The first planet in a globular cluster was discovered orbiting the 11-millisecond pulsar/white-dwarf binary PSR B1620-26 in Messier 4. As more data have become available on this extraordinary object, the permitted range of parameters for this system has become increasingly tightly constrained, and the most recent data require an outer planet with mass of between 1 and 2 Jupiter masses in a hierarchical orbit, with orbital period of a few decades and with high orbital inclination relative to the plane of the inner pulsar/white-dwarf binary, yet relatively modest orbital eccentricity. The signatures of the Newtonian interaction between the planet and the white dwarf are clearly seen in the data. We discuss some of the implications of the current data, including constraints on scenarios for formation of the system and their implications. The data strongly favour a relatively recent exchange of the planet mass tertiary, and we may infer that the planet originally orbited a star which was a member of the cluster, at an orbital radius of a few AU, and that the planet is likely coeval with the cluster stars, having formed about 12.7 billion years ago.
Extreme Solar Systems
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