Why are planets round?

Tamela - That's a great question. It comes back to this idea that - well, this knowledge that gravity is a central force, it's acting radially inwards and pulling mass, anything with mass towards its centre of mass. So, to go back to the formation of planets, we expect that stars and planets would've condensed and collapsed down from these massive clouds of gas and dust, these nebulas. As they were collapsing, they were attracting other bits of matter and the natural equilibrium state is for everything to be as close to that centre as possible and to be in a sphere. It wants to be equidistant in a sense. Bigger objects have more mass and that strength of gravity is greater. So, it really starts to smooth without the surface. A smaller object might be a cube and material properties of that cube can keep it up without gravity pulling it into a sphere.

Chris - It's a bit like raindrops I suppose isn't it because you've got a droplet of rain coming down. If it can forms a sphere, but then it sort of smears out a bit with the air current pushing it. It's the same sort of phenomenon - the water sticking, pulling itself together.