Small bodies · Space Codex
Oort cloud
A vast, never-seen sphere of icy bodies thousands to perhaps 100,000 AU from the Sun, the source of long-period comets.
The Solar System's distant icy shell
At a glance
Key points
Voyager 1 will need about 300 years to reach it.
- Never directly observed
- May reach a third of the way to the nearest star
- Voyager 1 arrives in about 300 years
How it formed
As the giant planets grew, they scattered countless icy planetesimals outward. Passing stars and the Milky Way's tide then lifted their orbits into a spherical cloud, far from the planets' influence.
How comets fall out of it
Out here the Sun's grip is so weak that small nudges matter. The steady tide of the Milky Way's disc slowly reshapes orbits, and passing stars can stir up many bodies at once. A few are knocked onto paths that drop into the inner Solar System as long-period comets, arriving from every direction because the cloud is a shell, not a disc. Most are seen only once in recorded history. The star Gliese 710 is expected to pass through the cloud about 1.3 million years from now.
Borrowed comets?
NASA notes that the cloud could hold objects that formed around other stars. One modelling study published in 2010 argued that much of it may have been captured from sibling stars while the Sun was still in its birth cluster. If so, some comets we call our own would be interstellar in origin, unlike passing visitors such as 3I/ATLAS, which are not bound to the Sun. The idea remains debated.
What we still don't know
Nobody has observed an Oort cloud object in place: its existence is inferred from the orbits of long-period comets. Its true size and population are uncertain. Distant bodies such as Sedna, which never comes closer to the Sun than about 76 AU, may belong to an inner part of the cloud, but how to classify them is still argued over.
Missions
- 1977–
Jupiter 1979, Saturn 1980; interstellar space since 25 Aug 2012.
Related entries
- The heliosphereThe bubble blown by the solar wind
- Voyager 1First spacecraft in interstellar space
- Kuiper beltThe icy disc beyond Neptune
- 1P/HalleyThe most famous comet
Sources
- Levison et al. 2010, capture of the Sun's Oort cloud, Science (opens in a new tab)
- Brown, Trujillo & Rabinowitz 2004, discovery of Sedna (arXiv) (opens in a new tab)
- NASA Science: Oort cloud facts (opens in a new tab)
- Nesvorný et al. 2025, ApJ (arXiv) (opens in a new tab)
- de la Fuente Marcos & de la Fuente Marcos 2022, RNAAS (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.