A cold ‘super-Earth’ may live around a nearby star

By Loren Grush

An artistic rendering of what the planet around Barnard’s star might look like.
Image: IEEC / Science-Wave

A planet three times more massive than Earth may be lurking around the second-closest star system to our cosmic neighborhood. Astronomers say they have found evidence for a so-called “super-Earth” around the famous Barnard’s star — a small, faint dwarf star located less than six light-years away. Though we don’t know what this potential planet looks like, its close proximity makes it an excellent target for future telescopes to study in the long-enduring hunt for extraterrestrial life.

This discovery, detailed today in the journal Nature, marks another planet found around a relatively nearby star. In 2016, astronomers claimed to have found a small, rocky world around Proxima Centauri, which is the closest star to our Solar System at just 4.2 light-years away. Now, this makes two possible planetary systems located right next door, further bolstering the idea that our galaxy might be awash in planets.

Plus, a planet around Barnard’s star is one that we could see someday soon. Though we’ve found thousands of planets outside our Solar System, most of these worlds — known as exoplanets — are usually impossible to see directly with our current telescopes. They’re so far away that they become completely overwhelmed by the light from their host stars. But planets around nearby stars — like Barnard’s star — are easier for our instruments to detect. One day, we could potentially distinguish the light reflected from this planet from the light of its host star, especially as bigger and more sophisticated observatories come online in the 2020s.

“That’s the key benefit of finding a planet this close,” Ignasi Ribas, an astronomer at the Institute of Space Sciences in Spain and lead author on the study, tells The Verge. “If we ever have the hope of being able to directly image planets, it’s got to be planets that are really close by because you need to separate them from their star’s light.”

A graphic showing the relative distances of the closest stars to our Solar System.
Image: IEEC / Science-Wave

Barnard’s star has long intrigued astronomers. Because of its relatively short distance from us, it appears to be moving through the sky faster than all the other stars. The star gets its name from E. E. Barnard, the American astronomer who measured this motion in 1916. Barnard’s star is also a very small and dim type of star known as a red dwarf. In fact, it is just 14 percent the mass of our own Sun, and it’s so faint that you need a telescope to see it. Though they are among the smallest stars in the galaxy, red dwarfs are also the most common. In the past few decades, astronomers have found more and more potentially habitable planets around these celestial objects.

Because of its closeness, Barnard’s star has become a popular target for exoplanet hunters. In the 1960s, an astronomer named Peter van de Kamp claimed to have found a Jupiter-sized world around Barnard’s star after studying how the red dwarf moved. At the time, it was a big claim since we did not have good confirmation that exoplanets existed yet. But the findings were eventually refuted and blamed on issues with the telescope van de Kamp used. Over the years, astronomers have ruled out the existence of planets in the habitable zone of Barnard’s star, which is the region around a star where temperatures are just right for water to exist as a liquid.

But this time, there may actually be something in orbit around the star just a little farther out from the habitable zone. An international group of astronomers used data from 20 years of observations of Barnard’s star, taken with seven different telescopic instruments, to find clues that a planet may exist. “We put together nearly 800 observations, using the most precise instrumentation in the world just to see this small planet,” says Ribas.

Specifically, the team looked at how Barnard’s star “wobbled” over time — a technique that astronomers use to find exoplanets when they can’t be seen. Since exoplanets are drowned out by starlight, astronomers have to use clever, indirect ways to tease out the existence of a distant world. One way is to watch as these planets pass between their host star and Earth — what’s known as a transit — which slightly dims the star’s light. However, this planet does not transit, so instead, astronomers looked at how the planet’s gravity is affecting Barnard’s star. Even small planets can push and pull on a nearby star, creating tiny “wobbles” that we can measure from Earth.

Ribas and his team noticed a pattern in Barnard’s star’s wobbling within the 20 years’ worth of data, which indicated a planet revolving around the star every 233 days. They estimate the exoplanet is about 3.2 times the mass of Earth, and its distance from the star puts it in what’s known as the snow line. That’s the region around a star where gases can condense into solids and water turns into snow and ice. This area is about four times farther from the habitable zone, and many astronomers think this is the best spot around a star for planets to form. “If it’s real, then this would be a strong confirmation, one of our leading theories of planet formation,” Keivan Stassun, an astronomer at Vanderbilt University who was not involved in the study, tells The Verge.

An artistic rendering of what the surface of the planet around Barnard’s star might look like (though we haven’t actually seen it yet).
Image by M. Kornmesser / ESO

Still, we haven’t seen this planet directly, and though Ribas is confident in his team’s findings, he admits there’s always a tiny chance that the wobbles they saw can be explained another way. “We are not 100 percent sure the signal comes from a planet, but 99.2 percent sure,” he says. “But being this confident is sufficient to claim detection of the planet.”

So the obvious next step is to get a signal from the planet itself, which may be possible soon. “At this very near distance, we should be able to separate the light from the planet from the light of the star,” says Stassun. Being able to see the light from an exoplanet is huge because then we can study what’s in the planet’s atmosphere. And the gases surrounding a planet could tell us a lot about what’s on the surface below — perhaps even indicating whether or not life is present.

Since this planet is near the snow line, it’s not a particularly promising candidate in the search for alien life. But it will be a good exoplanet for astronomers to study. It could help them refine their techniques for distinguishing planets from stars and figuring out what’s in their atmospheres. And the tools to do this should be coming online within the next decade. NASA’s James Webb Space Telescope, which is supposed to launch in 2021, should have enough sensitivity to study this planet further. And big telescopes, such as the Thirty Meter Telescope planned for Hawaii, may also be able to spot the planet.

Ribas says the best bet for seeing this world is a future telescope NASA is working on called WFIRST. The space telescope could potentially study large planets using a special instrument known as a coronagraph, which helps to block out the light of stars. Due to budget concerns, NASA has been looking at ways to reduce the cost of the coronagraph by reducing the number of masks and filters the instrument uses, according to Space News. But Ribas says a coronagraph is the most certain way to see this world. “It would be very efficient at removing the starlight,” he says. “If that kind of machinery is in place, we’ll be able to image the planet.”

For now, Ribas and his team will continue to monitor the wobbles of Barnard’s star to further strengthen their claim — one that now aligns with numerous works of science fiction. Stories like The Hitchhiker’s Guide to the Galaxy and The Garden of Rama feature planets around Barnard’s star as way stations for interstellar travelers. “It has for decades been a favorite imagined destination for human civilization at some point in the future,” says Stassun. “How tantalizing that this neighboring star is a neighboring solar system.”