[3][4] Observations of variability related to its ~42-day rotation indicate a low level of stellar activity with only few surface spots, dominated by plages.
[3][4] However, the observed density is not consistent with a thin, hydrogen-depleted atmosphere expected from evolutionary models for a planet forming this close to the star.
Planetary migration is a plausible explanation, meaning that the planet could have formed further away from the star and only more recently moved to current location.
[4] TOI-421 b was selected as a James Webb Space Telescope target early on as a planet similar in size and density to the well-studied sub-Neptune GJ 1214 b, but thought to be too hot for formation of atmospheric haze, which would allow for bulk chemical characterization of atmopshere.
This is in contrast to the observed composition of sub-Neptunes around red dwarf stars, which typically show a much higher fraction of heavier gas species.