The underlined pronoun that has a stated antecedent is the pronoun "It" of the phrase " (It) is a book about planets. "
In this sentence, the pronoun "It" behaves as a relative pronoun, which is that which refers to a term previously mentioned which we call antecedent. The relative pronoun is given this name because it relates the antecedent to the following terms. In other words, "It," this is referring to a term that was quoted earlier and is an antecedent of "It".
