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(Abreviated) Continued radial velocity monitoring of the nearby M4V red dwarfstar GJ~876 with Keck/HIRES has revealed the presence of a Uranus-mass fourthplanetary companion in the system. The new planet has a mean period of$P_e=126.6$ days (over the 12.6-year baseline of the radial velocityobservations), and a minimum mass of $m_e\sin{i_e}=12.9\pm 1.7\,M_{\oplus}$.Self-consistent, N-body fits to the radial velocity data set show that thefour-planet system has an invariable plane with an inclination relative to theplane of the sky of $i=59.5^{\circ}$. The fit is not significantly improved bythe introduction of a mutual inclination between the planets ``b'' and ``c,''but the new data do confirm a non-zero eccentricity, $e_d=0.207\pm0.055$ forthe innermost planet, ``d.'' In our best-fit coplanar model, the mass of thenew component is $m_e=14.6\pm1.7\,M_{\oplus}$. Our best-fitting model placesthe new planet in a 3-body resonance with the previously known giant planets(which have mean periods of $P_c=30.4$ and $P_b=61.1$ days). The criticalargument, $\varphi_{\rm Laplace}=\lambda_c-3\lambda_b+2\lambda_e$, for theLaplace resonance librates with an amplitude of $\Delta\varphi_{\rmLaplace}=40\pm13^{\circ}$ about $\varphi_{\rm Laplace}=0^{\circ}$. Numericalintegration indicates that the four-planet system is stable for at least abillion years (at least for the coplanar cases). This resonant configuration ofthree giant planets orbiting an M-dwarf primary differs from the well-knownLaplace configuration of the three inner Galilean satellites of Jupiter, whichare executing very small librations about $\varphi_{\rm Laplace}=180^{\circ}$,and which never experience triple conjunctions. The GJ~876 system, by contrast,comes close to a triple conjunction between the outer three planets once perevery orbit of the outer planet, ``e.''

The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration