An international team of astronomers have discovered an Earth-sized planet orbiting the nearest star to the Sun. The rocky world, which is the closest exoplanet ever found, is only 1.3 times as massive as our own planet. Its orbit means it is warm enough to have liquid water on its surface, raising the intriguing possibility that it might be home to alien life. 

However, it should be stressed that no one knows for certain what it is like, and the existence of water and an atmosphere is still speculation. It could also be more like Venus than Earth, which would make it totally inhospitable.

The planet is in orbit around Proxima Centauri, a red dwarf star that lies “only” 4.2 light-years away – equivalent to 40 trillion km. It is part of a triple star system, which means the new planet will have three suns in its sky, with two resembling a bright double star.

The planet was found by a team led by Dr Guillem Anglada-Escudé, who is originally from near Barcelona, but now teaches at Queen Mary University of London. He analysed data that had been collected by two powerful telescopes operated by the European Southern Observatory in Chile.

The first indications that Proxima Centauri had a planet were spotted in February 2013 by Dr Mikko Tuomi, of the University of Hertfordshire. He found its “fingerprints” when he looked back at archived data collected since March 2000.

The observations showed a wobble in the star’s light that showed another body was tugging on it gravitationally. The team got permission to commandeer the telescopes to make a concerted study between 19 January and 31 March this year, using a powerful spectrograph called HARPS to measure the wobbles.

Professor Hugh Jones, from the University of Hertfordshire, said: “We didn’t have enough evidence to conclusively support such a major discovery. Dr Anglada-Escude then joined the University of Hertfordshire team and coordinated major observing campaigns. These gave us enough observations to unequivocally confirm the planetary signal.”

The data revealed tiny wobbles in the starlight from Proxima Centauri. Careful measurement of these wobbles over time allowed the astronomers to calculate the gravitational pull and size of the planet, which has no name and is simply labelled Proxima b.

The observations show that the new planet is only slightly bigger than the Earth, and orbits its home star every 11.2 days at a distance of 4.7 million miles. Although that is only about a twentieth the Earth’s distance from the Sun, Proxima Centauri is a much cooler star, being a red dwarf.

That means that planet Proxima b is lying within the so-called Goldilocks zone where it is not too hot and not too cold, but just right for liquid water to exist. However, the scientists do not know whether it is actually inhabited.

Dr Guillem Anglada-Escudé said in a statement: “Succeeding in the search for the nearest terrestrial planet beyond the Solar System has been an experience of a lifetime. The search for life on Proxima b comes next.”

His own studies, made with the European Southern Observatory’s 3.6-metre telescope at La Silla, Chile, were supported by research made by astronomers and observatories worldwide. The discovery is announced this week in the scientific journal Nature.

Professor Richard Nelson, head of Queen Mary’s School of Physics and Astronomy, told Skymania News: “This is the sort of discovery that you kind of dream of as an astronomer.

“It is very important, not only for what it can tell us about planet formation theory, but also from the more philosophical point of view about what it says about the existence of habitable planets within the Galaxy, and the potential for life in the Universe.

“It touches upon some of the most profound scientific questions that people are interested in asking.”

At a pre-announcement Nature briefing, which described the new world as “Earth-like”, Skymania’s reporter asked whether it might not be more like Venus whose surface is like a poisonous furnace. Dr Ansgar Reiners, a member of the discovery team from the Institut für Astrophysik, Georg-August-Universität Göttingen, Göttingen, Germany, replied: “It may well be like Venus. We have no idea about the actual state of the atmosphere. We can only compute probabilities for it to have an atmosphere that is like Earth of anything else. You can come up with formation scenarios that end up with an Earth-like atmosphere, a Venus-like atmosphere, or no atmosphere at all.”

Professor Nelson told Skymania News that it was important to make further observations to tell if Proxima b is genuinely like Earth. But he said: “We know that it is approximately the same mass as the Earth, so that means it is very unlikely to be a gaseous world like Neptune or Jupiter.

The professor said that studies of planetary formation showed that Proxima b might have formed further away from its star, where there was more icy material, before migrating in to its present orbit. He said: “Under those conditions we would expect the planet to be endowed with very large amount of water – perhaps 50 per cent of the planet itself could be made of water. So it may turn out to be an ocean world where there is an enormous amount of water on its surface.

“And if it is a watery planet, then it is very likely to have an atmosphere, partly made of steam because of the water vapour, but also its constituent parts, the hydrogen and the oxygen. Because hydrogen is 16 times lighter than oxygen, it will tend to evaporate from the planet more easily, leaving behind an atmosphere that could actually be quite oxygen rich. So this atmosphere could very well have a mixture of both water and oxygen in the atmosphere, possibly with some carbon dioxide as well, though that is really in the realms of speculation. But if it is water-rich, then almost certainly it will have a significant atmosphere as well.

Though Proxima b enjoys mild temperatures, its red dwarf host star is known to fire off ultraviolet and X-ray flares that are much more intense than the Earth experiences from the Sun. The possibility of life is therefore likely to depend on whether it has a strong enough magnetic field, as the Earth does, to act as a protective shield.

Despite being the closest alien planet, Proxima b is so far away that a conventional spacecraft would take many thousands of years. But earlier this year, Professor Stephen Hawking and Facebook chief Mark Zuckerberg launched the Starshot Project to send a fleet of tiny robotic probes that could accelerate to a quarter of the speed of light and get there in just 20 years.