And the first extrasolar planets

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On 5 October 1995, two Swiss astronomers - Michel Mayor and Didier Queloz -announced their discovery of an extrasolar planet in orbit around a Sun-like star, 51 Pegasi.

Another false alarm? Since the mid-twentieth century, reports of such sightings had, like stories of sea serpents, been bandied about, only to be discounted by other research teams. Results could not be reproduced, and each 'new planet' vanished in a puff of smoke...

Undaunted in the face of controversy, a few teams of researchers ploughed on doggedly. They were confident that their equipment was up to the task of detecting the presence of large planets of the size of Jupiter. Of course, there was no question of actually 'seeing' such a body: imaging an exceedingly faint planet orbiting a star billions of times brighter than itself is truly impossible. One might just as well try to detect a candle placed alongside a lighthouse, and, for fair comparison, the lighthouse would have to be observed from hundreds of kilometres away!

Detection programmes have therefore to rely on methods involving the minute perturbations induced in a star's motion by the presence of a nearby planet, involving the displacement of a star's apparent position compared with the positions of other stars, or modifications in a star's velocity within our galaxy. These perturbations are regular, with a period reflecting that of the revolution of the planet around the star.

In the case of Jupiter, its presence causes the velocity of the Sun through our galaxy to vary by 13 m/s over a 12-year period - a variation detectable with the best spectrometers since the 1990s.

Moreover, far from having detected planets around ordinary stars, astronomers have detected two around a late-stage star: a pulsar. A pulsar is a star which emits radio 'bursts' at very regular intervals. They are neutron stars - the remnant of the explosion of a star which is much more massive than the Sun: a supernova. In 1992 astronomers detected two planets a little larger than the Earth orbiting

1.2 First clues 5

The Crab Nebula - the remnant of a supernova explosion observed in 1054 and reported by Chinese astronomers. At its centre lies a pulsar - a very dense neutron star in rapid rotation (30 times per second). The nebula is all that remains of the rest of the progenitor star. Matter ejected during the explosion forms reddish filaments, and the cloud is expanding at around 1,500 km/s. The first two exoplanets to be found were detected near a pulsar created by a supernova explosion. These planets certainly formed after the explosion; but it is difficult to imagine how a planetary system could survive the enormous burst of energy of such an event. It is not known whether the Crab pulsar possesses any planetary companions. (Photograph courtesy CFHT, J.-C. Cuillandre.)

the pulsar PSR 1257+12. The explosion of a supernova is an event of unimaginable violence, releasing vast tides of energy into space. A shock wave would pass through its planetary system at velocities of several thousand kilometres per second. It is impossible to conceive how planets could survive a hurricane of this nature. However, the data were irrefutable: the pulsar PSR 1257+12 really did possess two planets, or perhaps three (the presence of the third planet being confirmed later). This most unexpected discovery led optimistic researchers to conclude that if planets can be found in such an inhospitable environment, how much more probable would it be that they could be found near ordinary stars?

So, the situation in 1995 was that, apart from the nine planets orbiting the Sun, two planets had been found near a pulsar, and a few teams were searching intently for other planetary bodies. At that time, few people would have bet on the likelihood of such a plethora of discoveries during the following years... which demonstrates that the course of scientific discovery is not always easy to predict.

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