Nearly 30 trillion kilometers from the Sun, at the outer edges of the Solar System, lies a spherical region called the Oort Cloud. This cloud, which surrounds our planetary system, is home to an estimated six trillion icy bodies, or long-period comets.

Dutch astronomer Jan Oort predicted the cloud’s existence in 1950, using the orbits of 19 comets to determine their source. Oort noticed that comets seem to approach from all directions, therefore, he believed, they must originate from some sort of sphere that surrounds our solar system. Oort’s prediction was later confirmed by the gathering of additional data. However, scientists still aren’t sure how the Oort Cloud originated. It is believed the objects in the Oort Cloud could not have formed there, because the area is too distant for matter to condense. One explanation is that the bodies formed elsewhere, probably around Jupiter, Saturn, Uranus, and Neptune, and then were hurled toward the cloud by the planets’ gravitational influences. Because this also means the bodies formed at varying times and locations, it explains why the compositions of long-period comets differ so greatly.

The Oort Cloud is believed to have a dense core, which holds most of the bodies found there. The rest, approximately one-sixth of the total, are in its outer region, and it is thought that all of the comets are millions of miles away from each other. The total mass of these comets is estimated to be about 40 times Earth’s mass. Because the Oort Cloud is so distant, the Sun has only a weak influence on the comets there. In fact, the cloud is considered the outer limit of the material part of the Solar System, and is thought to mark the limit of the Sun’s gravitational pull. The orbits of Oort Cloud comets are easily disturbed by passing stars, and other outside forces such as giant molecular clouds (masses of cold hydrogen in which stars and solar systems form). If their orbits are disrupted, the comets can be sent hurtling toward either the inner solar system, or the outer reaches of the Solar System.