( It ( ( is ( ( the ( sun 's ) ) ( gravity ( that ( keeps ( ( the Earth ) ( in ( its orbit ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (DT the) (NN sun) (POS 's)) (NN gravity) (SBAR (WHNP (WDT that)) (S (VP (VBZ keeps) (NP (NP (DT the) (NN Earth)) (PP (IN in) (NP (PRP$ its) (NN orbit))))))))) (. .)))
It is the sun's gravity that keeps the Earth in its orbit.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ attracts) (NP (DT the) (NN earth)) (PP (TO to) (NP (DT the) (NN sun)))) (. .)))