A Planet with a 3.1 Day Period around a Solar Twin

Doppler measurements from the Keck/HIRES spectrometer of the G3 V star HD 187123 reveal Keplerian variations with a period of 3.097 days and a semiamplitude of 72 m s^-1. An orbital fit yields a companion mass of M=0.52 M_JUP/ sini, a semimajor axis of a=0.042 AU, and an eccentricity of e=0.03 (consistent with zero). HD 187123 appears similar to the Sun in mass, age, chromosphere, and rotation rate. Although unlikely, non-Keplerian explanations for the Doppler variations, such as spots and pulsation, cannot be ruled out and require future photometry and spectroscopy.