The Kuiper belt and the Oort cloud

In recent years astronomers have discovered that there is a belt of millions of small objects beyond Neptune from 30 AU to 50 AU.

The solar system formed from a disc of material around the Sun. The inner dense part of the disc formed the planets whilst the less dense outer part of the disc is what we know now as the Kuiper belt. many of the objects in the Kuiper belt are very large and some would classify Pluto (at 39 AU) not as a planet but just another Kuiper belt object. Triton, one of Neptune's satellites, may also have originated from the Kuiper belt. The Kuiper belt is certainly the source of short term comets.

Where do long term comets come from? Their orbits suggest that they originate from a region about 50,000 AU from the Sun ( a thousand times the orbit of Pluto !). They also come from all directions, not just from the same plane like Kuiper belt objects. In 1950 Jan Oort suggested that there was an enormous spherical cloud surrounding our solar system, now called the Oort cloud. It would extend out into space perhaps to a distance of a light year and contain many millions of objects of different sizes. Again it may be remnants of the early solar system, perhaps the planetary nebula from which our solar system formed. There is no direct evidence, as yet, for the existence of the Oort cloud.