Smaller and cooler than the gas giants, Neptune and Uranus are classified as ice giants. It’s a good name, since they do have large quantities of water ice mixed in with a largely hydrogen and helium atmosphere. There’s very little water at the cloud tops, but the percentage of water increases as you descend towards the heavier core. Could there be a layer on Neptune with enough pressure and temperature for liquid water to form into vast oceans? And if not Neptune, what about a Neptune-like planet orbiting another star? First, a little about Neptune. This “ice giant” planet orbits...