@badastro "K2-18b (note the lower-case “b”, denoting it’s a planet) is about 2.6 times the Earth’s diameter and 8.6 times its mass. Some people call it a super-Earth, but in my opinion that’s misleading; it’s more likely a mini-Neptune at that size and mass (so a small gas giant instead of gigantic rocky world)"
8.6x Earth mass with only 2.6x diameter is way too dense to be a gas giant.
Hm, I take that back. volume = 4/3 pi * r^3, 2.6x diam is 17.5x larger volume. Half density of earth, ok.