Final answer:
The underlined phrase 'proposed many years ago' in the given sentence is a participial phrase, acting as an adjective to describe 'The idea of alien life.' Participial phrases are among several types of phrases, including prepositional, gerund, and infinitive phrases, which play different roles within sentences.
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. A participial phrase includes a participle and acts as an adjective, giving more information about a noun or pronoun in the sentence. Here, 'proposed many years ago' is describing 'The idea of alien life' and is using the past participle 'proposed'.
Participial phrases are just one type of phrase students may encounter. Others include prepositional phrases that begin with a preposition and function as either adjectives or adverbs, gerund phrases that start with a gerund and function as nouns, and infinitive phrases that start with an infinitive and can act as nouns, adjectives, or adverbs. Correctly identifying these phrases helps in understanding sentence structure and the relationships of ideas within a sentence.