What is the Oort Cloud?


Question posed by Jennie


Our solar system (and therefore, probably, most others) has a number of sources of asteroids, comets and other bodies. One of these, for example, is the asteroid belt that orbits the Sun between the orbits of Mars and Jupiter somewhere between 2 and 5 AU from the Sun.

Another one of these repositories is...


The Oort Cloud
The Oort Cloud, sometimes called the Öpik-Oort Cloud after Ernst Öpik, the Estonian astronomer who first postulated its existence in 1932 and Jan Hendrik Oort, a Dutch astronomer who revived the idea in 1950, is another such region of space that contains a higher than usual concentration of small solar system bodies. The existence of the Oort Cloud has not been confirmed by observation, but most scientists and astronomers accept it as the best bet for the source of long-period comets (such as Halley's comet).


The Oort Cloud can be separated into two parts:

  • The inner Oort cloud (also known as the Hills cloud), which is a doughnut-shaped ring surrounding the solar system and orbiting in the same plane as the planets.
  • The outer Oort cloud, which is a spherical shell of bodies surrounding the solar system in every direction.
The inner Oort cloud is though to extend from around 2,000 AU to 20,000 AU from the Sun - it starts around 50 times further from the Sun than Pluto. The outer cloud is thought to extend from around 20,000 AU to 50,000 AU (this is almost a light year, and is about a quarter of the way to the Sun's nearest neighbour, Proxima Centauri), although some estimates put the outer extent as far as 100,000 to 200,000 AU away.


The Oort cloud is thought to have been formed from leftovers of the solar system's formation and was pushed further out by gravitational interaction with the gas giants. The bodies in the Oort Cloud are not densely packed: on the contrary, if you were to stand on one of the bodies you wouldn't have much chance of seeing another one, even if you were using a decent telescope.
The outer cloud is thought to contain a few trillion cometary nuclei, estimated to have a combined mass of around five times that of the Earth. The inner cloud may contain hundreds of times more material.



The objects in the Oort Cloud are so far away from the Sun as to be only loosely bound by its gravity. This means that they are prone to being affected by gravitational sources from outside the solar system such as passing stars and even tidal effects from the Milky Way and are sometimes knocked out of their largely circular orbits and sent deeper into the solar system. This one possible source of long-period comets such as Halley's comet. Another possible source is the Kuiper belt.








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