A famous example of someone who believed in other planets and life on these planets was Giordano Bruno.

He was a Dominican friar (defrocked) who was tried and killed for heresy in 1600.

Here is the exact quote, translated from Latin:

"Innumerable suns exist; innumerable earths revolve around these suns in a manner similar  to the way the seven planets revolve around our sun. Living beings inhabit these worlds."

This is from his work "On the Infinite Universe and Worlds".