[truculent]

I think this is supposedly a Hycean (ocean + hydrogen atmosphere) planet, rather than rocky. Unless those categories aren’t exclusive.

[billforsternz]

Sorry, didn't notice this in a timely way. I went back to the library to check, and indeed the planet I was reading about was K2-18b. The book at least says that K2-18b is a Super-Earth and will almost certainly have a molten rocky core.