosol-free H2-dominated atmospheres with H2O or CH4

Favors a possible H2 atmosphere with HCN and clouds/haze, but inconclusive.

Abstract

With the increasing number of planets discovered by TESS, the atmospheric characterization of small exoplanets is accelerating. L98-59 is a M-dwarf hosting a multi-planet system, and so far, four small planets have been confirmed. The innermost planet b is $\sim15\%$ smaller and $\sim60\%$ lighter than Earth, and should thus have a predominantly rocky composition. The Hubble Space Telescope observed five primary transits of L98-59b in $1.1-1.7\ \mu$m, 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 $\sim20$ 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, H$_2$-dominated atmosphere with H$_2$O or CH$_4$ are inconsistent with the data. The transmission spectrum also disfavors, but does not rules out, an H$_2$O-dominated atmosphere without clouds. A spectral retrieval process suggests that an H$_2$-dominated atmosphere with HCN and clouds or haze may be the preferred solution, but this indication is non-conclusive. Future James Webb Space Telescope observations may find out the nature of the planet among the remaining viable scenarios.