Question

add a comma or semicolon if needed. otherwise, submit the text without any additional punctuation.

we now know that life can exist in much harsher environments than previously believed
consequently, it is possible that extraterrestrial life forms live in places once thought
uninhabitable.

Explanation:

The two independent clauses "We now know that life can exist in much harsher environments than previously believed" and "consequently, it is possible that extraterrestrial life forms live in places once thought uninhabitable" are connected by the conjunctive adverb "consequently". When a conjunctive adverb connects two independent clauses, a semicolon should be used before the conjunctive adverb. So we need to replace the blank (which is a period - like break here) with a semicolon.

Answer:

We now know that life can exist in much harsher environments than previously believed; consequently, it is possible that extraterrestrial life forms live in places once thought uninhabitable.