A pale streak with a faint tail moving slowly against the fixed stars is a familiar sight, but the true origin of a comet is difficult to grasp. These icy visitors arrive from a distance so vast that the Sun is merely a bright star among many. To understand where comets come from, we have to travel outward past the planets and the Kuiper Belt to the Oort Cloud, a spherical shell of frozen mountain-sized bodies suspended in absolute silence at four degrees above absolute zero.

This episode examines the structure of the Oort Cloud and explains how these frozen remnants of the early solar system were ejected into the outer darkness by the gravity of giant planets like Jupiter. We explore what causes these bodies to fall inward after billions of years of stillness. The gravitational pull of passing stars and the tidal force of the Milky Way cre ...