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FEELINGS among planet hunters are running high this week. While one team grabbed headlines with the first sighting of a solar system similar to our own, another has quietly firmed up their claim to an even greater prize: the first Earth-like planet orbiting a star other than the Sun.

Finding planets outside our Solar System has become a relatively common occurrence鈥攁round 90 have been found so far. But what astronomers really want to know is whether there are more planets like our own out there, and whether they might be inhabited.

Planets orbiting distant stars are far too dim to be seen directly. Instead, most planet hunters look for the effect of a planet鈥檚 gravity on the host star. But the influence of small planets would be undetectable, so most of the planets found so far weigh in at several times the mass of Jupiter. This means they are too large and gaseous to support life.

A New Zealand and Japanese team headed by Philip Yock at the University of Auckland relies on a totally different technique called microlensing. When one star passes in front of another, it bends the light from the distant star like a lens, magnifying it tens of times. If the foreground star has planets, even small ones like Earth, the shape of the 鈥渓ens鈥� should be warped in a way that depends on the planets鈥� motion. Although relatively untested, so far it鈥檚 the only method capable of spotting Earth-sized planets.

In 1999, Yock鈥檚 team saw a blip in a microlens that could have been caused by a small Earth-sized planet, with an orbit between one and three times as far from its star as Earth is from the Sun. Critics said the blip was probably down to statistical noise. But the researchers have published their latest calculations this week, using a new, more accurate, image-analysis technique. They show that the chances of the blip being random noise are less than 1 per cent.

鈥淭he signal is stronger,鈥� says microlensing expert John Bennett of the University of Notre Dame in Indiana. But he warns that there could be other explanations, for example a third dark object coincidentally passing in front of the foreground star. Yock鈥檚 colleague Ian Bond disagrees. 鈥淚鈥檝e been through the data so many times looking at other possibilities and that signal won鈥檛 go away,鈥� he says.

However, competing teams refuse to acknowledge the findings of Yock and his colleagues. Planet hunter Geoff Marcy at the University of California, Berkeley, has left the planet off his almanac of sightings at , and refuses to comment on the microlensing result.

Marcy himself has been in the headlines this week after announcing the discovery of a solar system that looks a little like our own. He and colleague Paul Butler have found a system, called 55 Cancri, which has a Jupiter-like planet orbiting its star at about the same distance as the real Jupiter orbits the Sun, a result that has taken 15 years of observations. There are also two other gas giants closer in, but computer modelling carried out by Greg Laughlin, also at Berkeley, suggests that a terrestrial planet could exist in 55 Cancri鈥檚 鈥渉abitable zone鈥� (see Graphic). Planets here would have liquid water, something that鈥檚 rare among other solar systems found so far.

Bond is positive about Marcy鈥檚 result. 鈥淭here鈥檚 no way of knowing if it has smaller planets like Earth, but it鈥檚 an exciting candidate,鈥� he says. He and Yock refuse to be frustrated that Marcy鈥檚 work has received so much attention when they believe they have already found an Earth-like planet. 鈥淭here鈥檚 a bit of competition here because our method has come along later,鈥� says Yock.

One problem for the microlensing team may be that they have been too lucky too soon. Astronomers have seen only a handful of microlensing events with as high a magnification as this one, and no one has found any other kind of planetary system to compare with the signal. Many feel that to jump straight in with such an ambitious claim is just not the done thing.

But one thing Yock and Bond can say is that their system is nothing like our Solar System, as they have only seen signs of one planet. If their other Earth is real, it鈥檚 on its own.