PSR B1620-26 - A binary radio pulsar with a planetary companion?

Abstract

PSR B1620-26, in the globular cluster M4, is a millisecond pulsar with an approximately 0.3 solar mass companion in an 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-yr 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 an approximately 10-U orbit or a star in an 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.