ESA Science & Technology - INFO 12-1999: Planets, planets everywhere

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. 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 (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."

Note(*)

The paper about this discovery is published in Nature, on 30 September 1999.

Footnote about ISO

The European Space Agency's infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations.

For more information please contact:

ESA Public Relations Division:
Tel: +33(0)1.53.69.71.55
Fax: +33(0)1.53.69.76.90

Martin F. Kessler (ISO Project Scientist):
Tel: +34 918131254
mkessleriso.vilspa.esa.es

Other science contacts:

Harm Habing, Leiden University, The Netherlands
Tel: +31 71 527 5916
habingstrw.leidenuniv.nl

Carsten Dominik, Sterrenkundig Instituut "Anton Pannekoek", Amsterdam (NL)
Tel: +31 20 5257477
dominikstrw.leidenuniv.nl
dominikastro.uva.nl

Rene Laureijs, ISO Data Centre, Spain
Tel: +34 91 813 1367
rlaureijiso.vilspa.esa.es

Marie Jourdain de Muizon, LAEFF, Madrid, Spain, and
Observatoire de Paris
Tel: +34 91 813 1261
muizonlaeff.esa.es