TOI-421 b: A Hot Sub-Neptune with a Haze-free, Low Mean Molecular Weight Atmosphere

Abstract Common features of sub-Neptune atmospheres observed to date include signatures of aerosols at moderate equilibrium temperatures (∼500–800 K) and a prevalence of high mean molecular weight atmospheres, perhaps indicating novel classes of planets such as water worlds. Here we present a 0.83–5 μ m JWST transmission spectrum of the sub-Neptune TOI-421 b. This planet is unique among previously observed counterparts in its high equilibrium temperature ( T eq ≈ 920 K) and its Sun-like host star. We find marked differences between the atmosphere of TOI-421 b and those of sub-Neptunes previously characterized with JWST, which all orbit late K and M stars. Specifically, water features in the NIRISS/SOSS bandpass indicate a low mean molecular weight atmosphere consistent with solar metallicity and no appreciable aerosol coverage. Hints of SO 2 and CO (but not CO 2 or CH 4 ) also exist in our NIRSpec/G395M observations, but not at sufficient signal-to-noise ratio to draw firm conclusions. Our results support a picture in which sub-Neptunes hotter than ∼850 K do not form hydrocarbon hazes owing to a lack of methane to photolyze. TOI-421 b additionally fits the paradigm of the radius valley for planets orbiting FGK stars being sculpted by mass-loss processes, which would leave behind primordial atmospheres overlying rock/iron interiors. Further observations of TOI-421 b and similar hot sub-Neptunes will confirm whether haze-free atmospheres and low mean molecular weights are universal characteristics of such objects.