A pair of resonant planets orbiting GJ 876

Precise Doppler measurements during 6 yr from the Lick and Keck observatori es reveal two planets orbiting GJ 876 (M4V). The orbital fit yields compani on masses of M sin i = 0.56 and 1.89 M-J, orbital periods of P = 30.1 and 6 1.0 days, semimajor axes of a = 0.13 and 0.21 AU, and eccentricities of e = 0.28 and 0.10, respectively. The orbital periods are nearly in the ratio o f 2: 1, unprecedented among major planets but common among moons and astero ids. Moreover, the axes of the elliptical orbits appear to be nearly aligne d. The inner companion was not recognized previously owing to the 2: 1 rati o of periods, which allowed its signature to masquerade as added orbital ec centricity of the outer planet. Dynamical simulations show that the system is stable within a subset of the observed orbital parameters. The stability may be provided by a mean-motion resonance and the apparent alignment of t he major axes. These planets pose unsolved questions about their formation and dynamical evolution, which brought them within 0.08 AU of each other an d locked them in resonance.