March 8, 1996. Copyright, 1996, Graphic News. All rights reserved

HUNTING DOWN ELUSIVE PLANETS
By Nicholas Booth, Science Editor

LONDON, March 8, Graphic News Ð ItÕs been described as significant a moment in astronomy as the discoveries with the first telescopes in the 17th century that showed the Earth was not the centre of the universe. In a subject replete with shocks and surprises which often lead to the rewriting of text books, none is so surprising as the recent discovery of planets around other stars. They are all the more tantalising because one of them appears to harbour water, an ingredient essential to the creation and nurturing of life.

The discoveries are all the more impressive because trying to locate a planet around another star has been likened to searching for a speck of dust in the beam of a searchlight. Stars are bright whilst planets are only visible by light reflected from them and they are many billions of times fainter. Though some astronomers believe we may be able to detect them by their faint heat signature, others are not so sure. The technique which has won out measures how stars wobble in space due to the presence of planets around them. 

Planets orbit a star Ð as the Earth does the Sun Ð owing to its gravitational pull, which keeps each planet in motion with almost clockwork precision. But the planets have a measurable effect on the star itself, so it will move, imperceptibly to the naked eye, towards and away from an observer on Earth. By making very precise measurements of the colour of stars, astronomers have been able to determine these tell-tale movements. The presence of planets is indicated from the way in which spectral lines Ð chemical fingerprints Ð are shifted due to their influence upon the star. 

Advanced computer techniques can sort through the mass of information contained in these measurements and determine motions as small as three metres per second, quite an achievement for light which has travelled many years to reach us. Yet caution is warranted, for astronomers recall the experience of a team from Jodrell Bank who announced in 1991 that they had found a planet around a pulsar, a rapidly rotating remnant of a star, only later to realise the wobble they had measured was that in the orbit of the Earth.

In 1992, three planets were discovered (and their existence confirmed) around an otherwise obscure pulsar called PSR 1257+12. But as pulsars are the burned-out shells of stars which have come to the end of their lives, there is no possibility for life upon them. For that, we would need to look at stars similar to the Sun, which is precisely what Michel Mayor and Didier Queloz from the Geneva Observatory did at the end of last year. A star in the constellation of Pegasus, 51 Peg, was found to be orbited by a planet half the size of Jupiter (the largest planet in our Solar System), very close to the surface of the star itself. With a temperature of 1,300¡C, the chance of life is remote but augurs well for the existence of greater numbers of planets.

To some extent, the Swiss duo stole a march on a pair of American astronomers in San Francisco who had been targeting 120 Sun-like stars in our galactic neighbourhood. Their reference books listed 51 Peg as an older star, so they ignored it. No matter: in January, Drs Geoffrey Marcy and Paul Butler announced the discovery of planets around two other stars, 47 Ursae Majoris and 70 Virginis, the former having spectral lines indicating the presence of water. It will be a while before telescopes large enough to observe these planets directly can be built, but it may occur in the next thirty years as part of a new effort by the U.S. space agency, NASA. NASAÕs Administrator, Daniel Goldin, believes that his grandchildrenÕs children may grow up to see photographs of seas and mountain ranges upon distant planets around other worlds.

Sources: Sky & Telescope, BBC Television, Time

EditorÕs Note: MondayÕs ÔHorizonÕ programme on BBC2 (March 11, 8:00pm), written and directed by Danielle Peck, looks at the topic of new planets. For further information contact BBC Publicity on (0181) 743 8000.