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 1 87123 reveal Keplerian variations with a period of 3.097 days and a semiamp litude of 72 m s(-1). An orbital fit yields a companion mass of M = 0.52 M- JUP/ sin i, 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, ag e, chromosphere, and rotation rate. Although unlikely, non-Keplerian explan ations for the Doppler variations, such as spots and pulsation, cannot be r uled out and require future photometry and spectroscopy.