What's the size limit of a planet?

We had this sizeable question from Craig, it was over to planetary geologist David Rothery to crunch the numbers...

David - Yes, there’s a limit to both. It’s really a mass limit rather than a size limit. We’re discovering exoplanets around other stars, and in the cases where you can see both their size because they pass in front of the star, and their mass because they disturb the star’s position, we can pin down the size and mass of these bodies.

There are bodies that are about the right density to be rocky planets up to about ten times the Earth’s mass. So from a hundreth of the Earth’s mass up to about ten times the Earth’s mass you can have a rocky planet. But given that,  Jupiter’s 3 hundred times the mass of the Earth, if you get up to about 13 times the mass of Jupiter we don’t regard the thing as a planet anymore. Because at 13 times Jupiter’s mass the pressure inside that body is going to be sufficiently high that the heavy form of hydrogen - deuterium - can be fused together and you can have deuterium fusion and it will be sort of a star called a brown dwarf. Not a proper star fusing common hydrogen, to do that you have to be about 80 times Jupiter’s mass. So that would be a real star. But certainly, 13 times Jupiter’s mass upwards it’s not really a planet, and the rocky planets have to be smaller than that.

And in between we’ve got ice giants like Uranus and Neptune. And a bit smaller than than that: not exemplified in this solar system we think of waterworlds. Bodies which have a lot of rock but surrounded by water, so very deep oceans.

Chris - So that would be a quite an interesting place to go surfing wouldn’t it?

David - No, because there wouldn’t be a beach so there’s nowhere for the waves to break so you could bob up and down and sail around them but you wouldn’t surf.

Chris - How much pressure would the water experience? Is it possible to get a planet that’s got so much water, such a deep ocean that the pressure is so high that the water would actually turn to a solid at the bottom of that ocean?

David - There are various high pressure forms of ice known and you don’t need terribly high pressure to do that. One of Jupiter’s moons - Ganymede - the largest moon in the solar system has ordinary ice. Ice - one at the surface. Below that it has a layer of liquid water and below that we’ve got higher pressure forms of ice there. So the right temperature conditions, you don’t need a very strong gravity field to get you into these high pressure ice forms.