Planet Hunters

A new breed of astronomers is scanning the skies. Between them they have discovered over 70 planets beyond our own Solar System, in orbit around other stars - and some of them are quite astounding.

Not too many years ago we only thought there were 8 planets in our own Solar System. Then in 1930 a ninth was discovered; but since then there has only been speculation about a mysterious planet X. But the astronomers of past generations had set their sights too low, or - to be more precise - too close.

Geoff Marcy (University of California at Berkeley) and Paul Butler (Carnegie Institution of Washington) started a much more ambitious search in 1987. They weren't interested in just scanning our own back yard. They wanted to see if there were any extrasolar planets orbiting distant stars. Their chosen method of planet hunting was to look for the secondary tell-tale effects of the planets.

Even with today's monster telescopes such as the Keck Telescopes on Hawaii and the Very Large Telescope in Chile, the possibility of directly seeing a planet is zero. They are just too small and faint. Instead, Marcy and Butler had to be content with the data they could measure from the parent star. They examined the starlight and looked for tiny wavelength shifts, a sure give away that a star was wobbling in response to a planet in orbit around it.

W M Keck Observatory The first Keck telescope has discovered tiny ice planets beyond Pluto

Any star with a planetary system should wobble. Our own star wobbles by as much as half a million miles as our largest planet Jupiter lumbers around it in a 12-year orbit. Marcy and Butler were sure that if Jupiter could have this effect on the Sun then a similar planet orbiting a distant world would produce a similar wobble. For years they toiled night after night scanning the heavens for a wobbling star. They found none. It was a depressing time. Despite their strong convictions that all galaxies, including our own Milky Way, should be teeming with planets they almost gave up. Then, almost a decade after their hunt began, astounding news arrived from overseas…

A young Swiss researcher had been studying the star 51 Pegasi for his PhD. Didier Queloz had no preconceptions about the orbits of extrasolar planets, and he was soon delighted to find that he had recorded a dramatic wobble. But it was nothing like the motion that Marcy and Butler had been looking for. They had expected to see a wobble that would takes years, maybe as much as the 12 year wobble of the Sun as Jupiter orbits it. What Queloz found was entirely different - he observed on a much shorter time scale and found a high frequency wobble. At first he thought it was a mistake, but after he checked his data he realised that he had found a planet the size of Jupiter racing around 51 Pegasi in just 4 days!