For over 70 years Pluto looked like the lonely last outpost of the solar system. This episode explores how astronomers discovered that it is actually the gateway to the Kuiper Belt, a vast circumstellar disk stretching from about 30 to 50 astronomical units that is roughly 20 times wider than the asteroid belt and made largely of frozen water, methane, and ammonia. The conversation covers the frost line, the dwarf planets that live out there, and why Neptune's backward-orbiting moon Triton is almost certainly a captured Kuiper Belt object.

The discovery story is full of irony. Gerard Kuiper, the belt's namesake, actually argued in 1951 that an Earth-sized Pluto would have cleared the region out. It was Uruguayan astronomer Julio Angel Fernandez who mathematically proved in 1980 that a reservoir of short-period comets had to exist, and David ...