The radio millisecond pulsar PSR B1620 - 26 is part of an extraordinary triple star system in the globular cluster M4 .
The inner companion to the neutron star is thought to be a white dwarf of mass m _ { 1 } \simeq 0.3 M _ { \odot } in an orbit of period \simeq 0.5 yr .
The nature and orbital characteristics of the second , more distant companion , have remained a mystery for many years .
A theoretical analysis of the latest available radio pulsar timing data is presented here , allowing us to determine approximately the mass and orbital parameters of the second companion .
Remarkably , the current best-fit parameters correspond to a second companion of planetary mass , with m _ { 2 } \sin i _ { 2 } \simeq 7 \times 10 ^ { -3 } M _ { \odot } , in an orbit of eccentricity e _ { 2 } \simeq 0.45 and with a large semimajor axis a _ { 2 } \simeq 60 AU .
The short dynamical lifetime of this very wide triple in M4 suggests that large numbers of such planets must be present in globular clusters .
We also address the question of the anomalously high eccentricity of the inner binary pulsar .
While this eccentricity could have been induced during the same dynamical interaction that created the triple , we find that it could also naturally arise from long-term secular perturbation effects in the triple , combining the general relativistic precession of the inner orbit with the Newtonian gravitational perturbation by the outer planet .