A planet similar to Earth that may have deep oceans and harbour life has been discovered in another solar system just 4.27 light years away - close enough to be reached by future space missions.
The new world, slightly more massive than Earth, orbitsProxima Centauri - our closest stellar neighbour.
It is now the primary target for Russian billionaire Yuri Milner's ambitious 100 million dollar (£75m) plan to send a fleet of miniature interstellar probes fitted with cameras on a 20-year mission to search for alien life.
Breakthrough Starshot - which is backed by Professor Stephen Hawking - aims to accelerate the tiny one gram "nanocraft" to 20% of the speed of light using "sails" pushed by a powerful laser.
Proxima Centauri is part of a triple system of stars in the constellation of Centaurus. It is the faintest of the three, which also include a much brighter pair of stars known as Alpha Centauri A and B.
From Earth, the system appears as a single bright star - the third brightest visible in the night sky.
Scientists have calculated that Proxima b is about 1.3 times more massive than the Earth and probably rocky.
It lies only 7.5 million kilometres from its parent star, five per cent of the distance between the Earth and the Sun, and takes just 11.2 days to complete one orbit.
But because Proxima Centauri is a dim red dwarf star radiating much less heat than the Sun, the planet occupies the "habitable zone" where temperatures are mild enough to permit liquid surface water.
Professor Richard Nelson, from Queen Mary, University of London - a member of the international team that announced the discovery in the journal Nature, said: "Finding a planet around the star which is the nearest to the Sun is a big event.
My own view is that this planet probably has a significant amount of surface water
"Finding that the planet has certain characteristics that make it potentially Earth-like and habitable is even more exciting.
"My own view is that this planet probably has a significant amount of surface water. It's likely that it formed further out from the star before migrating in, and may have picked up a lot of icy material. I wouldn't be surprised if it had fairly deep global oceans."
One possible obstacle to life on Proxima b is that the planet would regularly be bathed in powerful ultraviolet radiation and X-rays from flares erupting on the star.
But Prof Nelson said he did not think conditions on the planet's surface were too inhospitable for life.
"We shouldn't over-play the radiation too much," he said. "The radiation is significantly larger than we experience on Earth, but it's not frying the planet and it's not continuous. It's associated with significant flaring events on the star.
"The radiation is not going to preclude the existence of life.
"We know that wherever life can evolve on Earth, it does so. We also know that life is very resilient."