Where do most long-period comets originate?

Most long-period comets—those with orbital periods exceeding 200 years—originate from the Oort cloud. This immense, roughly spherical reservoir of icy bodies surrounds the solar system at distances typically extending beyond 1,000 times the orbit of Neptune, with new comets often originating from a region between 40,000 and 50,000 astronomical units (AU) from the Sun.12

The Oort cloud is believed to have formed from icy planetesimals that were scattered outward by the gravity of the giant planets during the early history of the solar system. These bodies remain in "cold storage" until their orbits are gravitationally perturbed—often by the passage of nearby stars, interstellar molecular clouds, or galactic tidal forces—which sends them inward toward the inner solar system. Because they arrive from such distant, random orientations, long-period comets are also referred to as "nearly isotropic comets" due to their totally random orbital inclinations.123