extraterrestrial life

Are we alone in the Universe? There are billions of stars in our galaxy, and 100 billion galaxies in our Universe. We don't know how many planets there are, or how many of these planets might have life on them. We've found some planets outside our solar system, but none seem to have conditions needed for life.

Image credits: main image, © AMNH/Jim Steck and Daryl Collins; Neil Tyson: AMNH.