Universe - Alien Worlds: The Search for Second Earth episode 2: Humans have long gazed up at the night sky, wondering whether other lifeforms and intelligences could be thriving on worlds far beyond our own.     Answering that question seemed fated to remain pure speculation. But over the last few decades, ultra-sensitive telescopes and dogged detective work have transformed alien planet-hunting from science fiction into hard fact. Gone are the days of speculation; the hunt for extraterrestrials has become…