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The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. [4] The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets.
The concept of such a cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was 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.
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The classic Oort cloud theory states that the Oort cloud, a sphere measuring about 50,000 AU (0.24 pc) in radius, formed at the same time as the solar nebula and occasionally releases comets into the inner Solar System as a giant planet or star passes nearby and causes gravitational disruptions. [44]
The Oort cloud is a hypothesized spherical cloud of comets that may lie nearly a light-year from the Sun. It is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud; the outer extent of the cloud defines the boundary of the Solar System.
The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs). [42] Two TNOs have been visited and studied at close range, Pluto and 486958 Arrokoth.
Positions of the main asteroid belt, Kuiper belt, and the Oort cloud in the Solar System. The current definition was included in the 2006 IAU resolution that defined the term planet, demoting the status of Pluto to that of dwarf planet.
For the purposes of this article, "asteroid" refers to minor planets out to the orbit of Neptune, and includes the dwarf planet Ceres, the Jupiter trojans and the centaurs, but not trans-Neptunian objects (objects in the Kuiper belt, scattered disc or inner Oort cloud).