Yesterday I read about a planet as is transit-close to red dwarf Gliese 1252. This planet has lost its atmo. Same time, I read about the marshmallow around TOI-3757, which - clearly - hasn't lost atmo.
Yeah, "TOI". TESS has been back in business for a week now, for those concerned.
As to the difference between airless lavaworld and fluff world: I expect a mass-constraint. TOI-3757b is 85 Earth-mass; this makes it a subSaturn (< 95). Saturn himself used to be presented to children as "could float on water!!".
So when and where did TOI-3757b form? Sure, the 1 AU region is chilly around this red dwarf now. But the very act of stellar-creation - you'd think - would have blasted all the system's hydrogen out to 3 AU. Earth hydrogen came from outside and even Ceres is thought to be a migrate from outside 5 AU. Maybe red-dwarfs form more quietly. Maybe the Saturnlike in 1 AU formed first and held onto its hat whilst the dust collapsed into a dwarf.
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