A frozen “super-Earth” has been discovered orbiting Barnard’s Star, the closest single star to the sun.
Despite surface temperatures of around minus 150C, scientists believe pockets of liquid water could lie beneath the ice capable of harbouring life.
Barnard’s Star is six light years from Earth – hardly any distance on astronomical scales.
The only closer star system is Alpha Centauri, which consists of three stars bound together by gravity.
The newly detected planet, Barnard’s Star b, is thought to be rocky and at least 3.2 times more massive than the Earth.
It circles a cool red-dwarf star, smaller and older than the sun, completing one orbit every 233 days.
Because of the lack of heat the planet remains deep frozen despite being much closer to its parent star than the Earth is to the sun.
But astronomers have not ruled out the chances of life evolving on the icy world.
Unlike many other red dwarfs, Barnard’s Star is relatively inactive and not so likely to blast nearby planets with radiation that would not give life a chance.
Professor Carole Haswell, head of astronomy at the Open University and a member of the international team that announced the discovery in the journal Nature, said: “While the starlight from Barnard’s Star is too feeble for Barnard’s Star b to have liquid water on its surface, Barnard’s Star b probably has a similar temperature to Jupiter’s moon Europa.
“Famously, Europa has a sub-surface ocean which has been considered as a potential habitat for life. It is possible Barnard’s Star b may offer similar niches for life.
“Tantalisingly, super-Earths like Barnard’s Star b probably sustain geothermal activity for longer than their lower mass counterparts. This could be helpful to life by providing sustained heat and the chemicals needed to build complex organic molecules.
“This new discovery offers exciting prospects to learn more about the galaxy’s diversity of planetary systems, starting with our own solar system’s near neighbours.”
The planet’s existence was confirmed after two decades of observations using several different ground-based telescopes and instruments.
One of them was the new state-of-the-art planet-hunting instrument Carmenes at the Calar Alto Observatory in Spain.
Even the most powerful telescopes in use today would not be able to image Barnard’s Star b directly.
Instead, astronomers used the tried and trusted “radial velocity” technique of looking for light frequency variations that betray the “wobble” an orbiting planet imparts on a star.
From these measurements they are able to estimate a planet’s mass and orbital period.
Two years ago the same astronomical team detected a planet orbiting the sun’s closest neighbour, Proxima Centauri, part of the Alpha Centauri system. The planet, Proxima Centauri b, is just 4.2 light years from Earth.
Although Proxima Centauri b has a surface possibly warm enough for liquid water, the chances of finding life there are reduced by hostile X-rays and ultraviolet radiation pouring out of its star.
Prof Haswell added: “Thirty years ago, as far as we knew, our solar system could be utterly unique, with planets formed by a freak event in the sun’s early history.
“Technological developments over the last 20 years have shown that planets are common, and we are now pretty sure – based on statistics – that the majority of stars have planets. This means we can begin to put our own solar system in context, and draw informed conclusions about just how special the Earth is.
“This discovery of Barnard’s Star b, orbiting the nearest single star to the sun, shows that the statistical conclusions are correct: most stars seem to have planets.
“At a distance of only six light years, Barnard’s Star b could conceivably be visited by people from Earth. Certainly, even with our current technology, we could launch unmanned probes to send back pictures of this planet.”
Lead astronomer Dr Ignasi Ribas, from the Institute of Space Studies of Catalonia in Spain, said the team had studied 771 measurements.
She said: “After very careful analysis we are over 99% confident that the planet is there. However we must remain cautious and collect more data to nail the case in the future.”