  The existence of multiple planetary systems involved in mean motion
conmensurabilities has increased significantly since the Kepler mission.
Although most correspond to 2-planet resonances, multiple resonances have also
been found. The Laplace resonance is a particular case of a three-body
resonance where the period ratio between consecutive pairs is n_1/n_2 near to
n_2/n_3 near to 2/1. It is not clear how this triple resonance can act in order
to stabilize (or not) the systems.
  The most reliable extrasolar system located in a Laplace resonance is GJ876
because it has two independent confirmations. However best-fit parameters were
obtained without previous knowledge of resonance structure and no exploration
of all the possible stable solutions for the system where done.
  In the present work we explored the different configurations allowed by the
Laplace resonance in the GJ876 system by varying the planetary parameters of
the third outer planet. We find that in this case the Laplace resonance is a
stabilization mechanism in itself, defined by a tiny island of regular motion
surrounded by (unstable) highly chaotic orbits. Low eccentric orbits and mutual
inclinations from -20 to 20 degrees are compatible with the observations. A
definite range of mass ratio must be assumed to maintain orbital stability.
Finally we give constrains for argument of pericenters and mean anomalies in
order to assure stability for this kind of systems.
