Evidence for a Long-Period Planet Orbiting ε Eridani

High precision radial velocity (RV) measurements spanning the years1980.8--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 takenwith four different telescopes, show convincing variations with a period of$\approx$ 7 yrs. A least squares orbital solution using robust estimationyields orbital parameters of period, $P$ = 6.9 yrs, velocity $K$-amplitude $=$19 {\ms}, eccentricity $e$ $=$ 0.6, projected companion mass $M$ sin $i$ = 0.86$M_{Jupiter}$, and semi-major axis $a_2$ $=$ 3.3 AU. Ca II H&K S-indexmeasurements spanning the same time interval show significant variations withperiods of 3 and 20 yrs, yet none at the RV period. If magnetic activity wereresponsible for the RV variations then it produces a significantly differentperiod than is seen in the Ca II data. Given the lack of Ca II variation withthe same period as that found in the RV measurements, the long-lived andcoherent nature of these variations, and the high eccentricity of the impliedorbit, Keplerian motion due to a planetary companion seems to be the mostlikely explanation for the observed RV variations. The wide angular separationof the planet from the star (approximately 1 arc-second) and the long orbitalperiod make this planet a prime candidate for both direct imaging andspace-based astrometric measurements.Comment: To appear in Astrophysical Journal Letters. 9 pages, 2 figure