Published online 25 January 2006 | Nature | doi:10.1038/news060123-5

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Astronomers use gravity lensing to spot homely planets.

How to spot a planet: watch for wiggles in the light coming from a far distant star as it curves around another sun. (artist's impression). Credit ESO

Astronomers say they have found the most Earth-like planet yet outside our Solar System. At just 5.5 times the mass of Earth it is one of the smallest extrasolar planets ever found, and orbits its star at a distance comparable to that of habitable worlds.

Similarly sized extrasolar planets have been found before. But the method used to detect them meant we could see smallish planets only when they were very close to their suns, and such bodies are battered by scorching radiation.

Planet OGLE-2005-BLG-390Lb looks much more like home. It lies about 390 million kilometres from its star: if it were inside our Solar System, the planet would sit between Mars and Jupiter.

“The search for a second Earth is the driving force behind our research.” Daniel Kubas

at the European Southern Observatory in Santiago de Chile, Chile.

It takes ten years for the planet to orbit its parent star, a common-or-garden red dwarf that lies about 28,000 light years from Earth, close to the centre of our Galaxy.

But sadly this Earth-like body probably isn't crawling with life. Its dwarf star is so dim that the surface temperature of this planet is thought to be about - 220 °C.

"The search for a second Earth is the driving force behind our research," says Daniel Kubas at the European Southern Observatory in Santiago de Chile, Chile, part of the team that made the discovery. They are optimistic that the clever method they used to spot the planet could soon uncover an alien twin to our own world.

Wobbly stars

Gravitational microlensing can find small alien worlds in places other techniques cannot reach. Click here to see enlarged image.

More than 170 planets have been discovered outside our Solar System. Astronomers usually detect them by watching how they make their parent star wiggle, a technique known as the Doppler method. This is ideal if you are looking for massive planets orbiting very close to their star, which induce a lot of wobble.

But there is no way this can be used to find small, blue-green planets approximately 150 million kilometres from a yellow sun. It is simply not sensitive enough, says Didier Queloz, an astronomer from Geneva Observatory in Switzerland who was part of the team that found the first extrasolar planet, just 11 years ago1.

The new sighting relies on an effect called gravitational lensing, where a massive object such as a star warps space so that it behaves like a lens. This means that it bends and slightly magnifies light from a more distant star before it reaches our telescopes. Adding a planet to the mix modifies the lensing effect by a tiny amount, just enough to work out its mass and orbit.

"Microlensing is the fastest way to find small, cool planets, down to the mass of the Earth," says Keith Horne, one of the planet's discoverers and an astronomer from the University of St Andrews, UK.

Spot the difference

The planet was found by a consortium of 73 astronomers from 12 different countries. Its star was first spotted by scientists working on the Optical Gravitational Lensing Experiment (OGLE), before the planet itself was noticed by astronomer Pascal Fouqué.

OGLE-2005-BLG-390Lb is only the third planet found using the microlensing technique so far, but astronomers expect to spot many more. "The other two microlensing planets have masses of a few times that of Jupiter, but the discovery of a five-Earth-mass planet is a strong hint that these objects are very common," says Jean-Philippe Beaulieu of the Astrophysics Institute of Paris. Beaulieu is lead author of the paper describing the find in this week's Nature2.

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