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We present a combined analysis of previously published high-precision radialvelocities and astrometry for the GJ876 planetary system using aself-consistent model that accounts for the planet-planet interactions.Assuming the three planets so far identified in the system are coplanar, wefind that including the astrometry in the analysis does not result in abest-fit inclination significantly different than that found by Rivera andcollaborators from analyzing the radial velocities alone. In this unique case,the planet-planet interactions are of such significance that the radialvelocity data set is more sensitive to the inclination of the system throughthe dependence of the interactions on the true masses of the two gas giantplanets in the system (planets b and c). The astrometry does allowdetermination of the absolute orbital inclination (i.e. distinguishing betweeni and 180-i) and longitude of the ascending node for planet b, which allows usto quantify the mutual inclination angle between its orbit and planet c's orbitwhen combined with the dynamical considerations. We find that the planets havea mutual inclination of 5.0 +3.9 -2.3 degrees. This result constitutes thefirst determination of the degree of coplanarity in an exoplanetary systemaround 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 likelyformed in a circumstellar disk, and that their subsequent dynamical evolutioninto a 2:1 mean motion resonance only led to excitation of a small mutualinclination. This investigation demonstrates how the degree of coplanarity forother exoplanetary systems could also be established using data obtained fromexisting facilities.

The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c