A planet three times the size of Earth has been spotted orbiting Barnard's Star, one of the closest suns to our Solar System.

Various science-fiction authors – notably Douglas Adams, Arthur C. Clarke and Michael Moorcock – have written about an alien world around Barnard's Star, which at six light years away is relatively close to our corner of the galaxy. If the futurists at the Breakthrough Starshot campaign are right, we could get a probe to such a system within 30 years from launch using today's tech.

“Barnard’s star is among the nearby red dwarfs that represents an ideal target to search for exoplanets that could someday actually be reached by future interstellar spacecraft,” said Steven Vogt, professor emeritus of astronomy and astrophysics at UC Santa Cruz in the US, and coauthor of the paper on the planet which will be published in Nature.

The planet, snappily dubbed GJ 699 b, is estimated to be 3.2 times the mass of Earth and orbits the dim red dwarf star every every 233 days. It's far enough away from the star to make it a most likely frozen planet, with an estimated surface temperature of -150C (-238 in Freedom units).

“After a very careful analysis, we are over 99% confident that the planet is there, since this is the model that best fits our observations,” added Ignasi Ribas, of the Institute of Space Studies of Catalonia (IEEC), who led the team that found the planet, on Tuesday.

“However, we must remain cautious and collect more data to nail the case in the future, because natural variations of the stellar brightness resulting from starspots can produce similar effects to the ones detected.”

Playing the long game

Getting proof of GJ 699 b's existence has taken over 21 years and involved some of the finest telescopes humanity has yet built.

In 1997 the team started scanning Barnard's star using the Keck Observatory’s HIRES instrument, which was designed by Vogt himself. For 16 years it charted the wobbles in Barnard's Star caused by an object orbiting the celestial body.

Two years ago this data was correlated with readings from the European Southern Observatory’s UVES and HARPS spectrometers, the Automated Planet Finder (APF) telescope at the University of California's Lick Observatory, the Carnegie Institution for Science’s Planet Finder Spectrograph (PFS) on the Magellan II Telescope at Las Campanas Observatory in Chile, and CARMENES, a new planet-hunter spectrograph at Calar Alto Observatory in Spain.

“For the analysis, we used observations from seven different instruments, spanning 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies. The combination of all data led to a total of 771 measurements,” Ribas said.

The discovery is also something of a vindication for Dutch astronomer Peter van de Kamp, who predicted that there were planets orbiting Barnard's Star after seeing wobbles similar to those which prompted today's announcement. The claims were hotly disputed and, while van de Kamp predicted the orbiting planets would be gas giants not rock worlds, he did at least get the planetary prediction right. ®