( ( The sun ) ( ( ( keeps ( ( every planet ) ( in orbit ) ) ) ( by ( its gravity ) ) ) ... ) )
(ROOT (S (NP (DT The) (NN sun)) (VP (VBZ keeps) (NP (NP (DT every) (NN planet)) (PP (IN in) (NP (NN orbit)))) (PP (IN by) (NP (PRP$ its) (NN gravity)))) (: ...)))
The sun keeps every planet in orbit by its gravity......
(ROOT (S (NP (NNS Planets)) (VP (VBP are) (VP (VBN held) (PP (IN by) (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity))))) (PP (IN in) (NP (NP (JJ elliptical) (NNS orbits)) (PP (IN around) (NP (NN sun))))))) (. .)))