The Kuiper Belt contains the icy remains of the solar system’s formation, and the objects within are made of a dazzling array of different materials. For instance, Pluto is larger by a slim margin than Eris (known for unseating the former planet from its status). But Pluto is mostly made of ice, so it has far less mass. Eris has a large proportion of rock, making it more dense. Some worlds out there seem to have a lot of methane, while others have a lot of ammonia. It’s a bit like a garage sale of dwarf planets, with bits and baubles from our early history that are key to understanding how our planets came to be—and how other planetary systems may or may not resemble our own.