  Aims. To search for extrasolar planets around intermediate-mass stars, we are
conducting a precise RV survey around K-giants. Methods. We present
high-accuracy RV measurements of {\alpha} Ari made from November 2003 to
February 2010. This star belongs to our sample of 55 K-giants studied for
extrasolar planet and pulsation searches using the fiber-fed Bohyunsan
Observatory Echelle Spectrograph (BOES) attached to the 1.8-m telescope at
Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. Results. We find a
planetary companion with long-period and low-amplitude radial velocity (RV)
variations in oscillating K2 III star {\alpha} Ari (HD 12929). We do not find
the correlation between RV variations and chromospheric activity indicators (Ca
II H & K region, H{\alpha} line). The bisector analysis also shows that the
bisector velocity span (BVS) does not show any obvious correlation with RV
variation but has periodic component that may be attributed to the rotation of
the star. If the RV variation is indeed caused by a planetary companion, an
orbital solution with a period of P = 381 days, a semi-amplitude of K = 41 m/s,
and an eccentricity of e = 0.25 fits the data best. Assuming a possible stellar
mass of M_star = 1.5 M_sun, we estimate the minimum mass for the planetary
companion of m2 sin i = 1.8 M_Jupiter with an orbital semi-major axis of 1.2
AU. Conclusions. Our finding of a likely exoplanet in {\alpha} Ari supports
searching for extrasolar planets around giant stars with multiperiodic
oscillations.
