( Gravity ( ( is ( ( ( ( ( the force ) ( that ( keeps ( planets ( orbiting ( their suns ) ) ) ) ) ) , ) and ) ( moons ( orbiting ( the planets ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Gravity)) (VP (VBZ is) (NP (NP (NP (DT the) (NN force)) (SBAR (WHNP (WDT that)) (S (VP (VBZ keeps) (NP (NP (NNS planets)) (VP (VBG orbiting) (NP (PRP$ their) (NNS suns)))))))) (, ,) (CC and) (NP (NP (NNS moons)) (VP (VBG orbiting) (NP (DT the) (NNS planets)))))) (. .)))
Gravity is the force that keeps planets orbiting their suns, and moons orbiting the planets.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ is) (ADJP (JJ responsible) (PP (IN for) (NP (NP (DT the) (NNS orbits)) (PP (IN of) (NP (NP (DT the) (NNS planets)) (PP (IN around) (NP (DT the) (NN sun))))))))) (. .)))
Gravity is responsible for the orbits of the planets around the sun.