Evidence for a long-period planet orbiting epsilon Eridani

High-precision radial velocity (RV) measurements spanning the years 1980.8- 2000.0 are presented for the nearby (3.22 pc) K2 V star epsilon Eri. These data, which represent a combination of six independent data sets taken with four different telescopes, show convincing variations with a period of app roximate to7 yr. A least-squares orbital solution using robust estimation y ields orbital parameters of period P = 6.9 yr, velocity amplitude K = 19 m s(-1), eccentricity e = 0.6, projected companion mass M sin i = 0.86 M-Jupi ter and semimajor axis a(2) = 3.4 AU. Ca II H and K S-index measurements sp anning the same time interval show significant variations with periods of 3 and 20 yr yet none at the RV period. If magnetic activity were responsible for the RV variations, then it produces a significantly different period t han is seen in the Ca II data. Given the lack of Ca II variation with the s ame period as that found in the RV measurements, the long-lived and coheren t nature of these variations, and the high eccentricity of the implied orbi t, Keplerian motion due to a planetary companion seems to be the most likel y explanation for the observed RV variations. The wide angular separation o f the planet from the star (approximately 1 ") and the long orbital period make this planet a prime candidate for both direct imaging and space-based astrometric measurements.