The region beyond Neptune is home to thousands of small, icy worlds that are “cosmic leftovers” from the material from which the Earth and the other planets formed. Because these objects are so distant and faint, they’re extremely difficult to detect: over 60 years passed between the discovery of Pluto in 1930 and the discovery of the next trans-Neptunian object. I’ll describe how we use telescopes on the ground and in space, together with the tools of modern data science, to discover hundreds of new objects and understand this vast “third zone” of the solar system.