WASHINGTON, D.C.--Which star is the sun's closest neighbor? Astronomy textbooks tell you it's Proxima Centauri, just 4.3 short light-years away. But according to Todd Henry of Georgia State University in Atlanta, the truth is that no one knows for sure. There is a fair chance that an extremely faint dwarf star or a cool brown dwarf--a "failed" star incapable of the hydrogen fusion that fuels our sun--haunts our cosmic backyard, closer than Proxima.
Over the past few years, Henry and his colleagues have conducted an ongoing search for close stellar neighbors. The goal is not just to establish a new entry in the astronomical Guinness Book of Records. A complete knowledge of all the red and brown dwarfs in our own locale will provide astronomers with a better understanding of the composition of the Milky Way, Henry says. The survey focuses on faint stars that move relatively swiftly across the sky, indicating that they are close by. By measuring their parallax--the apparent shift in their position as Earth orbits the sun--the team is able to determine the stars' distances.
The survey has identified 13 previously unknown stars, including 12 that Henry introduced here on 10 January at the 199th meeting of the American Astronomical Society. One of the stars is a mere 12 light-years away, making it the 20th closest star to the sun. Another surprising system contains one white dwarf and two red dwarfs, orbiting each other. An interesting aside, says Henry, is that possible planetary companions of these low-mass stars will be very easy to find with future instruments. "It's my bet that the first Earth-like planet will be found around a [nearby] red dwarf," he says.
Mark Skrutskie of the University of Virginia in Charlottesville agrees that there may be undiscovered dwarfs within a few light-years of the sun. Skrutskie is the principal investigator of the 2MASS infrared sky survey, which already has spotted numerous brown dwarfs at farther distances. According to the most recent conservative estimates, he says, the chance that Proxima's proximity record will be shattered within a few years is at least 10%.