Atmospheric Characterization of the Temperate Planet LHS1140b with JWST/NIRISS – Is LHS1140b a Mini-Neptune or a Water-World?

LHS1140b is one of the very few nearby transiting exoplanets with a temperate equilibrium temperature suitable for habitable surface conditions. A recent study (Cadieux et al. 2023) revealed that this planet is less massive and larger than previous estimates strongly suggestive of a significant volatile-rich envelope. More specifically, internal structure modelling informed by stellar abundances of refractory elements (Fe, Si and Mg) supports two exciting new possibilities for its nature: LHS1140b is either a mini-Neptune with a small (0.1%) H/He mass envelope or a water world with 10–20% of its bulk mass made of water. Global Climatic Modelling suggests liquid condition at the sub-stellar point whose extent depends on CO2 concentration. Discriminating the mini-Neptune and water-world scenarios requires atmospheric characterization with JWST through transmission spectroscopy over the widest possible wavelength range to assess the potential issue of stellar contamination shown to be a serious challenge for active M dwarfs like Trappist-1. We will present the results of a reconnaissance transmission spectroscopy program of LHS1140b with JWST/NIRISS SOSS, two visits with a simultaneous wavelength coverage from 0.6 to 2.8 microns. We will discuss the implications of these results for future in-depth atmospheric characterization of this keystone planet.