Hubble WFC3 Spectroscopy of the Terrestrial Planets L 98-59 c and d: No Evidence for a Clear Hydrogen Dominated Primary Atmosphere

The nearby bright M-dwarf star L 98-59 has three terrestrial-sized planets. One challenge remaining in characterizing atmospheres around such planets is that it is not known a priori whether they possess any atmospheres. Here we report on study of the atmospheres of L 98-59 c and L 98-59 d using near-infrared spectral data from the G141 grism of Hubble Space Telescope (HST)/Wide Field Camera 3. We can reject the hypothesis of a clear atmosphere dominated by hydrogen and helium at a confidence level of ~3σ for both planets. Thus they could have a primary hydrogen-dominated atmosphere with an opaque cloud layer, or could have lost their primary hydrogen-dominated atmosphere and re-established a secondary thin atmosphere, or have no atmosphere at all. We cannot distinguish between these scenarios for the two planets using the current HST data. Future observations with the James Webb Space Telescope would be capable of confirming the existence of atmospheres around L 98-59 c and d and determining their compositions.