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Thursday, August 12, 1999 Published at 20:08 GMT 21:08 UK


Sci/Tech

When a star swallows a planet

Impression of a planet's last few moments

By BBC News Online Science Editor Dr David Whitehouse

Stars must swallow planets all the time, say astronomers who have seen the after effects.

Following the recent discoveries of Jupiter-sized planets orbiting nearby stars, astronomers estimate that as many as 100 million of the sun-like stars in our galaxy may have close-orbiting gas planets.

They add that many of them are doomed to be gobbled up by their parent stars.

Hubble Space Telescope astronomer Mario Livio detected evidence that some giant stars once possessed giant planets that have been swallowed up. The devouring stars radiate excessive amounts of infra-red light, spin rapidly, and contain a higher than expected amount of the element lithium.

About 8% of the stars in our galaxy display these characteristics, according to Dr Livio. This is consistent with estimates of close-orbiting giant planets.

An ageing solar-type star will expand to a red giant and in the process will engulf any close-orbiting planets. If the planets are like Jupiter they will have a profound effect on the red giant's evolution.

According to Dr Livio's calculations, such a star is bigger and brighter because it absorbs gravitational energy from the orbiting companion. This heats the star so that it puffs off expanding shells of dust, which radiate excessive amounts of infra-red light.

The orbiting planet also transfers angular momentum to the star, causing it to rotate at a faster rate.

Finally, a chemical tracer is the element lithium, which is normally destroyed inside stars. A newly devoured Jovian planet would provide a fresh supply of lithium and this shows up as an excess in the star's spectrum.

Such drama will not happen in our solar system, when our Sun swells up in five billion years' time. Jupiter is too far from the Sun to be affected.



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