PSR-B1620-26 - A BINARY RADIO PULSAR WITH A PLANETARY COMPANION

PSR B1620-26, in the globular cluster M4, is a millisecond pulsar with a approximately 0.3 M. companion in a approximately 0.7 AU orbit. It was recently realized (Backer 1993) that timing observations of this pulsar show, in addition to a linear spin-down, a large-period second derivative naturally interpreted as evidence for a varying acceleration, or jerk, of the pulsar binary. We describe timing observations of PSR B1620-26 taken over a 5 year period. The measured acceleration and jerk are inconsistent with acceleration by the mean gravitational field of M4, and an encounter with another star close enough to cause the observed acceleration is very unlikely. The data are consistent with acceleration by either a planet in a approximately 10 AU orbit or a star in a approximately 50 AU orbit. We therefore believe that PSR B1620-26 is likely to be either the first pulsar in a triple stellar system or part of the first binary star system with a planet, possibilities that can be distinguished with further observations over the next decade or two.