Two Substellar Companions Orbiting HD 168443

Precise Doppler measurements during 4.4 yr from the Keck/HIRES spectrometer reveal two superimposed Keplerian velocity variations for HD 168443 (G6 IV). A simultaneous orbital fit to both companions yields companion masses of Msini=7.7 and 17.2 M_JUP, orbital periods of P=58 days and 4.8 yr, semimajor axes of a=0.29 and 2.9 AU, and eccentricities of e=0.53 and 0.20. An upper limit to the mass of the outer companion of 42 M_JUP is derived from the lack of astrometric wobble. The outer companion was not detected with Keck adaptive optics in the near-IR. Dynamical simulations show that the system is remarkably stable for all possible masses of both companions. The two orbiting companions have masses that are probably near and slightly above the upper end of the observed mass distribution of ``planets'' at 10 M_JUP. Formation in a protoplanetary disk seems plausible. But these objects present a puzzle about their formation and dynamical history, as well as about their possible kinship with planetary systems and triple-star systems. Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology.