( ( ( ( ( This force ) , ) ( known ( as gravity ) ) ) , ) ( ( ( keeps ( the planets ) ) ( in ( ( ( orbit ( around ( the sun ) ) ) and ) ( ( the moon ) ( around ( the earth ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT This) (NN force)) (, ,) (VP (VBN known) (PP (IN as) (NP (NN gravity)))) (, ,)) (VP (VBZ keeps) (NP (DT the) (NNS planets)) (PP (IN in) (NP (NP (NP (NN orbit)) (PP (IN around) (NP (DT the) (NN sun)))) (CC and) (NP (NP (DT the) (NN moon)) (PP (IN around) (NP (DT the) (NN earth))))))) (. .)))
This force, known as gravity, keeps the planets in orbit around the sun and the moon around the earth.
(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.