Update on Pulsar B1620−26 in M4: Observations, Models, and Implications

The combination of ever more precise radio timing data and serendipitous HST data has confirmed that the outer companion to PSR B1620−26 is a planet. Here we summarize the observational situation, including preliminary new timing solutions and the implications of the measured system parameters. We detail the proposed formation scenarios, discussing the advantages and problems of each for explaining the origin of the triple, and we speculate on some of the implications for planet formation in the early universe. Future data on this system will provide additional constraints on fundamental modes of planet formation. We predict that many more exchanged planets will be discovered orbiting recycled pulsars in globular clusters as the sensitivity and duration of radio timing increases. Strong observational tests of some of the alternative formation models should be possible with additional data. 1. Historical Background The long series of radio timing data of the triple pulsar PSR B1620−26 (Lyne et al. 1988) has now confirmed that the second companion (Backer et al. 1993), is a 1−3MJ substellar object—a planet—in a low eccentricity, wide circumbinary orbit about the inner pulsar–white dwarf binary. The outer orbit is significantly inclined to the inner orbital plane, and HST photometric data confirms that the white dwarf is young, and is a proper motion member of the cluster (Thorsett et al. 1999; Sigurdsson et al. 2003). The mass of the object is very well constrained. The timing data puts a firm lower mass limit of a jupiter mass; as more data has come in, the upper bound on the mass has steadily shrunk. When timing first hinted at the presence of the third object in the system, the mass was only weakly constrained (c.f. Michel 1995), although various arguments suggested that a low mass solution might be preferred (c.f. Thorsett et al. 1993, Sigurdsson 1993, Phinney 1993, Rasio 1994, Joshi and Rasio 1997). By 1999 Thorsett et al., using measurements of the precession of the inner orbit produced by the tidal field of the outer object, had effectively excluded stellar mass companions, and the main issue was