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 21 . An orbital fit yields a companion mass of , a semimajor axis of AU, and an eccentricity of M 5 0.52 M /sinia50.042 e 5 0.03 JUP (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. 1. INTRODUCTION Of the 10 planetary companions found from precision Doppler surveys, seven have semimajor axes of less than 0.25 AU (Mayor et al. 1998; Marcy Butler 1998; Noyes et al. 1997; Cochran et al. 1997). Four of these are 51 Peg cases with semimajor axes of #0.05 AU. While precision Doppler surveys are most sensitive to planets in small orbits, this piling-up of planets near host stars may be a r...