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Science: The truth About... Gravitational Microlensing

THE DISCOVERY of a planet orbiting a distant star does not, these days, arouse much comment. The first was identified in 1995, but there are now 17 listed; and so the addition of another, announced by a team from Japan and New Zealand at the weekend, might not seem unusual. But what was interesting was the method they used to detect it - and what it implies about our future ability to detect Earth-sized planets that could harbour life.

Called "gravitational microlensing", this technique relies on the fact that light passing close to a massive object will be bent by a degree related to its mass. Exactly this method was used in 1919 to verify Einstein's theory of relativity. During a total solar eclipse, starlight that should have been hidden behind the Sun was actually visible, because it had been bent around the star's massive body by its gravity.

Microlensing, however, is a more subtle application of this effect. It requires two stars - a very distant one and a nearer one, around which the extrasolar planet revolves. When the distant star passes behind the nearer one, a ring of light is formed, as the distant star's light is equally bent around the nearer star's mass.

The diameter of this ring is measured in "arcseconds", subdivisions of a degree in a circle. But such events usually produce rings which are only micro-arcseconds in size; hence the name "microlensing", and typically last 40 days.

If the nearer star has a planet orbiting it, then there will be an extra peak in the microlensing ring's intensity, lasting perhaps a few hours for an Earth-sized planet. That was what the team observed.

Previously, extrasolar planets were detected - or their existence extrapolated - by examining the movements of stars for "wobble". This wobbling would be caused by massive objects moving elliptically around its parent star. But to induce enough wobble to be detectable from Earth, the planet had to be very big, and the star relatively close. Thus most of the 17 extrasolar planets so far identified lie less than 100 light years from Earth, and have masses at least as great as our own Jupiter - about 300 times greater than the Earth. They are unlikely candidates to foster life, as they are gas giants.

Microlensing, by contrast, can identify candidate stars thousands of light years away, and infer the existence of Earth-sized planets by the length of the peaks of intensity in the light emitted. The process can even be automated, and is the most effective method for detecting planets that range in mass up to 20 times that of Earth. Though the Search for Extraterrestrial Intelligence (Seti) has suggested putting systems into orbit which would watch stars for "transits" - the passage of a planet in front of a star - microlensing is accepted as the most effective tool available in the hunt for planets beyond our Solar System.