A Surprise Discovery Around a Lonely Star

A planet more than three times the mass of Earth may be orbiting in a star system long thought to be barren.

M. Kornmesser / ESO

In the 1960s, the astronomer Peter van de Kamp announced that he had discovered a planet orbiting Barnard’s star, one of the nearest stars to Earth at just six light-years away. It was, at the time, the first credible claim of an exoplanet—a planet outside our solar system—and it riveted the astronomy community. The thought of another planetary neighborhood so close to our own fueled science-fiction daydreams. Perhaps someday, if human beings had to abandon Earth, they could travel to the planet around Barnard’s star, a short journey on cosmic scales.

The thrill didn’t last. Although van de Kamp published a series of papers refining his findings, other astronomers couldn’t verify his measurements, and papers questioning van de Kamp’s claim began to appear in the ’70s. In 1973, astronomers discovered that the signal van de Kamp had detected was actually just a glitch in the telescope he had used to photograph the star.

Van de Kamp’s findings were abandoned, and astronomers grappled with the disheartening thought that our cosmic neighbor may not have any planets at all.

Astronomers continued to study Barnard’s star anyway, amassing hundreds of observations using instruments mounted on telescopes around the world. Now, decades after van de Kamp shocked the astronomy community with his claim, they have one of their own.

Astronomers announced Wednesday that they have detected a super-Earth—a planet about three times the mass of our own—orbiting Barnard’s star.

The research relied on 20 years’ worth of observations of the star, gleaned from seven different instruments. When they stitched all the data together, an intriguing signal emerged—the distinct mark of a planet. “It kept getting stronger as measurements accumulated,” says Ignasi Ribas, the lead researcher and scientist at the Institute of Space Studies of Catalonia and the Institute of Space Sciences in Spain. “A slowly growing signal, right before our eyes.”

The planet, referred to as Barnard’s star b, completes one orbit in 233 days. The findings are published in Nature.

It’s too soon to say whether Barnard’s star b definitely exists: The research must be independently verified, by other astronomers, with more data. But some astronomers are hopeful. “I think they’ve genuinely detected a planet,” says Leslie Hebb, an astronomy professor at Hobart and William Smith Colleges, who was not involved in the study.

If confirmed, Barnard’s star b will become the second-closest known exoplanet to Earth, after Proxima Centauri b, which orbits one of the three stars in the Alpha Centauri system, about 4.4 light-years away.

The distance between the alleged planet and its parent star is about the same as the distance between Mercury and the sun. But these celestial bodies have little in common. Barnard’s star is much cooler and smaller than our sun, which means it doesn’t blast its planet with punishing heat. Despite its close proximity, Barnard’s star b is a frozen world, with an estimated surface temperature of -170 degrees Celsius (-274 degrees Fahrenheit). Brrr.

While astronomers have estimated the mass of Barnard’s star b, they don’t know its size or its density, properties that can provide clues about an object’s composition. Ribas says he suspects the planet is mostly made of rock; most known exoplanets with a similar mass to Barnard’s star b have turned out to be rocky, according to findings from the Kepler mission, the exoplanet-hunting telescope that ran out of fuel last month after nearly a decade of operations.

An artist’s concept of the view from Barnard’s star b (M. Kornmesser / ESO)

Barnard’s star b orbits well outside of its system’s habitable zone, where temperatures would be just right for liquid water to exist on a surface. Instead, it resides near the edge of what astronomers call the “snow line,” where starlight is minimal and gases, including water vapor, become solids. Astronomers believe this type of region, which falls somewhere between Mars and Jupiter in our own system, is quite conducive for planet formation.

Ignasi and his colleagues found the exoplanet in the data using the radial-velocity method, a similar technique to the one van de Kamp used in the 1960s. Sometimes, when a planet orbits a star, the planet’s gravity tugs on the star and makes it wobble slightly. The wobbling motion produces changes in the star’s light that can be detected with instruments on Earth, even at great distances. Astronomers use this phenomenon to find planets and estimate their orbits. Van de Kamp had mistaken a technical error in his observations for this tell-tale wobble.

Small, cool stars like Barnard’s star, known as red dwarfs, are popular targets for exoplanet searches using this method. “If a planet is orbiting that star, the planet is able to tug on the star more than if the star were heavier, and so the amount of wobble of the star that you can measure is larger,” says Keivan Stassun, an astronomer at Vanderbilt University, who was not involved in the new study.

There’s a catch. Large planets close to their star are easier to spot than small planets farther out, like Barnard’s star b. “The farther they are, the smaller the perturbations, and the harder [it is] to notice them,” says Abel Méndez, the director of the Planetary Habitability Laboratory at the Arecibo Observatory, where observations of Barnard’s star are ongoing. “That is why most of the detected exoplanets are hot worlds. I was surprised by the detection of this planet around Barnard. It was not easy to detect.”

There’s also the matter of starspots, which can flare up on a star and cause fluctuations in its brightness and mimic the wobbling signal. Barnard’s star is an old star, perhaps twice as old as the sun, so it exhibits less of this erratic stellar activity. But the researchers still had to take star spots into account. “We are 99.2 percent sure” the signal is a planet, Ribas says, “but there’s still that 0.8 percent chance that this could be due to the spots on the surface. We cannot claim this is for sure, and that’s why.”

Ribas says his team took great pains to account for starspots and the kind of technical mistake that felled van de Kamp. “These authors take a very cautious approach,” Stassun says. “They’re not claiming anything definitively here, but it is very exciting.”

Astronomers will have to wait for more research to determine whether Barnard’s star b is the real deal. They know to be skeptical—more skeptical than usual—but they’re optimistic. “It’s been a 60-year-long emotional roller coaster,” Stassun says. “And maybe we finally got it.”