Oort cloud

The Oort cloud is a hypothesized, roughly spherical reservoir of icy bodies surrounding the Solar System far beyond the planets. Gravitational disturbances can send some of its objects inward as long-period comets.

Long-period comet: A comet with an orbital period longer than 200 years, often arriving from the distant Solar System. Perturbations of Oort-cloud objects can place them on these elongated orbits.
Solar nebula: The rotating cloud of gas and dust from which the Sun and Solar System formed. The cloud's proposed source population formed in the same nebula as the planets.
Astronomical unit: The mean Earth–Sun distance, about 150 million kilometers, used as a standard unit of Solar System scale. It provides a familiar unit for expressing the cloud's immense proposed distances.
Kuiper belt: A disk-shaped population of icy bodies beyond Neptune's orbit. Unlike the distant, roughly spherical cloud, this reservoir occupies the planetary plane beyond Neptune.
Cometary orbit: The path followed by a comet under the gravity of the Sun and other bodies. The distribution of incoming comet orbits provides the main evidence for a distant reservoir.
Galactic tide: The gravitational influence of the Milky Way's mass distribution on objects within it. Its gradual pull can alter distant Oort-cloud orbits enough to send bodies sunward.
Planetesimal: A solid body formed by the accumulation of dust and smaller solids in a young planetary system. Objects scattered outward during planet formation began as planetesimals.
Orbital eccentricity: A dimensionless measure of how much an orbit departs from a circle. Long-period comets' highly elongated orbits point back to a distant source.
Scattered disc: A population of trans-Neptunian objects on eccentric, inclined orbits shaped by Neptune. It is a nearer, flattened reservoir that can supply some comets without invoking the Oort cloud.
Comet shower: A temporary increase in the rate at which comets enter the inner Solar System. Shower histories may reveal how stellar passages and galactic tides disturb the cloud.

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