What is the Oort cloud

Oort cloud

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Oort cloud (not to scale)

The Oort cloud, sometimes referred to as the circumolar comet cloud or Öpik-Oort cloud, was postulated in 1950 by Jan Hendrik Oort as the place of origin of the long-period comets, following a suggestion made by the Estonian astronomer Ernst Öpik in 1932.

Oort came to his hypothesis on the basis of the investigation of comet orbits and the consideration that an obvious contradiction can be found in the assumption that the comets originated from the solar system itself: Since comets are destroyed after several passages through the solar system, none should today under this assumption Comets can be observed.

Formation of the Oort cloud

The Oort cloud encloses the solar system in the shape of a shell at a distance of 300 to about 100,000 astronomical units, which corresponds to about 1.5 light years (cf. distance to Proxima Centauri 4.2 light years), and contains bodies of rock, dust and ice of various sizes that were left over from the creation of the solar system and that did not combine to form planets. These leftover planetesimals were hurled into the outer reaches of the solar system by Jupiter and the other major planets. Due to the gravitational influence of neighboring stars, the orbits of the objects were disturbed over time in such a way that today they are almost isotropically distributed in a shell around the sun. The objects of the Oort cloud are gravitationally bound to the sun despite their great distance, i.e. fixed components of the solar system. Presumably the Oort cloud merges continuously into the Kuiper belt, whose objects are however concentrated in the ecliptic. Estimates of the number of objects in the Oort Cloud are between 1011 and 1012.


Due to the influence of the gravitational fields of the neighboring stars, the objects in the Oort cloud are disturbed and some of them end up in the solar system. There they appear as long-period comets, with a period of several thousand years. Short-period comets cannot form from comets in the Oort cloud, as the disturbance required for this by the large planets is too ineffective.

The Oort cloud is not the only place of origin of comets: comets with an average period length also come from the Kuiper belt.

Unclear delimitation

Direct evidence of the Oort cloud through observation is not expected in the near future either, but there are enough indirect indications that its existence is considered certain.

An 'Inner Oort Cloud' was postulated by the discoverers of the extremely far outward orbiting asteroid Sedna to explain its origin, since even the perihelion of Sedna lies far outside the Kuiper Belt. Sedna's orbit inclination towards the ecliptic is only 11 °, but the orbits in the Oort cloud should be evenly distributed in space. However, a single value has no statistical significance. Nevertheless, the existence of this 'Inner Oort Cloud' remains very hypothetical for the time being. It is also doubtful whether accretion (see accretion disk) of such a large body, perhaps up to 1800 km in diameter, would have been possible in the circumolar cloud itself.

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