How Do We Know The Gas Planets Don't Have A Solid Surface?

[FuzzyPlum]

Originally Posted by singertobe

Something I've been thinking about for a while. Nobody has ever been straight through the center of any of the gas planets so how do we know there isn't a solid surface buried all the way underneath all the gas clouds? There must be something that differentiates a gas planet from a star that's made of the same gases...

I think some or all of them do have a rocky core deep inside (or at least they are believed to have rocky cores).