Discovery of Earth-like planet brings hope of finding alien life

A planet similar to Earth has been found orbiting a distant star by astronomers who believe they are getting closer to discovering an alien world inhabited by extraterrestrial life.

The new planet is five times the size of Earth but is itself unlikely to harbour life because it is probably covered in frozen oceans with average temperatures of around minus 220C.

However, the scientists behind the discovery believe the find marks a breakthrough in the search for relatively small, rocky planets such as Earth where temperatures are neither too hot nor too cold for life.

The scientists said that the discovery showed it was technically possible to discover a planet in a temperate "habitable zone" around a far-away sun that would permit the existence of liquid water, which is believed to be necessary for life.

The new planet, designated OGLE-2005-BLG-390Lb, is the smallest and the coldest planet yet discovered beyond the solar system. It orbits a star towards the centre of the Milky Way galaxy located 20,000 light years away in the constellation Sagittarius.

"This has huge implications for finding life," said Stephen Kane of the University of Florida, one of the 73 astronomers from the 32 institutions around the world involved in the study, published today in the journal Nature. "The good thing about this is it shows that planets this size might be quite common in habitable zones," Dr Kane said.

More than 150 planets are known to exist outside our solar system but the vast majority are large, gaseous planets, like Jupiter. The latest planet has an orbit three times the distance between the Earth and the Sun, meaning its temperatures are similar to those of permanently-frozen Pluto.

The discovery was made last summer using a technique called gravitational microlensing. Light from a bright, distant star is bent by the gravity of an intervening star to make the distant star appear larger than it is. This affect can be distorted when a planet is orbiting the intervening star.

Bohdan Paczynski at Princeton University said: "We may predict with reasonable probability that gravitational microlensing will discover planets with masses like that of Earth at a similar distance from their stars and with comparable surface temperatures."

A number of telescopes in the southern hemisphere - which has the best view of the "galactic bulge" of stars at the centre of the Milky Way - took part.

The robotically-controlled telescopes surveyed millions of stars in the hope that a second star with a nearby planet would intervene during the period of observation.

"With this method, we let the gravity of a dim, intervening star act as a giant natural telescope for us, magnifying a more distant star, which then temporarily looks brighter," said Andrew Williams of the Perth Observatory in Australia.

"A small 'defect' in the brightening reveals the existence of a planet around the lens star," Dr Williams said.

Professor Keith Horn of the University of St Andrews said gravitational microlensing was a good way of identifying smaller, rocky planets and has already discovered three exoplanets.

"Microlensing is the fastest way to find small cool planets, down to the mass of the Earth. Our first three planet discoveries indicate small cool planets are abundant," Professor Horne said.

"If we can deploy robotic telescopes at additional sites in the southern hemisphere, they could make the first detection of extra-solar Earths," he said.