( ( ( The ( Sun 's ) ) gravity ) ( ( is ( what ( keeps ( ( the planets ) ( in orbit ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNP Sun) (POS 's)) (NN gravity)) (VP (VBZ is) (SBAR (WHNP (WP what)) (S (VP (VBZ keeps) (NP (NP (DT the) (NNS planets)) (PP (IN in) (NP (NN orbit)))))))) (. .)))
The Sun's gravity is what keeps the planets in orbit.
(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.