"This is so fascinating because Kepler 452b receives the same kind of spectrum and intensity of light as we do on Earth," said Dr Daniel Brown, an astronomy expert at Nottingham Trent University.
"This means plants from our planet could grow there if it were rocky and had an atmosphere. You could even get a healthy tan like here on holiday." But because it's 1.5 billion years older, scientists say it gives a "peek into a crystal ball showing a possible future for Earth" as it reaches a point where it is no longer habitable.
"If Kepler 452b is indeed a rocky planet, its location could mean that it is just entering a runaway greenhouse phase of its climate history," said 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.
"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."
The Kepler spacecraft has been looking for signs of new worlds outside the solar system since May 2009, and has so far found more than 4000 planets in the so-called Goldilocks Zone -- neither too hot, nor too cold to sustain life.
Yesterday Nasa announced it had found 500 new possible planets to add to the 4175 already found by the telescope, and 12 were "Earth-like". But Kepler 452b is the first of the 12 to be confirmed as a planet and Nasa said it was the "closest" to Earth that has ever been seen.
It is 60 per cent larger in diameter than Earth and is considered a super-Earth-size planet.
"Today the Earth is a little less lonely because there is a new kid on the block," said Jon Jenkins, Kepler data analysis lead at Nasa's Ames Research Centre in Moffett Field, California. "If you travelled to this star with an arkful of plants ... the plants would photosynthesise just perfectly fine. It would feel a lot like home from the standpoint of the sunshine.
"It is six billion years old. That is considerable opportunity for life to arise on its surface and its oceans should all the necessary conditions for life have appeared on this planet.
"This is the closest thing that we have to another planet like the Earth. And the Earth follows nearly in the footsteps of its older cousin and will be there in 1.5 billion years time."
The discovery gives new hope that alien civilisations may exist beyond the solar system.
This week Professor Stephen Hawking and the Astronomer Royal Lord Martin Rees announced they were joining a US$100 million ($151.7 million) project to seek out signs of extra-terrestrial intelligence in the Milky Way.
"We won't be going to this planet but our children's children's children might be," said Jeff Coughlin, Kepler research scientist at the SETI Institute in California.
Kepler's task is to look for rocky planets between half and twice the size of Earth where water could still exist on the surface. The size of the planet also means it has enough gravity to pull in gases like hydrogen and helium to form an atmosphere.
The space observatory detects planets as their orbits cross in front of their star and cause a very tiny but periodic dimming of the star's brightness. Nasa is also trying to determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.
