The European Space Agency's infrared space observatory, ISO has shown that the formation of extrasolar planets must be a very common event. As explained in today's issue of the journal Nature (30 September), ISO has found that almost all young stars are surrounded by a disc of debris - a requisite for planet making - while most above a certain age do not have discs.

Caption Above: Computer rendering, based on a hydrodynamic model which calculates the evolution of a protostellar disk as a giant protoplanet forms. Image courtesy of: Geoffrey Bryden, Lick Observatory, University of California, Santa Cruz

Correlating these data and certain events in the history of our own Solar System, such as the formation of the Moon's craters, astronomers postulate that the discs of older stars have vanished because they have already condensed into planets.

The authors, an international team led by Harm Habing, from Leiden University in The Netherlands, wanted to know if stars belonging to a particular class were more likely than others to form planets. In our own Solar System planets formed out of a disc of small particles of dust, so every star surrounded by such a disc is a potential planet-forming star.

The astronomers therefore chose a sample of 84 nearby stars, all of them very common and in the most stable phase of their lives - the 'main sequence' - but of different ages. Which ones would have discs? Discs are difficult to see because they emit very faintly; only a few had been positively detected so far.

Using ESA's highly sensitive infrared space observatory, ISO, the international team found that 15 stars in their sample did have a disc. Then they analysed the ages of the stars: it turned out that most of those younger than 400 million years had discs, while the great majority of the older ones did not.

"We show for the first time that the presence of a disc around a main sequence star depends strongly on the star's age. Why do those above a precise age not have discs? We searched for clues in our own Solar System, and realised that it was just when the Sun was that age (about 400 million years) that planets were forming", Habing says.

In our Solar System, several facts demonstrate that very soon after the formation of the planets the disc orbiting the Sun disappeared. Some evidence comes, for instance, from Moon craters.

These 'scars' on the lunar surface were made while the planets were completing their formation phase and the Sun was losing its own disc of debris, during the 'clean-up phase' of the Solar System.

The newly-born planets scattered the remaining planetesimals, which were ejected from the system, fell into the Sun or collided with other large bodies - such as the Moon. The age determinations of lunar rocks brought back by the Apollo missions prove that all this happened when the Sun was 300 to 400 million years old.

In the light of these facts, the authors postulate that the young stars in their sample - those with a disc - are now undergoing their 'heavy bombardment' period. When this process finishes, the disc will vanish and proto-planets will orbit the star instead.

Does this theory mean that all stars for which a disc cannot be observed are surrounded by planets?

"This is something we cannot say. That's where the knowledge barrier is", Habing answers. "However, we think the Sun has the same history as the other planetary systems. When the planets form they destroy the disc".

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