Hubble WFC3 Spectroscopy of the Rocky Planet L 98–59 b: No Evidence for a Cloud-free Primordial Atmosphere

Abstract

We are using archived data from HST of transiting exoplanet L 98-59 b to place constraints on its potentially hot atmosphere. We analyze the data from five transit visits and extract the final combined transmission spectrum using Iraclis. Then we use the inverse atmospheric retrieval code TauREx to analyze the combined transmission spectrum. There is a weak absorption feature near 1.40 μm and 1.55 μm in the transmission spectrum, which can be modeled by a cloudy atmosphere with abundant hydrogen cyanide (HCN). However, the unrealistically high abundance of HCN derived cannot be explained by any equilibrium chemical model with reasonable assumptions. Thus, the likeliest scenario is that L 98-59 b has a flat, featureless transmission spectrum in the WFC3/G141 bandpass due to a thin atmosphere with high mean molecular weight, an atmosphere with an opaque aerosol layer, or no atmosphere, and it is very unlikely for L 98-59 b to have a clear hydrogen-dominated primary atmosphere. Due to the narrow wavelength coverage and low spectral resolution of HST/WFC3 G141 grism observation, we cannot tell these different scenarios apart. Our simulation shows future higher precision measurements over wider wavelengths from the James Webb Space Telescope can be used to better characterize the planetary atmosphere of L 98-59 b.

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

“…Theoretical calculations suggest that the retention of a pure-H 2 O atmosphere is also difficult because of a high escape efficiency (Johnstone 2020). All these scenarios agree well with the HST observations (Damiano et al 2022;Zhou et al 2022). L 98-59 c and d are also near the cosmic shoreline, and have likely experienced a runaway greenhouse phase (Demangeon et al 2021;Pidhorodetska et al 2021).…”

“…Demangeon et al (2021) found evidence for a fourth non-transiting planet, and a possible fifth non-transiting small planet in the system. The atmospheric properties of the innermost planet L 98-59 b were studied by Zhou et al (2022) and Damiano et al (2022). Both studies rejected the hypothesis that L 98-59 b has a cloud-free hydrogen-dominated primary atmosphere.…”

Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab

“…With the data presented in this work we rule out a clear, lowmean-molecular-weight atmosphere at 10 bars of surface pressure (3.2σ) and 1 bar of surface pressure (3.1σ). This result joins previous work on GJ 1132b (Diamond-Lowe et al 2018;Mugnai et al 2021;Libby-Roberts et al 2022), TRAPPIST-1a-f (de Wit et al 2016), LHS 3844 (Diamond-Lowe et al 2020b), L 98-59b-d (Damiano et al 2022Zhou et al 2022Zhou et al , 2023Barclay et al 2023), andLHS 475b (Lustig-Yaeger et al 2023) to strongly suggest that highly-irradiated terrestrial exoplanets orbiting M dwarfs are not capable of retaining low-mean-molecular-weight atmospheres, if they are able to accrete them in the first place. It is possible, however, that LTT 1445Ab possesses a high-meanmolecular-weight or cloudy/hazy atmosphere, which would fall below our detection limits, or no atmosphere at all.…”

Potential Melting of Extrasolar Planets by Tidal Dissipation

“…It is possible that the nonzero eccentricities of the planetsdespite the significant tidal heating-could be explained by forced eccentricity from the mean motion resonance. Measurements obtained with the Hubble Space Telescope (HST) of L 98-59 b were consistent with no atmosphere (Damiano et al 2022;Zhou et al 2022). However, Barclay et al (2023) presented preliminary and marginal evidence of an atmosphere on L 98-59 c with HST measurements, although this has since been debated (Zhou et al 2023).…”