Oort cloud

Oort cloud

The **Oort cloud** (/ɔːrt, ʊərt/), sometimes called the **Öpik–Oort cloud**, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). It is first described in 1950 by the Dutch astronomer Jan Oort. 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 are in interstellar space. The Kuiper belt, the scattered disc and the detached objects, the other three 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 cosmographic 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 short-period comets may have originated from the Oort cloud.

## Hypothesis

There are two main classes of comet: short-period comets (also called ecliptic comets ) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 au, and follow the ecliptic plane, the same plane in which the planets lie. All long-period comets have very large orbits, on the order of thousands of au, and appear from every direction in the sky.

A. O. Leuschner in 1907 suggested that many comets believed to have parabolic orbits, and thus making single visits to the solar system, actually had elliptical orbits and would return after very long periods. In 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System. Dutch astronomer Jan Oort independently revived the idea in 1950 as a means to resolve a paradox

## Origin

The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. 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. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.

Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun. Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material. A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.

## Exploration

Space probes have yet to reach the area of the Oort cloud. *Voyager 1*, the fastest and farthest of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years and would take about 30,000 years to pass through it. However, around 2025, the radioisotope thermoelectric generators on *Voyager 1* will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by *Voyager 1.* The other four probes currently escaping the Solar System either are already or are predicted to be non-functional when they reach the Oort cloud.

In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, called *TAU*; among its missions would be to look for the Oort cloud.

In the 2014 Announcement of Opportunity for the Discovery program, an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the "Whipple Mission" was proposed. It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away. The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission. It was also suggested that the Kepler observatory could have been capable of detecting objects in the Oort cloud.