GENESIS OF A PLANET IN MESSIER-4

The anomalous spin period second derivative of the binary millisecond pulsar PSR 1620-26 in the globular cluster M4 (Backer 1993; Backer, Sallmen, & Foster 1993; Thorsett, Arzoumanian, & Taylor 1993) is best explained by a sub-Jovian mass planet in a moderately eccentric approximately 7 AU orbit about the pulsar binary (Lyne et al. 1987; McKenna & Lyne 1988; Gross, Kulkarni, & Lyne 1988; Phinney 1993a). 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 that 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 unbiquitously around low-mass main-sequence stars, even stars of low metallicity.