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The BBC's Julian Siddle
"The star has engulfed a planet"
 real 28k

Wednesday, 9 May, 2001, 23:44 GMT 00:44 UK
Star 'eats' a planet
Planets BBC
The star must have consumed planetary material
Artist's impression

By BBC News Online science editor Dr David Whitehouse

Astronomers finally have evidence that Sun-like stars with planets are guilty of swallowing the planets in orbit around them.

Garik Israelian, at the Astrophysics Institute of the Canary Islands, and colleagues have caught a planet-swallowing star (HD82943) red-handed with lithium-6 in its atmosphere.

This particular type, or isotope, of lithium should not be in the outer layers of any Sun-like star because it is rapidly destroyed in the early stages of stellar evolution. However, the isotope does remain intact in planets.

The researchers report their find, made using a sensitive spectrometer, in the journal Nature.

Planetary billiards

The team says that the only way the presence of lithium-6 in HD82943 can be explained is if the star swallowed one or more of its planets whole. That HD82943 has two planets around it, one in a highly elliptical orbit, adds weight to the idea that at least one of its original planets was devoured.

HD82943 is a star very similar to our Sun. It is only a few percent larger and brighter. It is 78 light-years distant, making it relatively close to our Solar System.

It has two planets in orbit around it. One is about twice the mass of Jupiter; the other has a mass slightly less than Jupiter's.

Since they were first detected in 1992, extrasolar planets - planets outside our Solar System - have puzzled astronomers because they orbit very close to their parent stars; regions of space in which they could not possibly have formed.

Weird systems

One way they could have got there is if multiple planets have interacted in a gravitational "billiards game", in which one planet might have been flung in towards the parent star (sometimes to be swallowed whole), while another might have been thrown into an eccentric orbit, or perhaps even ejected from the system altogether.

Just over 50 extrasolar planetary systems have so far been discovered and analysis of them shows just how unusual our local system is with its giant planets (Jupiter and Saturn) positioned well away from the Sun.

Astronomers are uncertain if this represents a genuine rarity of systems like our own, or a simple self-selection problem in which our current technology only has the sensitivity to pick up the weird systems.

Certainly, statistical analyses would suggest there are very many stars out there with terrestrial bodies in orbit about them. One recent study of more than 450 Sun-like stars within about 325 light-years of Earth indicated most had high iron content in their photospheres, or on their surfaces - again, evidence the stars had gorged themselves on planetary material.

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