Twin Hearts

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The possibility of extraterrestrial life just went up with the discovery of the most Earth-like planet ever spotted outside our solar system.

The planet -- sporting the unwieldly name OGLE-2005-BLG-390Lb -- is roughly 23,000 light years from Earth, right near the middle of the Milky Way galaxy and visible in the constellation Sagittarius.

A light year is roughly 10million million kilometres.

"We're getting closer to discovering an Earth-twin," said Australian National University astronomer Penny Sackett, a co-author of today's report in the journal Nature.

Astronomers Andrew Williams from Perth Observatory and John Greenhill and his colleagues at the University of Tasmania in Hobart were also among the 73 discoverers. They observed the planet continuously for a month last year to confirm its existence.

The team is thrilled because the rocky planet -- far smaller than Neptune -- is only five times more massive than Earth, making it the most home-like of the 170 "exoplanets" detected so far. It also circles its sun in a ten-year orbit corresponding to one between Mars and Jupiter.

In contrast, most exoplanets are Jupiter-sized balls of gas with hot noxious air, said astronomer Seth Shostak with the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California.

"This discovery is only one planet, but it strongly suggests that small (Earth-like) worlds might be as common as flies in the outback," Dr Shostak said.

Dr Williams said he agreed. He said an unpublished analysis showed that as many as one in three or four stars may have planets like ours.

To spot the planet, Dr Williams said the team used a "microlensing" technique, based on an effect predicted by Einstein in 1912.

"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," he explained.

"A small 'defect' in the brightening reveals the existence of a planet around the lens star," he added. "We don't see the planet, or even the star that it's orbiting, we just see the effect of their gravity (on the starlight)."

But that was enough to determine the position and characteristics of the planet, down to an icy surface about 220C below zero.

Only two other, much larger, planets have been detected with the method. The rest were found because they're so big or close to their star, their gravitational field causes it to "wobble".