Super-Earth may lurk around nearby star

An international team of scientists has found evidence that suggests a large, rocky world — a super-Earth — may be orbiting a nearby star.

Barnard's Star is the fourth closest star to us, coming after the Alpha Centauri triple-star system, comprising Proxima Centauri and Alpha Centauri A and B. Named after American astronomer Edward Emerson Barnard, the star is quite special: It moves across our sky — a measure called proper motion — faster than any other star.

In order to detect the planet, the team of astronomers relied on more than 20 years of data from multiple telescopes and a method called radial velocity, which measures incredibly tiny wobbles in a star caused by the gravitational pull of a planet orbiting it. 

"This is a small planet causing a small wobble of an amplitude of only 1.2 metres per second, which is similar to the speed of a person walking," said Ignasi Ribas, the study's lead author and an astronomer at the Institut d'Estudis Espacials de Catalunya in Barcelona, Spain. "So this means you need to accumulate hundreds and hundreds of measurements to be able to see this tiny motion."

The newly found planet has a mass at least 3.2 times that of Earth, orbits its parent star once every 233 days and is located six light-years away, the researchers suggest in the study published Wednesday in the journal Nature.

Icy world

The exoplanet orbits in a region around the star that is known as the snow line, one of the most favourable areas for planetary formation.

While very little is known about this super-Earth, Ribas says researchers can use what they do know to make some speculations.

(It's important to note that when astronomers use the term "super-Earth," it has nothing to do with habitability, but rather references a planet's size and composition.)

"The most likely scenario is that we're facing a frozen, rocky planet," he said. "But again this is speculation, we don't really know if this is the case."

As for habitability, that is an unknown. Though stellar activity on Barnard's Star — activity that could strip a planet of any potential atmosphere — is relatively low, the planet's location isn't in the traditional habitable zone, where liquid water can exist on a planet's surface and what many believe strengthens the chance for life.

Still, Ribas said, we are learning that some planets or moons we never thought to be habitable could be, such as the moons of Jupiter and Saturn.

"For this planet, … we don't have the knowledge to say, 'Well, we can absolutely confirm and we're absolutely 100 per cent sure that there are no oceans on that planet under the surface.' It's hard to say that because there may be scenarios we're not accounting for," Ribas said. "But if I had to choose a planet to go on vacation, this would not be my first option."

But not everyone is convinced about the finding. Mike Reid, an associate professor at the University of Toronto's Dunlap Institute for Astronomy and Astrophysics, said that the challenge is distinguishing the signal in the data from stellar activity.

"There are exoplanets where the data is incontrovertible," Reid said. "Here they're trying to pull the planet out of the noise in the statistics. It's reasonably convincing, but there are some uncertainties."

Galactic neighbourhood

Barnard's Star is a red dwarf, the most abundant type of star in our galaxy. These stars are small and low mass: Barnard's Star has just 16 per cent the mass of our sun and roughly 17 per cent of its radius.

The nearby star is also no stranger to exoplanet claims. In the 1960s, American astronomer Peter van de Kamp suggested that Barnard's Star was home to two giant planets, reaching the conclusion after he had observed slight wobbles in the star's motion. Unfortunately for him, it was later discovered that the lens of his telescope had shifted, and once that was taken into account, the wobbles disappeared.

And Proxima Centauri — our nearest neighbour at just over four light-years away — is another red dwarf star known to be home to an exoplanet. But in a study published last February, researchers detailed a flare that increased the star's brightness by 1,000 times over 10 seconds, followed by a second, smaller flare, meaning it would be unlikely that any atmosphere would be left over from that cataclysmic event.

"Over the billions of years since [exoplanet] Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water," astronomer and lead author Meredith MacGregor said at the time.

Despite these two planets not being ideal candidates for life as we know it, Ribas is excited by the exoplanets we are finding.

"We're living at a time when we're finding planets around nearby stars, and it's this nice feeling of getting a census of our next-door neighbours," he said.

And who knows? We could one day send a probe to those systems: A project known as Breakthrough Starshot is already working on a plan to send a light-powered spacecraft to the Alpha Centauri system.