 I'm Brad here at A&U and I'm with Nathalia and Eloise and Adam from ABC Canberra talking planets and stars and moons. So how do we actually define these? What is the difference between a planet and a moon? So like you said, in the solar system we have many different objects. We have stars, planets, asteroids, moons, but what makes a planet a planet? So you may have heard that Pluto is no longer classified as a planet. It's now classified as a dwarf planet. What does that actually mean? So in 2004 the International Astronomical Union decided upon three rules as to what defines a planet. Do you think you can guess what they are? One, that it has to have a sun or be near a sun. Yes, so they all need to go around a star. So in our solar system that star is going to be the sun. But in another solar system if we've got an exoplanet it's going around a different sun. So it'll have a different star in its system. What do you think the next one might be? That it has to be a certain size or big enough. Yes, so the second rule of being a planet is that it has to be spherical when it collapses under its own gravity. So it can't be potato shaped like an asteroid. What about the third and final? We've done a star, we've done its size. Oh the third one must be a bit of a toughie. It is actually quite tricky so it needs to have cleared its orbit. Cleared its orbit? What does that mean? Well because that's a little bit difficult we're going to explain it in more detail here. So this is where Pluto fails here. It hasn't cleared its orbit because it's not the biggest thing in its orbit. It crosses over Neptune's orbit so that's not quite right. Here we're going to demonstrate clearing an orbit with a bouncy ball and a marble and some sugar which is going to be asteroids or other objects in the system. So as Natalaya moves around the marble you can see it's starting to clear a gap and that's clearing out its orbit. So now it's going to be the biggest thing in its orbit. Do all planets have to orbit around suns? Yes and no. So if a planet is orbiting around a sun it's not necessarily orbiting around the sun but rather the sun and the planet are orbiting around each other. And this is the centre of mass. So what we're going to do is explain this using a spoon, a bouncy ball which is going to be Pluto and a marble which is going to be Charon. So this is a good, because Pluto and Charon are similar masses, it's a nice example. So here I'm balancing the spoon on my fingers and you can see that's where the centre of mass is. So you can try this at home with a normal spoon, whoops, as I'm doing now, which is the centre of mass is there. So it's closer to Pluto than it is to Charon because Pluto is a bit more massive but it's outside of both of the objects. So if we do a similar example but rather with the sun being this shiny yellow ball and this little marble being Jupiter, which is actually to scale, if I try and balance it to the centre of mass we can see that it is a lot closer to the sun than what it is to Jupiter. So if we're going to do this with the Earth, you wouldn't see Earth on this scale, it'd be too small but we'd actually be having to hold the sun and that is because the centre of mass between the Earth and our sun is inside the sun. So now that we've spoken a bit more about Pluto, planets, orbits and the centre of mass you now understand how astronomers define what a planet is and ultimately how Pluto is a dwarf planet.