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.

by Stuart Carter

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!

Jupiter is 1300 times the size of Earth

The world of astronomy was stunned. As they began to confirm his observations the implications of the discovery began to sink in; the planet must be very close to the star and it must be very, very hot. Since the 51 Pegasi discovery, Marcy and Butler have gone on to find many more of these 'hot Jupiters', all in tight fast orbits.

Around the star, Tau Boötis is the hottest planet every found - its surface temperature is more than 1200 degrees Celsius. Orbiting around the star HD 195019 is a planet 1100 times the size of the Earth but 8 times closer! And the planet that goes around HD 187123 races around in just 3 days.

The astronomers were both excited and disappointed. They had proved that there must be many planets across the Cosmos, but nowhere that could be thought of as the slightest bit hospitable or desirable. These 'hot Jupiters' would have annihilated any Earth like planets. The theory of planet formation says these big giants must have been born much further out. As they migrated in towards the parent stars, they would 'bulldoze' any 'Earths' in front of them, and push the smaller planets into the fiery star.

NASA The Perseus Cluster of Galaxies - The Universe is accelerating away from us and is growing a billion miles in every direction

The astronomers knew that if they were to stand any chance of finding a planet like our own, then they would have to find an entire suite of planets. Their wobble technique is far too crude to directly detect the presence of an Earth-sized planet. But they realised that if they could find a number of gas giants orbiting at a safe distance from their parent star then there could theoretically at least be a place where a small Earth-like planet could survive.

Debra Fischer, a colleague of Marcy and Butler, has in recent years been one of the leading names in the planet-hunting pack. She uses the Lick Observatory in California to collect her raw data. In 1998 she announced her discovery of a three-planet system orbiting the star Upsilon Andromedae. The wobble she detected had baffled her for months; it was complex and irregular and certainly wasn't the result of a single planet. In fact, what she had discovered were 3 gas giants, one in a tight circular orbit and two in wild eccentric orbits shooting past their host star before slinging out into the icy depths of deep space.

It was a start - but this celestial pinball machine was still far too violent for an Earth-like planet to stand any chance of surviving. Fischer knew that what she needed was a multiple planet system where the orbits are circular and where, like our own Solar System, the gas giants keep their distance. After years of painstaking research Fischer struck gold in 2001. She found two large planets in circular orbits around the star 47 Ursae Majoris. Their positions and sizes are similar to our own Jupiter and Saturn. Between these gas giants and the host star is a large gap, possibly a place where small rocky Earth-like planets could also be hiding.

NASA Artist’s impression of the Next Generation Space Telescope: its exposed mirror is three times the size of Hubble’s.

But will the astronomers ever be able to 'see' a planet as small as Earth? A number of new projects make it highly probable that in the next few years just such a discovery will be made. The biggest and best is the billion-dollar Kepler telescope scheduled for launch in 2005. This high-tech eye in the sky will monitor thousands of stars simultaneously from the unobstructed view of space. But Kepler won't watch for wobbles. It will scan for any brief dip in light when planets pass in front of their parent stars. During its three year vigil, Kepler should be able to pick out even the tiniest star blink - indicating that a star has a planet as small as Earth.

The planet hunters are confident of success. Watch this space.

Damon Wright is a Staff Writer for FirstScience.com. Copyright (c) FirstScience.com

Click here to read the follow up article to this one. Called 'Distant Wanderers' - An article extract by Bruce Dorminey from his book of the same name.

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This lavishly illustrated book explores the mysteries of the planets, the birth of the universe and other strange, but true, phenomena of space and time.

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