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Why do Jupiter-family comets usually orbit close to the plane of the solar system?

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Comet Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.

Jupiter-family comets typically orbit close to the plane of the solar system, known as the ecliptic, because they originate from a flattened source of icy bodies rather than a spherical one.1

Dynamical simulations indicate that this concentration of orbits can only be explained by a flattened disk-shaped reservoir. That source is the Kuiper belt, a disk of icy bodies extending beyond the orbit of Neptune. More specifically, these comets are sourced from the scattered disk—a population of Kuiper belt objects with orbits that have been perturbed by Neptune, which can gravitationally scatter them inward to become Jupiter-family comets.1

In contrast, comets originating from the Oort cloud—a vast, spherical region surrounding the solar system—exhibit random orbital inclinations and can approach the planetary system from all directions.12