In 1930, Clyde Tombaugh serendipitously discovered Pluto: the first object in our Solar System out past Neptune. For 48 years, it was the only object known out there, until Charon -- its giant moon -- was discovered. But the 1990s brought with it a slew of Kuiper Belt objects, and in 2006, Pluto was officially demoted to a "dwarf planet." But also in 2006, NASA's New Horizons mission was launched: the first dedicated mission to the outer Solar System. In 2015, it flew by both Pluto and Charon, discovering two vastly different worlds. Here's what we've learned.