Cameron Forrest, of Balclutha Primary School, asks :-

What makes the planets spin?

William Tobin, an astronomer at the University of Canterbury, responded.

We believe that planets form as part of the process that forms stars, so most stars will have planets. Space is full of enormous clouds of gas. The densest of these clouds is about one thousand million million millionth as dense as the air in your classroom, but it is from these clouds that stars form. Like the air in your classroom, these clouds normally have no tendency to collapse, but when a star forms, something makes them collapse. It may be the gravity of the cloud itself, or a shock wave. The shock may arise when the cloud passes through a spiral arm in the Milky Way galaxy, or it could come from an exploding, dying star called a supernova: we do not know for sure. For whatever reason, the cloud collapses. It also flattens and begins to spin. This is because the initial cloud always has some slight spin, and the spin gets faster in just the same way that an ice skater's spin gets faster when she brings her arms closer to her body.

The collapsing cloud also heats up, in just the same way that a bicycle pump heats up when you compress the air. At the centre of the cloud, the pressures and temperatures become hot enough for nuclear reactions to start and a star is born.

Tiny grains of rock-like dust condense out in the outer, cooler parts of the cloud, and where it is coldest, grains of water ice form togther with exotic ices composed of substances like ammonia. The grains collide and stick together to make bigger grains, which collide further, building up particles that are kilometres across---and still spinning. These planetesimals, as they are called, grow by collision into the spinning planets. Closer to the star, the planets are rocky like the Earth; but further out they are less dense because of the ices and may also attract and retain gas, like Jupiter. The orbits of the planets lie in a plane reflecting the flat disc they formed from. Planets also often have moons orbiting around them.

Planetary formation takes less than a thousand million years. The solar system is almost five times older than that, and the only remaining planetesimals are a few comets. However, early in the history of the solar system, planetesimals colliding with planets created lots of craters, which we still see on airless worlds like Mercury and the Moon, where there has been no erosion to wear them away.

So, to use some long words to summarize: planets like the Earth form by collapse, condenstaion and aggregation; and they spin because they formed from spinning material.