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