Oort cloud

The Oort cloud (pronounced ORT or OORT), sometimes called the Öpik–Oort cloud, is theorized to be a cloud of billions of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). 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. The cloud is thought to encompass two regions: a disc-shaped inner Oort cloud approximately aligned with the solar ecliptic (also called its Hills cloud) and a spherical outer Oort cloud. The innermost portion of the Oort cloud is more than a thousand times as far from the Sun as the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects. Both regions lie well beyond the heliosphere, one of two boundaries chosen by planetary scientists at the end of the Solar system. The Oort cloud extends to the limits of the Sun's gravitational sphere of influence, the other boundary chosen by planetary scientists for the end of the Solar system. This area is defined by the Sun's Hill sphere, and hence lies at the interface between solar and galactic gravitational dominion. The outer Oort cloud is only loosely bound to the Sun and its constituents are easily affected by the gravitational pulls of passing stars, the Milky Way itself and the cloud's own microgravity. These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during its early phases of development. The circular orbits of material in the Oort disc are largely thanks to...