What is the difference between terrestrial planets and gaseous planets?

Baloo55th

Rock. We have a rock and iron core and a reasonably solid structure. OK, there are sort of molten layers that the continental plates skid around on (India is still heading north, for example). But Saturn is made up of gas. To use a rather silly analogy, it would float on water (I've never liked that one, but can't think of a better way of putting it). They used to think that planets like Jupiter were failed suns - they never ignited - but they've changed their minds about that now. The gaseous ones can have hydrogen as they are so big that their gravity will keep the hydrogen. It leaks off from smaller places like us - we can keep oxygen, nitrogen and so on because they're denser. Mercury (the planet) is too small to keep any gases in its atmosphere. In some ways, the gaseous planets are all atmosphere, but don't quote me on that! Feb 18 06, 3:50 PM