But this frozen “super Earth,” the second-closest exoplanet known to science, is a tantalizing clue to what else might be out there. And in the not-so-distant someday when telescopes become capable of photographing planets around other stars, it may well be the first new world we see.
The exoplanet’s sun, a tiny body known as Barnard’s Star, is one of our solar system’s nearest neighbors. The only stars closer are the triplet stars of the Alpha Centauri system, which is mainly visible in the southern sky. One of those stars, Proxima Centauri, is orbited by a small planet, but the star’s tendency to spew flares of deadly radiation means its planet is unlikely to be habitable.
Barnard’s Star has long been “the great white whale” of exoplanet hunting, said Carnegie astronomer Paul Butler, a co-author on the Nature paper. It’s just six light-years from our sun, and possibly twice as old. One of the main architects of exoplanet research, the astronomer Peter van de Kamp, proposed more than 50 years ago that this star could host a planet. In the 1970s, British astronomers studied the possibility of sending an uncrewed starship to probe the alien system — even though there wasn’t any evidence a planet existed to be explored.
But it wasn’t until the first exoplanet discovery was confirmed in 1995 that the search for a world around Barnard’s Star began in earnest.
This red dwarf is a 10th the mass of our sun and too faint to be seen with the naked eye. But its low mass makes it ideal for analysis using the radial velocity technique of exoplanet detection, which exploits the way a planet’s gravitational pull makes a star wobble as it orbits around it.
A slew of telescopes on three continents have set their sights on Barnard’s Star, allowing researchers to accumulate some 800 observations over the course of 20 years. The study authors also drew on data collected by amateur astronomers.
It took the combined efforts of more than 50 researchers at some two dozen institutions, but “slowly a signal in our data came out of all the noise,” said astronomer Ignasi Ribas, who is director of the Institute of Space Studies in Catalonia, Spain, and the lead author of the Nature paper.
Barnard’s Star’s periodic wobble suggests it is circled by a large planet once every 233 days. Very few exoplanets have been found so far from their stars (planets with short orbital periods generate more frequent signals, making them easier to detect).
Because Barnard’s Star is so dim, the planet’s long orbital period puts it at the “snow line,” where sunlight is so faint that its surface is perpetually frozen. Its average surface temperature is possibly a brisk -150 degrees Celsius (-238 degrees Fahrenheit).
This puts the planet outside the traditional “habitable zone,” where conditions are thought to be ripe for life. But Teske pointed out that microbes are resilient creatures; if there is water on the planet and if other necessary ingredients are present, it’s feasible that organisms might lurk in an ocean beneath the ice.
Yet much about the planet around Barnard’s Star remains uncertain. Astronomers aren’t sure if it’s rocky like Earth or built of gas and ice, like Neptune. They know it must be at least three times as massive as Earth, but it could be even larger.
They aren’t even 100 percent certain the planet is there, Ribas noted. The research pushed the limits of the radial velocity detection technique, which becomes more difficult the farther a planet is from its star. Mathematical models suggest there is still a 0.8 percent chance that Barnard’s Star’s apparent wobble is caused by some other factor, like sunspots. For that reason, the exoplanet is deemed a “candidate,” rather than a confirmed discovery.
“Difficult detections such as this one warrant confirmation by independent methods and research groups,” Rodrigo Diaz, an astronomer at the University of Buenos Aires who was not involved in the research, wrote in a commentary for Nature. But if it’s confirmed, the “remarkable planet” would give “a key piece in the puzzle of planetary formation and evolution,” he wrote.