The anomalous spin period second derivative of the binary millisecond pulsar PSR~1620-26 in the globular cluster M4 (Backer 1992, Backer \etal 1993, Thorsett \etal 1993) is best explained by a sub--jovian ma ss planet in a moderately eccentric $\sim 7\, AU$ orbit about the pulsar binary (Lyne \etal 1987, McKenna and Lyne 1988, Goss \etal 1988, Phinney 1993). We consider formation scenarios for PSR~1620-26. A planet scavenged from a single main--sequence star during an exchange encounter naturally produces systems such as PSR~1620-26. The position of the pulsar just outside the core of M4 is shown to fit naturally with the preferred formation scenario and permit a planet to have survived in the inferred orbit about the binary. It is possible the orbital eccentricity of the binary was induced by the planet. A confirmation of a planet in eccentric orbit about PSR~1620-26 would strongly suggest that planets form ubiquitously around low mass main--sequence stars, even stars of low metallicity.
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