Oort Cloud

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Oort cloud, named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 50,000 to 200,000 AU (0.8 to 3.2 ly). It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the extent of the Sun’s Hill sphere. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner Solar System.Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.

Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System’s evolution. Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well. — Wikipedia

Oort Cloud (Encyclopædia Britannica)

Oort Cloud (Eric Weisstein’s World of Astronomy, Wolfram Research)
Oort Cloud (Wolfram Alpha)




Solar System Exploration: Oort Cloud (NASA)
Oort Cloud (Jet Propulsion Laboratory, NASA)




The Man Who Knew Comets (Adam Frank, NPR)


WorldCat, Library of Congress, UPenn Online Books, Open Library




Where does the solar system end? (Space Place, NASA)

Oort Cloud (Cosmos4Kids)

MERLOT: Multimedia Educational Resource for Learning and Online Teaching


Oort Cloud (Astronomy Magazine)
Oort Cloud (Phys.org)
Oort Cloud (NPR Archives)


Oort Cloud (ISBNdb)



Oort Cloud (USA.gov)



Here are links to pages about closely related subjects.

Knowledge Realm


Law (Constant) Relativity
Force Gravity, Electromagnetism (Light, Color)
Matter (Microscope) Molecule, Atom (Periodic Table), Particle

Universe (Astronomical Instrument)
Galaxy Milky Way, Andromeda
Planetary System Star, Brown Dwarf, Planet, Moon

Our Neighborhood
Solar System Sun
Terrestrial Planet Mercury, Venus, Earth (Moon), Mars
Asteroid Belt Ceres, Vesta
Jovian Planet Jupiter, Saturn, Uranus, Neptune
Trans-Neptunian Object
Kuiper Belt Pluto, Haumea, Makemake
Scattered Disc Eris, Sedna, Planet X
Oort Cloud Etc. Scholz’s Star
Small Body Comet, Centaur, Asteroid


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