Credit: ESO/M. Kornmesser
This image shows an artist’s impression of the planet’s surface.

Astronomers have found strong evidence of a frigid alien world about 3.2 times more massive than Earth circling Barnard’s Star, the dim red dwarf that lies just 6 light-years from the sun.

Barnard’s Star is our sun’s nearest neighbor, well, apart from the three-star Alpha Centauri system that is, located about 4.3 light-years away. Classified as an M dwarf, its mass is only 15% that of the Sun with just one-fifth its diameter and likely more than twice its age, and just 3 percent as luminous.

Astronomers have long suspected there could be a planet around Barnard’s star due to the high frequency of planets around M dwarf stars revealed by NASA’s now-defunct Kepler space telescope.

Credit: IEEC/Science-Wave-Guillem Ramisa
A graphic representation of the relative distances to the nearest stars from the sun. Barnard’s star is the second-closest star system, and the nearest single star to us.

The newly discovered exoplanet, described in the journal Nature, is unlike anything in our own solar system, larger than Earth but smaller than Neptune, and far enough from its dim, red sun that any water on its surface is locked away in ice.

“In the end, we believe firmly enough the object is there,”

“We always have to remain a bit cautious, and of course more data should strengthen the case for it, but we were sure enough that we were willing to go forward with publication.”

– Ignasi Ribas of the Institut de Ciències de l’Espai and lead author of the paper says.

The first hints that Barnard’s planet was there came in 2015, when one of the co-authors of the paper, Mikko Tuomi, looked at existing radial velocity data from the star and found something changing according to a 233-day period. Barnard’s Star’s periodic wobble suggests it is circled by a large planet once every 233 days, but very few exoplanets have been found so far from their stars (planets with short orbital periods generate more frequent signals, making them easier to detect).

 

 

The researchers discovered the planet using the radial velocity method – unlike the transit method used by the Kepler telescope – it is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet’s parent star. Measuring the slight wobble of the parent star when orbiting planets interact with the star, as they orbit around their center of mass.

“We used observations from seven different instruments, spanning 20 years of measurements, 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 – a huge amount of information!”

“After a very careful analysis, we are over 99 percent confident that the planet is there. However, we’ll continue to observe this fast-moving star to exclude possible, but improbable, natural variations of the stellar brightness which could masquerade as a planet.”

– Ignasi Ribas

Definitive confirmation of Barnard’s Star b is unlikely to come from additional radial-velocity measurements, however. But super-precise measurements of star positions, such as those now being made by the European Space Agency’s Gaia spacecraft, may do the job in the next few years, or using the now active TESS telescope’s transit method to observe the light from the parent star and the faint irregularity caused by the planet as it passes by.