Shell of small bodies and dust enveloping the solar system at a distance of approximately 1.5 light years. It's believed to be made up of remnants from the formation of the solar system and the origin of most comets, possibly of spare "leftover" planetesimals ejected by gravitational forces during the formation of the major planets under the same principles that helped send Pioneer and Mariner space probes out of the solar system. Named after Dutch astronomer Jan Hendrik Oort who hypothesized its existence after observing the high eccentricity of comets' orbits.

The cloud itself has not been seen or measured but the hypothesis of its existence is generally accepted as fact by most astronomers. Its exact composition, density and size remain uncalculated and with current technology incalculable and unobservable. The best educated guess is that it contains 10-12 billion comet-sized objects. Unlike all other parts of the solar system, the Oort cloud is spherical and not flat in line with the Sun's orbital plane.

The Oort cloud is a large sphere surrounding our solar system, with an approximate radius of 50,000 astronomical units. This is considered to be the maximum range of our sun's gravitational influence.

Inside this cloud, comets orbit the sun. Distances between comets can be measured by tens of millions of kilometers. Although these comets are weakly bound to the sun, another star passing by or another large object can change their orbits. When this happens, the comet may be sent into the inner solar system, or it may be catapulted out of the Oort Cloud into space. Most scientists envision the Oort Cloud as having a relatively dense "core" that creates a stable state in the Cloud by gradually replenishing the outer boundaries. It is estimated that the Oort Cloud contains six trillion icy comets, with about five sixths of them within the dense core region.

Although most disturbances in the sun's Oort Cloud are caused by the Oort Clouds of other stars passing near or through, comets can also be affected by molecular clouds of hydrogen that pass through. The largest of these molecular clouds are much more massive than the sun. These clouds are frequently the birthplaces of new stars. Although they only appear in the Oort Cloud about once every 400 million years, their presence can quickly redistribute the comets in the Oort Cloud.

According to scientists, the mass of the comets in the Oort Cloud exceeds that of 40 Earths. Since the matter originated at varying temperatures and distances from the sun, different comets have different compositions.

The average temperature in the Oort Cloud is approximately four degrees Kelvin, which is near absolute zero. Of course, molecules within the cloud will never reach absolute zero, because that would require all molecular motion to cease.

The Oort cloud is the largest source of comets in our solar system, with the Kuiper Belt being the second. Although most of the comets in the Oort Cloud have very long periods, occasionally the gravity of our planets will pull them into a smaller orbit, as was the case with the Swift-Tuttle and Halley comets. Orbits can also shift when jets of dust and gas are emitted as the comet approaches the sun. The initial period of a comet in the Oort Cloud prior to being pulled into an intermediate or shorter period can range greatly, from orbiting the sun every 200 years to every million years. The average distance a comet comes from on its first trip around the sun is 45,000 AU.

The Oort Cloud was discovered in 1950 by Jan Hedrick Oort. At first, Oort inferred the existence of the cloud by studying the orbits of only 19 different comets. Since Oort postulated the Cloud, new data has confirmed Oort's theory, and the Cloud, which Oort had neglected to name, was named in his honor. Oort's discovery was significant in astronomy, because scientists now knew that comets were permanent members of the solar system and not just random bodies.

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