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

Abstract

The combination of ever more precise radio timing data and serendipitous {\it 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.