LHS 1903

Thomas Wilson and-Co. published DOI 10.1126/science.adl2348 last February. Science Daily, late as ever, has delivered the ESA' press release. Adam Mann at Science News might have the better summary.

The LHS 1903 system is 116 ly away, a red dwarf half Solar mass. Its four planets are all too hot as well as tidally-locked. The outer planet e is raising the hype because it is 1.7 Earth radii. Up to now, we tend to get Earthlikes up to 1.6 and then the "superEarths" from 1.8 up, more like miniNeptunes.

LHS 1903 e turns out to be Earthlike in density, not a Neptunian at all. It could have held onto a Venereal cloudy Sudarsky II layer but, it seems, not.

Wilson posits that the system simply ran out of gas, if I may: the "Gas Depleted Formation" / G-DF. The planets formed from the inside, outward. The fourth, and whatever might exist in the habitable zone on out, did not have anything left.

That G-DF model assumes planetary migrations at least for e. And yeah: no migrations for systems like this tend, I think, to produce orbital resonances. The periods are 2.2, 6.2, 12.6, and 29.3 days. Perhaps for b:c:d, 6:2:1 be close-enough (libration?). Planet e as the literal outlier, like Callisto outside Jupiter, demands a different theory. They ponder if it got smashed up early, such that on its final formation out there the ices weren't available.