( ( A Mercury ) ( ( is ( ( the ( closest planet ) ) ( to ( the sun ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NNP Mercury)) (VP (VBZ is) (NP (NP (DT the) (JJS closest) (NN planet)) (PP (TO to) (NP (DT the) (NN sun))))) (. .)))
A Mercury is the closest planet to the sun.
(ROOT (S (NP (NNP Mercury)) (VP (VBZ is) (ADJP (ADJP (JJS closest)) (PP (TO to) (NP (DT the) (NN sun))))) (. .)))