An international team of astronomers reports the discovery of two new planets orbiting Kapteyn’s star, a nearby red dwarf with a long history. One of its newly discovered worlds, dubbed Kapteyn b, circles the star at the right distance to allow seas to survive on its surface, where water is seen as a key ingredient of life as we know it.

The astronomers spotted the planets from the La Silla Observatory and the Las Campanas Observatory, both in Chile, with help from the W. M. Keck Observatory in Hawaii. To make the discoveries, they measured tiny periodic wobbles of the star as its planets gravitationally tugged at it. From these periodic wobbles, the scientists figured out the masses and orbital times of the planets.

Artistic representation of the potentially habitable exoplanet Kapteyn b as compared with Earth. Credit: PHL @ UPR Arecibo

Depiction of the potentially habitable exoplanet Kapteyn b compared with Earth. Courtesy of PHL @ UPR Arecibo

It turns out that Kapteyn b possesses at least five times Earth’s mass and has a year only 48 Earth-days long. It circles closer to the star than Mercury orbits the sun in our own solar system. But because its star is a cool red dwarf, the planet likely bathes in just enough heat to keep water in a liquid state on its surface.

The second planet, Kapteyn c, however, is more massive and takes 121 days to circle its host star. The team believes that it lies outside of the star’s habitable zone, where water would simply freeze.

Storied Star

Named after Jacobus Cornelius Kapteyn, the Dutch astronomer who discovered it, Kapteyn’s star resides only 13 light-years from Earth and ranks as the 25th nearest one to our sun.

It belongs to a stellar class known as red dwarfs, or M dwarfs. A fraction of the size of our sun, they shine in the much cooler, red part of the spectrum and appear to be the most numerous type of star by far in our Milky Way galaxy, easily numbering in the billions.

“Finding a stable planetary system with a potentially habitable planet orbiting one of the very nearest stars in the sky is mind-blowing. This is one more piece of evidence that nearly all stars have planets, and that potentially habitable planets in our galaxy are as common as grains of sand on a beach,” said Pamela Arriagada, the second author, and a Carnegie postdoctoral researcher.

And while its classification may be humdrum, the origin of Kapteyn’s star is unusual, to say the least.

This diminutive star—weighing in at only one-third the mass of our sun—is the second fastest moving star ever recorded. This suggests it may have originated in a dwarf galaxy cannibalized by our much larger Milky Way galaxy some 11.5 billion years ago. The likely remnant core of that long dead dwarf galaxy is a giant globular cluster called Omega Centauri, which contains a mass of hundreds of thousands of stars some 16,000 light-years away.

This storied pedigree would make Kapteyn’s star and its family of worlds some two-and-a-half-times older than Earth, making this super-Earth world, Kapteyn b, the oldest potentially habitable planet yet found.

On a cautious note, a separate team led by Ofer Cohen of the Harvard-Smithsonian Center for Astrophysics reported on Monday that red dwarfs might subject planets to much harsher space weather than the sun deals out to Earth. That would make habitable-zone planets circling red dwarfs more Mars-like, the team reported at the ongoing American Astronomical Society meeting in Boston.

Nevertheless, “it does make you wonder what kind of life could have evolved on those planets over such a long time,” said Guillem Anglada-Escude, part of the Kapteyn b discovery team and an astronomer at the Queen Mary University of London.

This work is published in the latest edition of the Monthly Notices of the Royal Astronomical Society.

See for Yourself

Like other red dwarf’s littering the heavens, Kapetyn’s star is too faint to be seen with just the naked eye at 8.9 magnitude. However it’s an easy target for binoculars from a dark sky or small telescope in the suburbs.

Located in the small southern constellation of Pictor, this red dwarf is best seen from the Southern Hemisphere.

This time of the year, it can be seen after nightfall using low magnification and wide-field eyepiece about midway up the southwest sky. The distinctively red-hued star is wedged between the constellations Dorado and Columba.