In 1950, Jan Oort published a statistical analysis of the orbital elements of long-period comets — comets with orbital periods exceeding 200 years. He noticed two things that demanded explanation. First, many comets had aphelia (farthest points from the Sun) clustered around 50,000 astronomical units — roughly a quarter of the distance to the nearest star. Second, the distribution of original orbital energies showed an excess of nearly parabolic orbits, comets falling in from very large distances on trajectories that suggested they had been sitting far from the Sun for billions of years and were now visiting the inner solar system for the first time. Oort proposed that a vast spherical shell of icy bodies surrounds the solar system at these extreme distances, bound gravitationally to the Sun but loosely enough that gravitational perturbations from passing stars or giant molecular clouds can nudge individual objects onto trajectories that bring them inward.
The Oort Cloud has never been directly observed. No telescope has the sensitivity to detect individual objects at distances of tens of thousands of AU — a kilometer-sized ice body that far from the Sun reflects so little light that it would be invisible to any current instrument. The cloud is inferred from the comets it produces, just as you might infer a mountain range from the rivers flowing from it without ever having seen the mountains themselves. The inference is solid — the orbital statistics of long-period comets demand a source reservoir at the right distances — but the total mass, detailed structure, and origin of the cloud remain uncertain.
Inner and outer Oort Cloud
The Oort Cloud is generally conceived as having two components. The outer cloud (from roughly 20,000 to 100,000 AU) is roughly spherical and thermally detached from the solar system's flattened disk geometry. It is this outer cloud that is most susceptible to stellar perturbations and the galactic tidal field, and most of the comets entering the inner solar system are thought to originate here. The inner cloud (from roughly 2,000 to 20,000 AU), sometimes called the Hills Cloud after astronomer Jack Hills, is more disk-shaped and more densely populated. It may contain 10 to 100 times more objects than the outer cloud but is harder to perturb into comet-producing orbits because its objects are more tightly bound.
The total mass of the Oort Cloud is estimated at between one and a few Earth masses, distributed among a population of objects ranging from small pebbles to bodies hundreds of kilometers across. The total number of Oort Cloud objects above a kilometer in size may be a trillion or more. These objects are thought to have been ejected to their current locations during the early solar system by gravitational interactions with the giant planets — Jupiter, Saturn, Uranus, and Neptune — as those planets migrated to their current positions and scattered the outer solar system's inventory of small icy bodies.
The comet pipeline
The pathway from the Oort Cloud to the inner solar system is not a single step. A perturbation from a passing star or from the galactic tidal field changes an Oort Cloud object's orbit enough to drop it into the inner cloud, where subsequent perturbations may eventually send it onto a near-parabolic trajectory that brings it past Jupiter. Jupiter's gravity then either ejects the comet from the solar system entirely, captures it into a short-period orbit, or deflects it into a trajectory that will send it back to the Oort Cloud to wait for the next perturbation. The long-period comets we see — Hale-Bopp, Hyakutake, and the interstellar interlopers like 3I/ATLAS that occasionally pass through — represent the most recent batch of deliveries from a reservoir so large and distant that it will be supplying the inner solar system long after the Sun has become a white dwarf.
