The Kepler 138 C and D are found to be so called water worlds with a small solid nucleus inside.

.Supernovas are supposed to produce gas, ice and rocks combined with starspot based comet micro black holes.and nuclear macro black holes. often leaving as singular Tadpoles forming dual Herbig Haro new star forming systems. ( Q-FFF Theory)

According to Q-FFF Theory, hot waterplanets seem ro be formed by multiple Comet alike micro black holes suggested to be former starspots, expelled by Nova explosions.
In Q-FFF Theory, each black hole will produce and expel water and gas around the Light horizon, by the Penrose so called trapped space horizon.
Astronomers find that two exoplanets may be mostly water around a small red star.
A team led by UdeM astronomers has found evidence that two exoplanets orbiting a red dwarf star are "water worlds,"
see:  https://phys.org/news/2022-12-montreal-astronomers-exoplanets.html
We might start finding a lot more water worlds like Kepler-138 c and d," Benneke concluded.