SETI [Fwd: Planets, planets everywhere]

Robert Owen (
Fri, 01 Oct 1999 18:30:30 -0400

Larry Klaes wrote:

> >Date: Fri, 1 Oct 1999 01:59:48 -0700
> >Reply-To:
> >Sender: News about Space from SEDS <>
> >From: "H. Alan Montgomery" <>
> >To:
> >
> >Nr. 12-99 - Paris, 30 September 1999
> >
> >Planets, planets everywhere
> >
> >Moon craters help us understand how extrasolar planets form
> >
> >More than a dozen planets orbiting other 'suns' have been found
> >in the last few years, but... are they the rule or the exception?
> >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.
> >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. Hyperlink to Nature:
> >
> >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 about ISO and ESA’s Science Programme visit
> >the ESA Science Website at: (a 'subscribe to
> >us' service to receive ESA Science News by e-mail is available),
> >where you will find another interesting ISO story on the giant
> >planets in our Solar System; an associated video clip is now
> >available for viewing on Tracker 2000 the multimedia service of
> >the ESA’s Science Website:
> >
> >
> >and can be ordered via an on-line form.
> >
> >To get even more on ISO, surf the ISO web-site:
> >
> >
> ==

Robert M. Owen
The Orion Institute
57 W. Morgan Street
Brevard, NC 28712-3659 USA

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