Oort cloud

The Oort cloud ( alternate spelling: Oort cloud ), sometimes referred to as circumsolar comet cloud or Öpik - Oort cloud is a hypothetical and not yet proven collection of astronomical objects in the outermost region of our solar system.

Theory

The cloud was postulated in 1950 by the Dutch astronomer Jan Hendrik Oort as origin of the long-period comets. Oort attacked so on a recommendation by the Estonian astronomer Ernst Öpik 1932.

Oort based his hypothesis on the study of cometary orbits and on the consideration that the comet could not come from the known regions of the solar system, as has been previously assumed. Comets are destroyed in the course of several passages of the range of the planet by the stronger solar wind and the formation of a comet's tail; to the old conditions, they should therefore no longer occur.

Extent

In theory, the resolution adopted by Oort "cloud" of the remaining zones of the solar system surrounds a shell at a distance from sun up to 100,000 astronomical units (AU ), which corresponds to about 1.6 light years. In comparison, the sonnenfernste planet Neptune only about 4.2 light hours ( 30 AU ), the nearest star Proxima Centauri, however, already 4.2 light years from the Sun. The significantly watered by the solar wind heliosphere has an estimated radius of about 110 to 150 AE. Estimates of the number of objects are between one hundred billion and a trillion. The Oort cloud probably goes continuously into the Kuiper Belt ( 30-50 AU), whose objects are, however, concentrated to the ecliptic.

Formation

The Oort cloud is currently considered out of rock, dust and ice bodies of various sizes that are left over from the formation of the solar system and the merger to form planets. These so-called planetesimals were ejected from Jupiter and the other giant planets in the outer regions of the solar system. Due to the gravitational influence of nearby stars the orbits of the objects have been disturbed over time so that they are now distributed almost isotropically in a shell around the sun. Because of the far greater distance to the neighboring stars, the Oort cloud objects are gravitationally bound to it, despite its relatively large distance from the sun, so integral parts of the solar system.

Comets

Through the influence of the gravitational fields of the neighboring stars and galactic tides, the orbits of Oort cloud objects are disturbed and some of them fall into the inner solar system. Once there, they appear as a long-period comet, with a period of several thousand years. Short-period comets can not make out the comet Oort cloud, since one needed for this disturbance by the large gas planet is too ineffective.

The Oort cloud is not the only origin of comets: At an average cycle length, these may originate from the Kuiper belt.

Unclear delineation

A direct proof of the Oort cloud by observation is not expected in the near future, but there is enough indirect evidence, so that their existence is considered to be virtually certain. Oort the cloud is divided into an inner and an outer Oort cloud, wherein the boundary is assumed to be a semi-major axis of the orbit of from 10,000 to 20,000 AU. Reason for this division is the Jupiter barrier that prevents comets can reach less than about 10,000 AU in the inner solar system with a semi-major axis. Therefore, the inner Oort cloud objects are not observable as de facto. Long-period comets with semi-major axis is less than about 10,000 AE that are observed in the inner solar system come from the outer Oort cloud, and have probably already experienced several Bahnumläufe through the inner solar system, with its semi-major axis was gradually reduced by planet by perturbations. These comets are therefore referred to as "dynamic old," as opposed to " dynamic new" comets, which come directly from the Oort cloud and reach the inner solar system for the first time.

From the explorers of the extremely wide outer circumferential planetoid Sedna - with an aphelion at about 926 AE - an affiliation of this object to the inner Oort cloud has been proposed, but this is not yet universally accepted.