What makes something a planet?

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

Matt - There’s basically three rules, there’s three things you have to fulfil in order to be a planet. You have to be something orbiting a star. You have to be big enough to be spherical so you’ve got to have enough mass that gravity can pull you into a sphere,  and you’ve got to have cleared your orbit of debris. And so that third one is the slightly tricky one that tripped Pluto up.

Orbiting a star is kind of obvious, you’ve just gotta be gravitationally orbiting a star. Being spherical: it means about being big enough. If you don’t have enough mass you can be a slightly strange shape. Like Mars has two moons called Phobos and Deimos and they’re both quite small, and they’re both a bit peanut shaped because they’re not big enough to be pulled into a sphere.

So you’ve got to be orbiting a star, you’ve got to be big. But you’ve also got to have cleared your orbit of debris which means if you imagine looking at the solar system from above and drawing a circle at the orbit, you have to be the most major thing in that orbit to be classed as a planet.

It was that third definition that was brought in in 2006 that tripped Pluto up because while the eight planets that we know of know are in nice clean orbits around the Sun, Pluto orbits in a very messy outer part of the solar system called the Kuiper Belt, and there’s all kinds of rocks and space junk out there. The definition was almost brought in because, if you come onto the second part of your question because out in the Kuiper Belt there are almost certainly tens of thousands of things, or even hundreds of thousands of things similar to Pluto. So without this third definition, this orbit clearing definition, you can end up with tens of thousands of planets in the solar system which is just going to be kind of exhausting. So we say that all of these things in the messy part of the solar system are dwarf planets, and then all the B8 inner planets are the classical planets.

Chris - Jason?

Jason - I was going to ask on the debris question, does that mean that the boundary of what is a planet is always going to be in relation to the size of the star and the distance from the star? Basically, if gravitational pull is low enough that you’re not clearing out a bunch of this debris does that just mean that if our Sun was bigger that there’d be less debris out at that level and then Pluto would be cleared to be a planet?

Matt - I think the debris isn’t so much a function of the gravitational pull of the star. It’s about the gravitational pull of the planet itself, so the debris is the stuff that’s left over from the formation of the solar system. Like gathering all the matter to itself and growing and, for the eight planets that we care about, like the A classical planets... I’m being very hard on Pluto now aren’t I? For the eight planets of the solar system they have all the matter that was around in their orbit as they were growing they accreted onto themselves. The outskirts of the solar system, the Kuiper Belt where Pluto lives, this is almost where planet formation didn’t fully complete if you like. So there’s a lot of this debris left over.

Chris - Quick question then, so I can understand say in the region of Jupiter, its own gravity is pretty powerful, so anything in the environs of the planetas it forms are going to be drawn into it. But what about the things on the opposite side of its orbit? So you know, the other side of the Sun form where Jupiter is, how would that get cleared and become part of Jupiter or wouldn’t it al all?

Matt - Well, you have to think about the early solar system as being a very very messy, chaotic place with all kinds of bodies forming and crashing into each other. And thing form heirarchally, they form bottom up in the solar system so individual molecules of gas and dust in the primordial clouds coalesce together. And then you go from molecules of gas to dust grains, to tiny pebbles, to slightly bigger pebbles, to rocks, to mountains, to planetesimals, to planets. And all the time this process is going on, you have all kinds of jostling and colliding. It’s a very chaotic process and it’s only over billions of years that you end up with just one single planet orbiting at that distance.