TOI-421 b: A Haze-Free, Low-Mean-Molecular-Weight Hot Sub-Neptune
A JWST 0.83-5 micron transmission spectrum of the hot (~920 K) sub-Neptune TOI-421 b, which orbits a Sun-like star, reveals water features indicating a low-mean-molecular-weight, roughly solar-metallicity atmosphere with no appreciable aerosols, unlike the hazy, high-metallicity atmospheres of cooler sub-Neptunes around M dwarfs. The finding supports the idea that sub-Neptunes hotter than ~850 K lack hydrocarbon hazes for want of methane to photolyze, and that such planets retain primordial envelopes over rocky interiors as expected if the radius valley is sculpted by atmospheric mass loss.
Draft version January 6, 2025 Typeset using LATEX twocolumn style in AASTeX631 TOI-421 b: A Hot
The authors present a 0.83-5 micron JWST transmission spectrum of TOI-421 b, a hot sub-Neptune (equilibrium temperature ~920 K) orbiting a Sun-like (FGK) star, in contrast to previously JWST-characte…