One of the closest stars to the Sun appears to host a planet at least three times as massive as our own where temperatures might be just low enough to freeze liquid water.
In the constellation Ophiuchus, just below the Summer Triangle, lies Barnard’s star, one of the closest and most well studied red dwarf stars in the Galaxy. Now after more than two decades of searches, an international team of astronomers reports November 14th in Nature that they have found signs of an exoplanet hiding in its light.
At just under 6 light-years away, Barnard’s star is the fourth closest star to the Sun (after the three stars of the Alpha Centauri system), though it’s too faint to see with the naked eye. 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. 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.
Hints of a super-Earth
The search for planets around Barnard’s star until now has come up empty, but this latest investigation, led by Ignasi Ribas (Institute of Space Sciences, Spain), combines both new and archival observations spanning 20 years from seven different instruments across the globe. The data record the star’s radial velocity, its relative speed toward and away from Earth. The result is “one of the largest and most extensive datasets ever used for precise radial velocity studies” Ribas says — a dataset that yielded a significant periodic signal with a period of 233 days.
After rigorous analysis and ruling out stellar rotation as the cause, the best explanation for this signal is a planet at least 3.2 times as massive as Earth — a.k.a. a “super-Earth” — orbiting Barnard’s star at a distance of 0.4 a.u. (60 million km). That’s a bit farther out from its star than Mercury is from the Sun. But given the star’s weak luminosity — its radiant energy is less than 4% that of the Sun — this orbit puts the planet, dubbed Barnard’s star b, close to the system’s snow line, where water exists only in its frozen form, making habitability unlikely. This jibes with some planet formation theories, which predict that super-Earths, the most common type of planet around low-mass stars, likely form close to the snow line.
This is the first time that the radial velocity method has detected a planet this small and this far away from its host star. The technique, which uses Doppler shifts of a star’s light to measure the tug of a planet on that star, is biased towards finding large planets in close orbits because their signals are much greater.
Don't buy a ticket just yet
“This is an extremely exciting new planet candidate,” says Sarah Ballard (MIT), who was not involved with this research. She believes the authors strike a good balance in the paper between being confident in their data and recognizing that even being 99% confident in their detection is not a guarantee that the planet is real. “A 1% probability that the signal is not a genuine exoplanet is many orders of magnitude above what the community typically designates a planet and not just a candidate," she says. “I wouldn't buy a ticket to Barnard's star just yet, knowing that there's a 1% chance the planet isn't there.”
Follow-up observations are already underway, according to co-author Guillem Anglada-Escudé (Queen Mary University of London). Additionally, the proximity of Barnard’s star to Earth means the separation between planet and star on the sky — about 220 milliarcseconds — is large enough to make it an ideal candidate for direct imaging as the next generation of instruments, such as NASA’s Wide Field Infrared Survey Telescope (WFIRST), come online over the next decade. The researchers say that the European Gaia satellite and even the Hubble Space Telescope might be able to directly measure the star’s planet-induced wobble on the sky, which could reveal the inclination of the planet’s orbit and therefore its true mass.
The discovery of Barnard b also shows how the radial velocity technique can be pushed to new limits by combining historical data with new observations from many high-precision instruments at multiple facilities. This could lead to the detection of many more super-Earths on relatively wide orbits. The researchers point out that previously the only hope for detecting these systems was chance alignments from microlensing, where the gravity of the planet and its sun momentarily magnify and brighten a distant star in the background.
“I was struck by the duration and intensity of the radial velocity campaign, across many of the best resources that exist in radial velocity,” Ballard says. “In person-hours and telescope-hours, this is a precious and hard-won planet (if true).”
I. Ribas et al. "A Candidate Super-Earth Planet Orbiting Near the Snow Line of Barnard’s Star." Nature. November 14, 2018.