  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 m1 ~0.3 Msun in an orbit of period ~0.5
year. 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 m2 ~0.007 Msun, in an orbit of
eccentricity e2 ~0.45 and with a large semimajor axis a2 ~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.
