Solar system · Outer edge

The Oort Cloud

The vast, unseen cloud of icy bodies at the edge of the solar system, reaching perhaps a third of the way to the nearest star, and the source of the long-period comets.

Kuiper Belt Inner Oort Cloud Outer Oort Cloud 1 AU 10 AU 100 AU 1,000 AU 10,000 AU 100,000 AU Earth Jupiter Neptune New Horizons Sedna, closest Heliopause Voyager 1 Sedna, farthest 1 light-year Proxima Centauri
Each step on the axis is ten times farther than the last. The heliopause, where the solar wind ends, is less than a thousandth of the way to the outer edge of the Oort Cloud. Distances to the cloud's edges are estimates from comet orbits and models.
Inner edge
about 2,000 AUinner Oort Cloud
Outer edge
about 100,000 AU1.5 light-years
Members
up to a trillionlarger than 1 km
Total mass
a few Earthsestimated
Seen directly
noneonly its comets

A shell of perhaps a trillion icy bodies, thousands of times farther out than Neptune, that no one has ever seen

The Oort Cloud is the source of the long-period comets, those that take more than 200 years to orbit the Sun. Its existence is inferred from their orbits: in 1950 the Dutch astronomer Jan Oort noticed that many arrive from 20,000 AU or more, from every direction in the sky, and proposed a vast spherical reservoir; Ernst Öpik had suggested the idea in 1932. Estimates put its population at hundreds of billions to a trillion or more bodies larger than 1 km, with a total mass of a few Earths. The inner Oort Cloud, sometimes called the Hills cloud, is thought to be a thick disk from about 2,000 to 20,000 AU; the outer cloud a sphere from 20,000 to about 100,000 AU, where the Sun's grip gives way to the pull of passing stars and the Milky Way.

The giant planets threw the cloud's comets out of the young solar system

The Oort Cloud's bodies formed among the giant planets, then were flung outward by close passes with Jupiter, Saturn, Uranus and Neptune. Most were thrown out of the solar system altogether; the ones that ended up barely bound had their orbits rounded off by the tide of the Milky Way and by passing stars, lifting their closest approach far beyond the planets. Simulations suggest the Sun may also have captured a large share of the cloud from sibling stars in the cluster where it was born.

A passing star or the galactic tide nudges comets inward, and a few reach the inner solar system

When a star passes within a light-year or so, or the tide of the galaxy slowly changes an orbit, some comets fall toward the Sun on paths lasting millions of years. Famous long-period comets include Hale-Bopp, McNaught and C/2023 A3 Tsuchinshan-ATLAS. Comet C/2014 UN271 Bernardinelli-Bernstein, with a nucleus about 120 km across, is the largest Oort Cloud comet known; it reaches its closest point to the Sun, beyond Saturn's orbit, in 2031. The star Gliese 710 will pass about 10,000 AU from the Sun in 1.3 million years, close enough to send a shower of comets inward. Comets

Simulations in 2025 found a spiral 15,000 AU long in the inner Oort Cloud

David Nesvorný and colleagues at the Southwest Research Institute found that the galactic tide twists the inner Oort Cloud into a two-armed spiral about 15,000 AU long, tilted about 30° to the plane of the planets. It turned up when the team's model was rendered in 3D for a Hayden Planetarium show in New York. The spiral would be very hard to confirm: it would take hundreds of known objects in the inner cloud.

Alpha Centauri probably has an Oort Cloud too; the two do not overlap today but may touch when the star passes closer

Alpha Centauri, the nearest star system, is 4.37 light-years or about 276,000 AU away. If it has an Oort Cloud scaled to its mass, a simple 2025 estimate from the University of Leicester puts the gap between the edges of the two clouds at 34,000 to 50,000 AU, so they do not overlap today. Alpha Centauri is approaching, though: in about 28,000 years it will pass within about 3 light-years, close enough that the outer edges of the two clouds could meet and its gravity could nudge more comets toward us. The two clouds may already be trading material. Cole Gregg and Paul Wiegert of Western University in Canada simulated what Alpha Centauri throws out and estimated in 2025 that about a million objects larger than 100 m from that system are now in our Oort Cloud, and that about 10 tiny meteors from Alpha Centauri enter Earth's atmosphere each year. Both numbers should rise about tenfold by the time of closest approach.

Sedna and a few other objects may be the first known members of the inner cloud

Bodies in the Oort Cloud are too faint and too far to see: a 100 km object at 10,000 AU would be fainter than magnitude 40. The closest candidates are the sednoids, such as Sedna and Leleākūhonua, whose orbits never come near Neptune. The Vera C. Rubin Observatory may find dozens more. At its speed of 17 km per second, Voyager 1 will take about 300 years to reach the inner edge of the cloud and about 30,000 years to pass through it. The Kuiper Belt · The heliosphere · Interstellar objects