  We present a combined analysis of previously published high-precision radial
velocities and astrometry for the GJ876 planetary system using a
self-consistent model that accounts for the planet-planet interactions.
Assuming the three planets so far identified in the system are coplanar, we
find that including the astrometry in the analysis does not result in a
best-fit inclination significantly different than that found by Rivera and
collaborators from analyzing the radial velocities alone. In this unique case,
the planet-planet interactions are of such significance that the radial
velocity data set is more sensitive to the inclination of the system through
the dependence of the interactions on the true masses of the two gas giant
planets in the system (planets b and c). The astrometry does allow
determination of the absolute orbital inclination (i.e. distinguishing between
i and 180-i) and longitude of the ascending node for planet b, which allows us
to quantify the mutual inclination angle between its orbit and planet c's orbit
when combined with the dynamical considerations. We find that the planets have
a mutual inclination of 5.0 +3.9 -2.3 degrees. This result constitutes the
first determination of the degree of coplanarity in an exoplanetary system
around a normal star. That we find the two planets' orbits are nearly coplanar,
like the orbits of the Solar System planets, indicates that the planets likely
formed in a circumstellar disk, and that their subsequent dynamical evolution
into a 2:1 mean motion resonance only led to excitation of a small mutual
inclination. This investigation demonstrates how the degree of coplanarity for
other exoplanetary systems could also be established using data obtained from
existing facilities.
