Pillar · Region · Updated May 2026
The Oort cloud.
The Oort Cloud is a vast spherical shell of icy bodies surrounding the solar system at distances of roughly 2,000 to 200,000 astronomical units (up to 1.6 light years from the Sun). It has never been directly observed, but its existence is inferred from the orbits of long-period comets, which appear to originate randomly distributed across the sky. It is thought to contain trillions of comet nuclei.
Open the 3D viewer →A spherical shell, not a disc
Unlike the asteroid and Kuiper belts, which are flat discs aligned with the ecliptic, the Oort Cloud is roughly spherical — comets come from every direction. This is because at such enormous distances, the gravitational pull of passing stars and galactic tides has stirred the orbits into all possible inclinations over billions of years.
Source of long-period comets
Comets with orbital periods longer than 200 years — sometimes millions of years — are believed to originate in the Oort Cloud. Occasionally a passing star or a gravitational nudge from the galactic disc perturbs an Oort Cloud body inward, where it eventually crosses the inner solar system and becomes a comet. Halley's comet, with its 76-year period, is short-period and from the Kuiper region. Comet Hale-Bopp (1997), with a period of about 2,500 years, is from the Oort Cloud.
The edge of the solar system
The Oort Cloud marks the gravitational outer limit of the solar system — the boundary where the Sun's gravity loses to the gravitational pull of nearby stars. The nearest known star, Proxima Centauri, sits at 4.24 light years (~268,000 AU). The Oort Cloud may extend to roughly 1.6 light years, which is more than a third of the way to the next star.
Never directly observed
No spacecraft has reached the Oort Cloud — Voyager 1, the most distant human-made object, is at about 167 AU and won't reach the inner edge for another 300 years. Its existence is inferred entirely from indirect evidence: the orbital distribution of long-period comets, the predictions of solar-system formation models, and the calculated reach of the Sun's gravity.
Frequently asked questions
How far away is the Oort Cloud?
Roughly 2,000 to 200,000 astronomical units from the Sun — the outer edge is up to 1.6 light years away, more than a third of the distance to the nearest star.
Has anyone seen the Oort Cloud?
No. Its existence is inferred from the orbits of long-period comets that appear to come from every direction in the sky.
How many comets are in the Oort Cloud?
Estimates range from a few hundred billion to a few trillion comet-sized bodies.