MIAMI • A planet believed to be the closest match yet to Earth has been discovered orbiting a distant sun-like star, bolstering hopes of finding life elsewhere in the universe, according to US scientists.

The potentially rocky planet, named Kepler-452b, is about 60 per cent larger than Earth and circles its star at the same distance as our planet orbits the Sun, Nasa said on Thursday. It could have active volcanoes, oceans, sunshine like ours, twice as much gravity and a 385-day year, scientists said.

"Today, we are announcing the discovery of an exoplanet that, as far we can tell, is a pretty good close cousin to the Earth and our Sun," Dr John Grunsfeld, associate administrator at Nasa's Science Mission Directorate in Washington, said on Thursday. "This is about the closest so far," he added, describing Kepler-452b as our "closest twin," or "Earth 2.0".

The planet is located 1,400 light years away in the constellation Cygnus. It was detected by the United States space agency Nasa's Kepler Space Telescope, which has been hunting for other worlds like ours since 2009.

This planet sits squarely in the "Goldilocks zone" of its star, where life could exist because it is neither too hot nor too cold to support liquid water, Nasa said. Kepler-452b orbits a star that is about 6 billion years old, compared with the 4.6 billion-year-old Sun.

"It's simply awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star," said astronomer Jon Jenkins, who is from the National Aeronautics and Space Administration's Ames Research Centre in Moffett Field, California.

"That's considerable time and opportunity for life to arise somewhere on its surface or in its oceans, should all the necessary ingredients and conditions for life exist on this planet," he added.

Kepler-452b's star is 1.5 billion years older, 4 per cent bigger and 10 per cent brighter than our Sun.

"This is really the first step - and I think humankind's first step - at answering that question of, 'Are we alone in the universe?'" said Kepler research scientist Jeff Coughlin, who is from the Search for Extraterrestrial Intelligence (Seti) Institute in Mountain View, California.

"You and I probably won't be travelling to any of these planets without some unexpected breakthrough but, you know, our children's children's children may."

If the planet is rocky - and scientists believe it has a better than even chance of being just that - then it could be headed for a fearful scenario, as the heat from its dying star evaporates Kepler-452b's lakes and oceans.

The planet also could have a thick atmosphere, cloudy skies and active volcanoes, Dr Jenkins said.

"Its location, vis-a-vis its star, could mean that it is just entering a runaway greenhouse phase of its climate history," said Dr Doug Caldwell, a Seti Institute scientist working on the Kepler mission.

"The increasing energy from its ageing sun might be heating the surface and evaporating any oceans. The water vapour would be lost from the planet forever," he added. "Kepler-452b could be experiencing now what the Earth will undergo more than a billion years from now, as the Sun ages and grows brighter."

Dr Jenkins was more optimistic, however, based on the planet's age, size, and higher mass and gravity than Earth. "This planet is protected at least for a little while longer, 500 million years or so, from experiencing the runaway greenhouse effect, assuming that it is six billion years old," he told reporters.

With the discovery of Kepler-452b, the telescope has found 1,030 confirmed planets and identified about 4,700 candidate planets.

The list of potential planets includes 11 other near-Earth twins, nine of which circle sun-like stars. The telescope cannot see planets directly but measures minute changes in light coming from target stars.

REUTERS, AGENCE FRANCE-PRESSE