Final answer:
The underlined phrase in the sentence is a participial phrase that modifies the noun phrase.
Step-by-step explanation:
The underlined phrase in the sentence, 'The idea of alien life, proposed many years ago, has not yet been proven,' is a participial phrase. Participial phrases consist of a participle (a verb form ending in -ing or -ed) along with any associated modifiers or complements. In this case, the participial phrase 'proposed many years ago' modifies the noun phrase 'The idea of alien life.'
Learn more about Identifying phrases in sentences