Our corner of the Milky Way is getting rather neighborly. In 2016, astronomers discovered a planet orbiting Proxima Centauri, the nearest star to our sun, just 4 light-years away. Now, they believe they have found an exoplanet around Barnard’s star, which at 6 light-years away is the second-closest star system. The planet—a chilly world more than three times heavier than Earth—is close enough that scientists could learn about its atmosphere with future giant telescopes. “This is going to be one of the best candidates,” says astronomer Nikku Madhusudhan of the University of Cambridge in the United Kingdom, who was not part of the discovery team.
Barnard’s star b, as the new planet is called, was excruciatingly difficult to pin down, and the team is referring to it as a “candidate planet” though it is confident it’s there. Most exoplanets, including the thousands identified by NASA’s recently retired Kepler space telescope, were found using the “transit” technique: looking for a periodic dip in starlight as a planet passes in front. But that method detects only the small fraction of planets that cross their star’s face when viewed from Earth. Despite decades of watching, astronomers haven’t detected any planets transiting Barnard’s star.
But astronomers can also look for planets by measuring their gravitational tug on a star. Hundreds of exoplanets have been found by looking for periodic Doppler shifts in the frequency of starlight. In 2015, astronomers saw hints of such shifts in the light from Barnard’s star. “Then we went hard for it,” says astronomer Ignasi Ribas of the Institute of Space Sciences in Barcelona, Spain, who led the new project.
His team made observations from two ground-based telescopes in Chile and Spain. They also observed with a spectrograph at Spain’s Calar Alto Observatory and added in archival data spanning 20 years from those and four other instruments, giving them a total of nearly 800 measurements. “It was a community effort,” Ribas says. As they report today in Nature, they found that the star’s light oscillated every 233 days, implying a planet orbiting with a 223-day year.
There’s a chance that the oscillations are caused by something that affects the way the star shines in a periodic way, such as star spots. The team has calculated that this is highly unlikely, although still possible. “We’re quite convinced” it is a planet, Ribas says. Madhusudhan isn’t quite so certain: “If confirmed, this will be very good. It shows how hard it is to do this thing.”
From this orbital information, the team calculates the planet must weigh at least 3.2 times as much as Earth. That puts Barnard’s star b squarely into a terra incognita between small rocky planets like Earth and larger gas planets like Neptune. The Kepler mission has shown that such intermediate planets are common across the galaxy, but with no examples among our eight home planets, astronomers have few ideas what they are like. Are they rocky super-Earths, or gaseous mini-Neptunes? “We just don’t know. It’s really hard to tell,” Ribas says.
Finding out more about Barnard’s star b will likely require telescopes able to detect light from the planet itself. That’s hard to do because, viewed from Earth, the planet is close to the star and swamped by its glare. A few telescopes with coronagraphs—devices for masking a star’s light—have directly imaged a few large planets in wide orbits, but something like Barnard’s star b will require the greater resolution of giant telescopes coming in the next decade, such as Europe’s 39-meter Extremely Large Telescope. Observations from these scopes could reveal the planet’s rotation rate, the composition and thickness of its atmosphere, and whether it has clouds. “This would be a dream. We would learn so much about this planet,” Ribas says.
Even if Barnard’s star b is rocky, life would have a hard time taking root on its chilly surface. Although the planet orbits its star much closer than Earth does to the sun, Barnard’s star, a red dwarf, is so dim that its planet gets only 2% of the energy that Earth does. The team estimates surface temperatures of –170°C.
Madhusudhan thinks the result is a sure sign that astronomers will soon find other arrivistes to the stellar neighborhood. “I’m willing to guess there are lots like this nearby,” he says. “The question is, how do we detect them?”
*Correction, 15 November, 9:50 a.m.: This story has been updated to correct the orbital period.