For thousands of years, humans thought that Earth was the      center of the universe. Thanks to our curiosity, imagination, and urge to explore, we now know that planets like Earth are nothing special in the cosmos. What we have found so far has surpassed the wildest expectations of scientists as well as authors of science fiction. The sun is just one star among hundreds of billions in our galaxy—and most of those stars have planets! A huge diversity of worlds is out there, just waiting to be discovered.