Epsilon Eridani is the closest star known to have a planet: and maybe two of them. It is the only system that has both precise Doppler data and long-baseline astrometric positional observations, which combined yield the orbital tilt and the true planetary mass. Given its closeness and the knowledge of where it is relative to its star, the planet may become the first ever actually imaged.
The circle shows the location of the class K (K2) dwarf star Epsilon Eridani (in the constellation Eridanus). At a distance of only 10.5 light years, the star is the closest to the Sun for which an extrasolar planet is known. With an average orbital radius 3.39 times that of the distance between the Sun and Earth (the Astronomical Unit, AU), 65% of the way from the Sun to Jupiter, the planet takes 6.85 years to make an orbit. Combination of stellar velocity and positional measures of the star show the planet to have a mass 1.55 times that of Jupiter with an uncertainty of only 15 percent. The orbital inclination of 30 degrees to the plane of the sky agrees well with the inclination of a dusty circumstellar disk. Unlike Jupiter, the planet is in a highly eccentric orbit that takes it from as far as 5.8 AU from the star (just farther than Jupiter is from the Sun) to as close as 1.1 AU. The planet will be farthest from Epsilon Eridani proper, 1.7 seconds of arc, late in the year 2010, when we have the best hope of imaging it directly even though it will be 20 million times fainter than the star. Slow changes in the apparent orbit suggest another "Jupiter" with a period greater than 50 years (which would place it at least 13 AU from the star).
Epsilon Eridani is a fourth magnitude (on the bright side, 3.73) class K (K2) ordinary, though relatively young, hydrogen-fusing dwarf. A mere 10.5 light years away, it is the 10th closest star system to the Earth. Its temperature of 5080 degrees Kelvin (88 percent that of the Sun) and low luminosity (34 percent solar) tell of a lower mass, just 83 percent that of the Sun. Though its rotation speed appears similar to that of the Sun, Epsilon Eridani is much younger (about 800 million years, as opposed to the Sun's 4.6 billion), and displays considerable solar- like activity. Unlike many stars so far found to have planets, Epsilon Eridani is not metal-rich, its metal content actually a bit less than that of the Sun (about 80 percent solar). The star achieved lasting fame as one of the first to be examined for radio signals produced by intelligent life, when Frank Drake turned an 85-foot radio telescope to it and to Tau Ceti in 1960. No signals were found. At least now the star does indeed appear to have a real planet, though as a Jupiter-like body, one incapable of supporting life. (For more information, see G. F. Benedict et al., Astronomical Journal, November 2006.)
- - Written by Jim Kaler