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