Is the gas in gas giants highly compressed?

Dominic - Well, planets like Jupiter certainly do have cores. Jupiter, we think, has a rocky core that's about 10 times more massive than Earth. Jupiter itself is a really vast planet. It's got about 300 times the mass of Earth and about 10 times the radius of the Earth and most of that volume, most of that mass is a mixture of hydrogen and helium gas. That gas is very heavily compressed and that's how Jupiter manages to be so very massive.

Actually it is in a state called metallic hydrogen, where these molecules are so compressed together that they form a lattice and the electrons, rather than orbiting around individual hydrogen nuclei, actually can flow freely through that metallic hydrogen. That's why Jupiter has such a strong metallic field - because the electrons flow through the hydrogen producing that electric field.

Chris - How did it get all of that gas in the first place? Hydrogen and Helium being so light, how did they manage to coalesce around Jupiter before it got big and had all of that gravity?

Dominic - That's an interesting question that people are actually still researching. But I think that the best theory at the moment is that when a planet gets to a mass of ten times that of the Earth, it's gravitational field is then so strong that it can pull in gas around it and you can get this sudden catastrophic fall of material onto this planet. So any planet that is less than ten times the mass of the Earth will tend to be rocky, like the inner planets of the solar system. Any planets that creep over that mass suddenly turn into these vast gas giants like Jupiter and Saturn.