When astronomers study the neighbors, Alpha Centauri usually takes center stage, and we neglect the second closest, Barnard’s Star. Only 6 light-years away, this red dwarf holds the record for highest proper motion: It zips across the sky at 10" per year. The star’s proper motion, combined with the 67 miles per second (108 kilometers per second) velocity toward us (deduced from Barnard Star’s spectrum), gives the star a total speed of 87 miles/s (140 km/s) relative to the Sun. That’s 10 times the speed of a normal star.
And this speed is slow compared with Kapteyn’s Star, another red dwarf 12.8 light-years away. It moves twice as fast as Barnard’s Star. Most local stars, including the Sun, revolve around the galaxy’s center in circular orbits. Both Barnard’s and Kapteyn’s, however, are tourists from the Milky Way’s halo that are just passing through. Their high relative speeds are caused by strikingly different galactic orbits.
Many red dwarfs have powerful magnetic fields that, like the Sun’s, collapse to produce bright, localized stellar flares. Unlike the Sun’s version, however, red dwarf flares can temporarily double the star’s brightness. Among the best known are Proxima Centauri and UV Ceti, the latter lying in a binary system 8.7 light-years away. Such flares are visible across the electromagnetic spectrum, from X-ray to radio, and can be fun for patient observers to spot.
Fifty years ago, astronomer Frank Drake thought we might find intelligent signals from possible planets belonging to G-class Tau Ceti. Although Drake had no clue at the time, at least four planets orbit Tau Ceti. These range in size from about 1.8 to 3.9 Earth masses. Unfortunately, none of them lies within the star’s habitable zone — the region where liquid water could exist on a rocky surface — and thus none has conditions conducive to life as we know it.
But these four are just the tip of the exoplanet iceberg. To date, astronomers have found some 30 planets orbiting the stars in our neighborhood, not counting the eight officially logged in our solar system. The only other star in our collection with four known planets is distant Ross 780 (Gliese 876), located 15.2 light-years away. Both YZ Ceti (12.1 light-years) and Wolf 1061 (13.9 light-years) have three planets.
But perhaps the most intriguing planet found to date orbits the Sun’s nearest neighbor, Proxima Centauri. This world, which weighs 1.3 Earths, circles its star in just 11.2 days at a distance of 0.0485 AU. But because Proxima is a cool red dwarf, its planet lies within the star’s habitable zone, and it’s not inconceivable that life could exist there. Still, Proxima’s stellar flares make advanced life unlikely on such a planet. But who knows? We can’t help but wonder if the Sun’s nearest stellar neighbors harbor hospitable planetary neighbors as well, some perhaps like Earth.