( ( More ( massive planets ) ) ( ( are ( able ( to ( ( ( ( form ( earlier ( in ( the ( nebula evolution ) ) ) ) ) ( when ( ( the ( disk gas ) ) ( is ( ( denser and ) hotter ) ) ) ) ) , ) ( while ( ( ( smaller planets ) ( such ( as ( ( Uranus and ) Neptune ) ) ) ) ( ( can only ) ( ( form gaps ) ( in ( the ( cold ( outer nebula ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJR More) (JJ massive) (NNS planets)) (VP (VBP are) (ADJP (JJ able) (S (VP (TO to) (VP (VB form) (ADVP (RBR earlier) (PP (IN in) (NP (DT the) (NN nebula) (NN evolution)))) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NN disk) (NN gas)) (VP (VBZ is) (NP (NN denser) (CC and) (NN hotter))))) (, ,) (SBAR (IN while) (S (NP (NP (JJR smaller) (NNS planets)) (PP (JJ such) (IN as) (NP (NNP Uranus) (CC and) (NNP Neptune)))) (VP (MD can) (ADVP (RB only)) (VP (VB form) (NP (NNS gaps)) (PP (IN in) (NP (DT the) (JJ cold) (JJ outer) (NN nebula)))))))))))) (. .)))
More massive planets are able to form earlier in the nebula evolution when the disk gas is denser and hotter, while smaller planets such as Uranus and Neptune can only form gaps in the cold outer nebula.
(ROOT (S (NP (NNS Uranus)) (VP (VBZ is) (NP (DT the) (ADJP (JJS least) (JJ massive)) (JJ outer) (NN planet))) (. .)))