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, orbits Proxima 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, 5% 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.
"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."
He added that the planet appeared to have a "normal" orbit - not a highly eccentric one, like that of a comet, which would not be conducive to life.
Colleague Dr Guillem Anglada-Escude, also from Queen Mary University of London - who led the team of around 30 astronomers, said: "Succeeding in the search for the nearest terrestrial planet beyond the solar system has been an experience of a lifetime, and has drawn on the dedication and passion of a number of international researchers.
"We hope these findings inspire future generations to keep looking beyond the stars. The search for life on Proxima b comes next."
The discovery was made after astronomers studied Proxima Centauri using a special instrument on the 3.6-metre telescope operated by the European Southern Observatory (ESO) at La Silla in Chile's Atacama desert.
The High Accuracy Radial velocity Planet Searcher (HARPS) was able to measure the tiny "wobble" in the star's position caused by its interaction with the planet's gravity.
Shifts in the star's light spectrum showed that at times the star was approaching Earth at around human walking pace - about 3mph - and at other times receding at the same speed.
From this data, the scientists were able to infer the presence of a planet around 1.3 times more massive than the Earth orbiting at a distance of 7.5 million kilometres.
Because red dwarfs can mislead planet hunters by giving false signals linked to "star spots" - the equivalent of sun spots - the scientists had to be sure of their findings.
Initial hints of a planet were observed in March 2000 and it took another 15 years before sufficient evidence was available to justify announcing the discovery to the world.
Co-author Dr John Barnes, from the Open University, said: "Once we had established that the wobble wasn't caused by star spots, we knew that there must be a planet orbiting within a zone where water could exist, which is really exciting.
"If further research concludes that the conditions of its atmosphere are suitable to support life, this is arguably one of the most important scientific discoveries we will ever make."
Professor Abraham Loeb, from Harvard University, US - who chairs the Breakthrough Starshot advisory board, confirmed that the mission now had Proxima b in its sights.
He said: "The discovery of the habitable planet around the nearest star, Proxima Centauri, is strategically important for motivating the Breakthrough Starshot initiative, since it provides an obvious target for a flyby mission.
"A spacecraft equipped with a camera and various filters could take colour images of the planet and infer whether it is green (harbouring life as we know it), blue (with water oceans on its surface) or just brown (dry rock). The spacecraft could also measure the mass of the planet and its magnetic field environment.
"We will celebrate this important discovery within the Starshot team. The discovery is likely to energise the project.
"The curiosity to know more about the planet (most importantly whether it hosts life) will give the Starshot initiative a sense of urgency in finding out more facts about the planet, especially those facts that cannot be inferred with existing telescopes from our current vantage point on Earth, at a distance of 4.24 light years from Proxima."
He said the first phase of the project over the next five to 10 years would involve a feasibility demonstration of the laser-driven light sail technology.
The aim was to launch the "nanocraft" towards Proxima Centauri within the next two to three decades.
"It will take 20 years to reach Proxima and four more years for the photos to get back to Earth," he added.
Prof Loeb raised the possibility of humans one day colonising Proxima b.
"The lifetime of Proxima is several trillion years, almost a thousand times longer than the remaining lifetime of the Sun," he said. "Hence, a habitable rocky planet around Proxima would be the most natural location to where our civilization could aspire to move after the Sun will die, five billions years from now."
James Cameron's blockbuster film Avatar depicts a habitable moon, Pandora, orbiting a Jupiter-like planet in the Alpha Centauri star system.
Scientists may find evidence of life on Proxima b long before the Breakthrough Starshot probes - or any other spacecraft - reach the planet.
If the planet can be spotted crossing or "transiting" the face of its star, astronomers may be able to look for chemical signatures of life by studying the light filtering through its atmosphere.
"It would open the way to absolutely fabulous new work," said Prof Nelson.