What's at the centre of a gas giant?

Chris Smith put this question to Cambridge University astronomer Matt Bothwell...

Matt - A gas giant is a big gassy planet. If you think of something like Jupiter or Saturn, so instead of what we call the terrestrial or rocky planets like Earth, they’re much bigger and they're not rocky, they’re gassy. The centre of them - they’re not 100% gas. They do have various weird and wonderful things in their centre. If you imagine falling down into Jupiter, the outside is mostly gas and that’s the outer cloud layer that we see.

Chris - What gas is it?

Matt - It’s mostly hydrogen in Jupiter. And then, as you descend into the planet, things get quite strange and the pressure goes higher and higher, and the temperature goes higher and higher and then, eventually, the hydrogen gets pressed into something called a Supercritical fluid. It’s a weird and wonderful state of matter that we don’t really have on Earth because you need very very high pressures.

Below that, the hydrogen is compressed even more into a state of matter called Metallic Hydrogen, so it’s hydrogen that’s been squashed so hard it basically becomes a metal. And that’s how you get massive magnetic fields flowing through there so that’s why Jupiter is like a big magnet in the sky, it’s because of all this metallic hydrogen in it.

And then, at the very centre, there is a solid core. At the very centre of Jupiter there will be a kind of rocky, icy core at the middle and we’ve been able to study that just gravitationally. So when we send missions to Jupiter, we look at the moons orbiting the planet and we study the gravitational interaction and that lets us make a very accurate map of Jupiter’s gravitational field that can only be explained by having a solid core in the centre.

Jason - Can I ask, do we know what the mineral composition of the core is, or is there a decent hypothesis for it?

Matt - I think the answer is no, we really don’t know to be honest. We know it has to be there gravitationally. We have theories based on how planets form so we expect it to be kind of rocky, icy. About 4½ billion years ago during the formation of the solar system there were lots of these rocky, icy planetesimals, and one of these probably acted like a seed, if you like. And then it was far enough out in the solar system, this rocky, icy planetesimal managed to accrete lots and lots of gas and grow very very large. So we think we understand how the solar system was formed reasonably well but yes, it’s not been directly confirmed.