Episode Details

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 Jewitt and Jane Luu who finally photographed 1992 QB1 after five years of searching with newly invented CCD sensors. The episode then explains how Neptune's gravity sculpts the belt into classical and resonant populations, why Pluto and Neptune can never collide, how the Nice model of planetary migration explains the dynamically hot and cold populations, and why the discovery of Eris forced the 2006 redefinition of the word planet.

• Why short-period comets like Halley's have an expiration date and what that revealed about a hidden reservoir
• How the switch from photographic plates to CCDs raised light capture from about 10 percent to 90 percent
• The 2:3 mean motion resonance that keeps Plutinos safely synchronized with Neptune
• Why fragile binary pairs in the cold population prove they are untouched 4.5-billion-year-old fossils
• What New Horizons revealed at Pluto in 2015 and at the pancake-shaped Arrokoth in 2019