Oort cloud

The Oort cloud (say "ORT" or "OORT"), also known as the Öpik–Oort cloud, is a theoretical swarm of billions of icy planetesimals that surrounds the Sun. It is thought to stretch from about 2,000 AU to 200,000 AU away (that’s 0.03 to 3.2 light-years). The idea was first put forward in 1950 by Dutch astronomer Jan Oort, after whom it was later named. Oort suggested that objects in this cloud serve as a steady source for long-period comets entering the inner Solar System, where they eventually get destroyed during close passes by the Sun.

The cloud is believed to have two parts: an inner, disc-shaped region roughly aligned with the solar ecliptic (sometimes called the Hills cloud) and a spherical outer cloud. The innermost edge of the Oort cloud lies more than a thousand times farther from the Sun than the Kuiper belt, the scattered disc, and the detached objects—three nearer reservoirs of trans-Neptunian objects. Both regions sit well beyond the heliosphere, one of two boundaries astronomers use to mark the Solar System’s edge.

The Oort cloud reaches out to the limit of the Sun’s gravitational influence, the other boundary chosen for the Solar System’s end. This limit is defined by the Sun’s Hill sphere, where solar gravity gives way to the galaxy’s pull. The outer Oort cloud is only loosely tied to the Sun, so its members are easily nudged by passing stars, the Milky Way’s gravity, and even the cloud’s own microgravity. These forces helped circularize the highly eccentric orbits of material flung out from the inner Solar System during its early days. The disc-shaped inner cloud owes its circular orbits largely to this galactic torque. At the same time, galactic interference can knock comets loose from the cloud, sending them toward the inner Solar System. Based on their orbits, most—but not all—short-period comets seem to come from the Oort disc; others may originate in the larger spherical cloud.

Astronomers think the material now in the Oort cloud formed much closer to the Sun, in the protoplanetary disc, and was then scattered far into space by the gravity of the giant planets. No one can observe the Oort cloud directly with current imaging technology. Still, it is believed to be the source that replenishes most long-period and Halley-type comets, which are eventually consumed by close approaches to the Sun. The cloud may also supply many centaurs and Jupiter-family comets.

**Development of theory**

By the early 1900s, astronomers recognized two main comet types: short-period (or ecliptic) comets and long-period (or nearly isotropic) comets. Ecliptic comets have small orbits near the ecliptic plane and rarely go beyond the Kuiper cliff at about 50 AU from the Sun (Neptune’s orbit averages 30 AU, and 177P/Barnard’s aphelion is around 48 AU). Long-period comets, however, travel in huge orbits thousands of AU from the Sun and appear from all directions in the sky, above and below the ecliptic.

In 1907, Armin Otto Leuschner showed that comet trajectories depended on observation time: short observation times suggested parabolic paths, while longer times pointed to elliptical orbits. He guessed that better data would reveal all comets had elliptical orbits and were permanent Solar System members, returning after long invisible periods. In 1932, Estonian astronomer Ernst Öpik proposed a reservoir of long-period comets in a cloud at the Solar System’s outer edge. Jan Oort revived this idea in 1950 to solve a puzzle about comet origins. The following facts clash with the highly elliptical orbits long-period comets always have:

- Over millions or billions of years, Oort cloud comet orbits are unstable. Celestial mechanics dictate that a comet will eventually be pulled away by a passing star, hit the Sun or a planet, or be ejected by planetary perturbations. - Also, comets’ volatile composition means that repeated close approaches to the Sun boil off these volatiles until the comet splits or forms an insulating crust that stops outgassing.

Oort reasoned that comets with orbits bringing them close to the Sun could not have been doing so since the protoplanetary disc condensed over 4.5 billion years ago. So long-period comets could not have formed in their current orbits and must have been stored in an outer reservoir for nearly all their existence. He also studied ephemeris tables for long-period comets and found a curious cluster of comets whose farthest point from the Sun (aphelion) gathered around 20,000 AU. This pointed to a spherical, isotropic reservoir at that distance. He also suggested that the rarer comets with orbits around 10,000 AU had likely made one or more trips into the inner Solar System, where planetary gravity pulled their orbits inward.

**Structure and composition**

The Oort cloud is thought to occupy a vast space from about 2,000 to 5,000 AU (0.03 to 0.08 ly) from the Sun out to as far as 50,000 AU (0.79 ly) or even 100,000 AU.