But Galileo was not credited for the discovery since he thought Neptune was a `` fixed star '' instead of a planet . Because of Neptune 's slow movement across the sky , Galileo 's small telescope was not strong enough to detect Neptune as a planet . On both occasions , Galileo mistook Neptune for a fixed star when it appeared very closeâ '' in conjunctionâ '' to Jupiter in the night sky ; hence , he is not credited with Neptune 's discovery .