Products for "TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere"

Common features of sub-Neptunes 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 µm JWST transmission spectrum of the sub-Neptune TOI-421 b.  This planet is unique among previously observed counterparts in its high equilibrium temperature (Teq  ~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 SO2 and CO but not CO2 or CH4) also exist in our NIRSpec/G395M observations, but not at sufficient signal-to-noise to draw firm conclusions.  Our results support a picture in which sub-Neptunes hotter than ~850 K do not form hydrocarbon hazes due 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.