Oort cloud
Theorized reservoir of icy bodies at the Solar System's edge.
Its existence was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was later named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
- type
- Theorized astronomical structure
- proposed_by
- Jan Oort (1950)
- also_called
- Öpik–Oort cloud
- composition
- Ices such as water, methane, ethane, carbon monoxide, and hydrogen cyanide; roughly 1–2% asteroids
- estimated_mass
- Roughly 3×10^25 kg (five Earth masses) for the outer cloud
- subregions
- Inner Oort cloud (Hills cloud) and spherical outer Oort cloud
Lore & Background
By the early 20th century, astronomers had identified two main types of comets: short-period comets (ecliptic comets) and long-period comets (nearly isotropic comets). Dutch astronomer Jan Oort revived this idea in 1950 to resolve a paradox about the origin of comets: long-period comets cannot have been in their current orbits since the formation of the protoplanetary disc over 4.5 billion years ago, as their volatile composition would have been boiled off by repeated approaches to the Sun.
Reader's Guide
The Oort cloud is significant as the theorized source that replenishes most long-period and Halley-type comets, which are eventually consumed by close approaches to the Sun. It also may serve the same function for many centaurs and Jupiter-family comets. The cloud is thought to encompass two regions: a disc-shaped inner Oort cloud (Hills cloud) approximately aligned with the solar ecliptic, and a spherical outer Oort cloud. The inner cloud is modeled to be much denser, with tens or hundreds of times as many cometary nuclei as the outer cloud, and is thought necessary to explain the continued existence of the Oort cloud after billions of years. No direct observation of the Oort cloud is possible with present imaging technology. Its legacy lies in explaining the persistence of long-period comets and the structure of the outermost Solar System, though its exact mass and composition remain uncertain.
Did You Know?
- The Oort cloud extends to the limits of the Sun's gravitational sphere of influence, defined by the Sun's Hill sphere.
- The outer Oort cloud may have trillions of objects larger than 1 km, and billions with diameters of 20 km.
Frequently Asked Questions
What is the Oort cloud?
It's a theorized spherical shell of trillions of small icy bodies that envelops the Sun at the outermost edge of the Solar System. Astronomers believe it serves as the reservoir that continuously feeds long-period comets inward toward the inner planets.
Who came up with the Oort cloud idea?
Dutch astronomer Jan Oort published the concept in 1950 after noticing that long-period comets arrived from every direction, implying a vast surrounding population. The structure is occasionally referred to as the Öpik–Oort cloud to credit Estonian astronomer Ernst Öpik, who had sketched a similar notion just before Oort.
What is the Oort cloud actually made of?
The individual objects are mostly frozen ices—water, methane, ethane, carbon monoxide, and hydrogen cyanide—with a small rocky fraction of about one to two percent. The outer cloud's total mass is estimated at around five Earth masses.
Why does the Oort cloud matter to astronomy?
It provides the natural explanation for why long-period comets keep showing up from random directions, keeping a steady trickle of visitors in the inner Solar System. It also marks the practical gravitational boundary of the Sun's domain and is a candidate source of the water and organics that may have helped seed early Earth.
More in Astronomy & Space 1-24
Elsewhere in the Astronomy & Space universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record