Unlike the belief held by many of their peers that our Earth and Sun

B changes the meaning of the original sentence.

The original sentence is talking about the belief held by the peers of astronomers Aleksander Wolszczan and Dale Frail.

B just talks "long-held belief" (and not the belief held by the peers of astronomers)


No, their is not ambiguous. The rule is:

When a pronoun appear as part of introductory modifier, the pronoun refers to the noun/noun-phrase immediately after the introductory modifier.

Since pronoun "their" appears as part of introductory modifier ("Even though many...solar system"), "their" would refer to the noun-phrase "astronomers Aleksander Wolszczan and Dale Frail".

p.s. Our book EducationAisle Sentence Correction Nirvana discusses this aspect of introductory modifiers. Have attached the corresponding section of the book, for your reference.