An "older cousin" of Earth has been discovered orbiting a distant sun-like star more than 1,000 light years away.
The world is 60% larger than Earth and lies in the star's "habitable zone" - the orbital region where temperatures are mild enough to be suitable for life.
No one knows if life has evolved on the planet, Kepler-452b. But since the parent star is 1.5 billion years older than the sun, any creatures living there could be far more advanced than they are on Earth.
That makes Kepler-452b a good candidate for scientists involved in the Search for Extra Terrestrial Intelligence (Seti).
On Tuesday, renowned physicist Professor Stephen Hawking helped launch a new £64 million project to hunt for radio signals from alien civilisations.
Breakthrough Listen, funded by Russian internet billionaire Yuri Milner, will use two of the world's most powerful radio telescopes to scour thousands of stars for intelligent transmissions over 10 years.
News of Kepler-452b's discovery was released by astronomers operating the American space agency Nasa's Kepler space telescope.
Dr Jon Kenkins, from Nasa's Ames Research Centre in Moffett Field, California, said: "We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth's evolving environment.
"It's awe-inspiring to consider that this planet has spent six billion years in the habitable zone of its star; longer than Earth. That's substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet."
The star system is located 1,400 light years away in the constellation of Cygnus, the Swan.
Evidence suggests the new planet is rocky, like the Earth. Its 385-day orbit is only 5% longer than an Earth year.
Ground-based observations from three US observatories confirmed that Kepler-452b was indeed a planet and helped to pin down its size and orbit.
Another 521 yet-to-be confirmed exoplanet candidates were also unveiled by the Kepler team.
Analysis of data from May 2009 to May 2013 has increased the number of potential planets identified by the space telescope to 4,696.
Twelve of the new planet candidates have diameters between one and two times that of the Earth and orbit in their star's habitable zones.
The habitable, or "Goldilocks" zone, is defined as an orbital path that is not too hot or cold, but just the right distance from a star to permit liquid surface water.
John Grunsfeld, associate administrator of Nasa's Science Mission Directorate in Washington DC, said: "On the 20th anniversary year of the discovery that proved other suns host planets, the Kepler exoplanet explorer has discovered a planet and star which most closely resemble the Earth and our Sun.
"This exciting result brings us one step closer to finding an Earth Two."
The new findings will be submitted for publication in the Astrophysical Journal.
Speaking at the Breakthrough Listen launch event held at the headquarters of the Royal Society in London, Prof Hawking said: "We believe that life arose spontaneously on Earth so in an infinite universe there must be other occurrences of life.
"Somewhere in the cosmos, perhaps, intelligent life may be watching these lights of ours aware of what they mean. Or do our lights wander a lifeless cosmos, unseen beacons announcing that here on our rock, the universe discovered its existence?
"Either way, there is no bigger question. It is time to commit to finding the answer - to search for life beyond Earth."
But he went on to warn of the possible risks involved in making contact with a much more advanced race of intelligent beings.
"A civilisation reading one of our messages could be billions of years ahead," said Prof Hawking. "If so, they will be vastly more powerful and may not see us as any more valuable than we see bacteria."