It was a discovery nearly a century in the making, but astronomers have finally detected a planet around the speedy little red dwarf known as Barnard’s Star. The existence of the planet is not particularly surprising given the vast harvest of exoplanets discovered since 1995 around all manner of stars. Nor is the planet a habitable world, to be sure. But it was welcome news nonetheless to find that the nearest single star to Earth has at least one planet in its relatively feeble gravitational embrace.
In many ways, Barnard’s Star was the “white whale” of exoplanet hunters. That’s because the star is close, just six light years away, the second-closest star system to Earth, which should make it easier to find an orbiting planet. And the star is prominent because of its speedy apparent motion across the sky. Barnard’s Star is also old, more than twice the age of our own solar system, so it’s had plenty of time to form a planetary system. But for nearly a hundred years, astronomers have examined the star for evidence of a planetary system using visual observation, photographic imaging, and finally using modern spectroscopic planet-hunting techniques. They came up empty every time.
There was, of course, an exhilarating announcement by the Swarthmore College astronomer Peter van de Kamp in the mid-1960s. He claimed to have detected a wobble in the star’s motion across the sky, which he presumed was caused by the gravitational influence of an unseen planet. Van de Kamp refined his results through the 1960s and 1970s and published a series of entirely serious scientific papers on the subject. But other astronomers were unable to reproduce his results, an essential step in the scientific enterprise. After much head scratching, it turned out his results were caused not by a planet around Barnard’s Star, but by periodic maintenance of the telescope’s objective lens by technical staff at the observatory. Still, the popular imagination was stoked by van de Kamp’s work, and Barnard’s Star figured prominently in many works of science fiction.
Despite so many null results, astronomers kept at it. In fact, they’ve been collecting data on Barnard’s Star for some twenty years using seven different telescopes to build up a spectroscopic signal on the star’s radial velocity, the slight wobble in the star’s motion that a planet might induce. The most recent study, announced in November 2018, compiled this data along with additional measurements and painstaking analyses to finally conclude with 99.2% certainty that Barnard’s Star does indeed have a planet.
Here’s a link to the scientific paper published in Nature that explains the details of the discovery.
What kind of planet is it? Barnard’s Star b, as it’s called, is about three times the mass of our own planet. The radius and density are still unknown, but most planets of this mass turn out to be rocky worlds like Earth or Mercury rather than gaseous worlds like Jupiter. The planet is also cold, with an estimated surface temperature of about -174oC. That puts it in the “snow line”, the position around the star at which volatile compounds like water, carbon dioxide, and ammonia condense into ices. It orbits Barnard’s Star in about 233 days at a distance equal to the distance between Mercury and our Sun. The planet is much colder than Mercury because Barnard’s Star is a small red dwarf, a dim-bulb star with a brightness only 0.04% that of our Sun.
Because of its low mass, the star sips its nuclear fuel frugally and will live to a great age. It’s already twice the age of our Sun, about 10 billion years, and has another many tens of billion of years to go before its demise. A planet around such a star would be a great place to build a long-lived civilization.
Barnard’s Star, of course, is also an accessible object to us amateur stargazers. Despite its proximity, the intrinsically dim star shines at just 9th magnitude. But you can see it with a pair of binoculars or any telescope. It’s located just off the horns of a little asterism called Taurus Poniatowski in the constellation Ophiuchus which is prominent in the sky from July through November. The star moves quickly across the sky at a rate of 10.4” per year, a motion plenty fast enough to track with a small telescope against the background stars. It’s great fun to see this speedy little star for yourself, especially now that we know it has a planetary companion that, in astronomical terms, is right next door to our own solar system.
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