Stellar parents that burn too brightly

June 18, 1999

Theoretical astronomer Cathie Clarke tells Alison Goddard how recent finds challenge long-held notions of star and planet formation NO PHYSICAL FILEThe formation of the stars and planets is incredibly interesting because it is to do with origins - where we come from," says Cathie Clarke, a theoretical astronomer at the University of Cambridge. "However, we have not really solved this problem at all because it is so difficult to see stars and planets forming."

Scientists believe that the matter from which stars and planets are made was created immediately after the Big Bang. As the universe expanded and cooled, pockets of hot, swirling gas formed. When these pockets grew cool enough, the gas began to coalesce into stars with orbiting planets.

"The way that stars and planets form goes back to an idea that French mathematician Pierre-Simon de Laplace had in the 18th century," says Clarke, who was the most recent lecturer in the young scientist series at the Royal Institution in London.

"The picture is of a gas cloud that is rotating slightly, and it collapses under its own gravity. As it is rotating, it forms a disc not a ball. Laplace argued that matter would accumulate at the centre of the disc, forming a star, and that there would be a small amount of matter left, containing the spin of the system, still in orbit, from which the planets form."

But it was 200 years before astronomers developed the technology to spot a fundamental problem with the theory. Laplace had assumed that all stars and their discs would be similar to the Sun and its solar system. But recent observations with the Hubble Space Telescope show that most stars form in crowded nurseries, while older stars tend to have a companion nearby.

"One of the big surprises recently was the discovery that most stars form in very crowded environments," Clarke says. "We have been biased until now. Our Sun's nearest neighbour is four light years away, but it would be more typical for it to be a few light weeks away, with quite a high chance that it might crash into us."

The discovery diminishes the chances of life existing elsewhere in the universe. "The implications for life are destructive - if one star is somewhere and another star passes through, that is catastrophic," Clarke says. "On the other hand, we are now starting to find planets around other stars, so the Sun is not unique in having escaped that fate."

The discovery of such planets is another indicator that theories of planet formation based on the solar system are flawed. Until recently, astronomers thought that Earth-like planets formed closest to a central star while gas giants - such as Jupiter and Saturn - formed further away.

"This is a good example of where theorists really should be chastened," Clarke says. "Theories of planet formation have been based on the Sun. So it was a real blow when it was found that there are a number of stars with Jupiter-like planets in the orbit of the planet Mercury, because astronomers had confidently stated that that could not happen. Some of them are still so confident that they believe these gaseous giant planets must have formed a long way out and somehow migrated in."

Given that older stars tend to orbit a companion star, what might their planetary system look like? "That is a problem we have not yet solved. We are working on it," Clarke says.

Using computers, Clarke tests different models of planet formation in binary systems. "We use the simulations to produce predictions of what they should look like," she explains.

"Structures are much more complicated when you have two stars rather than one. You have two stars, and they each have a disc round them, much like two single stars. Then you get a big rim of material around the outside, called the circumbinary disc, and what are called tidal streamers - S-shapes of gas coming out from the two discs in the middle. It is quite pretty.

"Astronomers have discovered a large number of binary stars that are young and they still have discs around them. In the next five years, they will be able to get good detailed images of what the gas and dust in a binary system looks like. We are just getting to the stage where observations are starting to pick up these features, so we will soon be able to take the simulations and compare them with what we see (through the telescopes)."

Whether such discs could eventually form habitable planets is unknown. Nevertheless, Clarke believes that extraterrestrial life is a possibility. "There are 100,000 million stars in our galaxy alone, so there must be many potentially habitable planets," she says. "Whether they are near enough for us to detect them is another question. I am agnostic on whether they are inhabited. It is a fairly good argument that if they were inhabited by technologically superior beings, they would have made their presence known to us."

The next RI lecture, "Can you tell a mathematician from a fish?", by Chris Budd of the University of Bath will be on June 23. Tickets: 0171-670-2985.

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