Our solar system does not end at Pluto. Surrounding it is a theorized shell of icy bodies stretching up to 3.2 light years from the Sun, never directly observed but demanded by the math. This episode unpacks the Oort cloud, starting with the puzzle of long-period comets that plunge in from every direction on unstable orbits and cannot have survived unchanged since the solar system formed. We trace Ernst Opik's 1932 proposal and Jan Oort's 1950 analysis of comet aphelia clustering near 20,000 astronomical units, along with the cometary fading problem and the idea of dead, crusted-over comets we can no longer see.

We then map the cloud's architecture, from the doughnut-shaped inner Hills cloud to the fragile spherical outer shell, and explain why trillions of objects add up to only about five Earth masses. We cover the 1996 PW discovery that  ...