Eating them alive

Almost 8 per cent of the stars in the universe have literally gobbled up planets

Published: Sunday 31 October 1999

our solar system with its star -- the Sun -- and the nine planets was thought for a long time to be a bit of an exception in the universe. No other planetary system had been discovered around the millions of stars observed by astronomers. This changed dramatically in 1995 when astronomers deduced that a star, 51 Pegasi, has a Jupiter-sized planet in orbit around it. After this, many other planetary systems were found. Now, M Livio and L Siess of the Space Telescope Science Institute, usa , conclude that almost 8 per cent of the stars in the universe have literally gobbled up planets.

The planets discovered around stars have peculiar properties. For one, they are all too close to the parent star, in some cases even closer than Mercury to the Sun. Such planets would be very hot with surface temperatures of around 1000 c . Though these planets are usually too small to be seen directly, their presence introduces a wobble in the orbit of the parent star which has been detected. One of the questions with regard to these extra-solar planets has been about their ultimate fate. Livio and Siess have some answers.

As the star gets older, its supply of nuclear fuel runs out. Our own Sun will meet this fate in a couple of billion years. At this point, the outer atmosphere expands in size and the star gets a reddish glow. This kind of star is called a Red Giant. The planet orbiting such a Red Giant is swallowed by the star. This engulfing of the planet by the star results in a tremendous release of gravitational energy which causes the star to throw out a shell of hot gas. This gas has a characteristic glow in the infrared part of the spectrum. The scientists show that there are some 8 per cent stars which glow in the infrared and cannot be accounted for by any other process.

The researchers contend that, apart from this infrared glow, there are other signatures of planet-bearing stars. One of them is the presence of absorption lines in the spectrum of the star corresponding to the elements in the planet which becomes part of the star now. One such element is lithium which they predict the Red Giants got from their planetary progeny.

Another way to recognise a star, which has eaten up its planet, is to look at its spinning rate. When a star gobbles up the planet, it starts to spin faster. Thus, if we find Red Giants with anomalous rates of rotation, it provides another clue to these gluttons.

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