Rebbeca Ching, of Port Chalmers School, asks :-

How were the planets formed?

William Tobin, an astronomer at the Institut d' Astrophysique, Paris, while on leave from the University of Canterbury, responded.

Planets probably form at the same time as the stars that they orbit about. For example, our star, the Sun, is about 5 billion years old, and the oldest rocks on the Earth have about the same age.

Astronomers have found huge clouds of very thin and very cold gas in outer space. Mixed in are light pepperings of tiny grains of carbon, iron, ice and other solid materials. A star forms when part of one of these giant clouds collapses down to the much-much-soupier and much-much-hotter gas of a star. What causes the collapse is gravity, the same force that is always pulling you and me towards the Earth. Gravity and swirling of the gas cause the new-born star to be surrounded by a flat spinning disc of left-over material.

The solid grains are attracted together by gravity into larger and larger clumps until finally we end up with planets. We know of nine major planets around the Sun. They all orbit the same way, like the flat disc from which they came. If these ideas are right, planets must be very common in the Universe because almost every star will have some.

At the Mount John University Observatory, near Lake Tekapo, we are making observations of a star called Beta Pictoris. We think this star has just formed planets, but the unused gas and grains have not yet been completely cleaned away. However, we absolutely do not understand what our observations are showing. There will still be lots for you to discover if you become an astronomer!