Planet building

Last updated 15:03 02/11/2009

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Connor Mills, of Balclutha Primary School, asks: How did the Earth get made?

William Tobin, an astronomer at the University of Canterbury, responds:

A few decades ago, astronomers thought that planets were very uncommon objects that were pulled out like tides, in the very rare case when two stars happened to pass close to one another.

But we have now found evidence for planets orbiting more than 100 nearby stars, so we know that they must form easily.

We believe that planets such as the Earth form as part of the process that forms stars, so most stars will have planets. Space is full of enormous clouds of gas.

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.

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 together 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.

These planetesimals, as they are called, grow by collision into the planets. Planetary formation takes less than a thousand million years, only one-fifth of the time that it took for the solar system to form.

Send questions to Ask-A-Scientist, PO Box 31-035, Christchurch 8444, or email

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