In 1932, Estonian astronomer Ernst Opik formed a theory that long-period comets were initiated in an orbiting cloud of comets at the edge of our solar system. Relying on Opik's theory for postulating his own hypothesis, Dutch astronomer Jan Hendrik Oort theorized that there was a spherical cloud of comets that exists at the outermost gravitational reaches of our solar system. Although technology still does not exist to prove or disprove that this cloud of comets exists, scientists and astronomers have accepted that it does exist and it is formally known as the Opik-Oort Cloud and less formally known as the Oort Cloud.
It may be that the Oort Cloud was formed approximately 4.6 billion years ago from the excess matter as the protoplanetary disc formed around the sun and the comets and icy bodies that make up the Oort Cloud were formed in much the same way as the planets and asteroids in our solar system. This excess matter may have been ejected from the inner solar system by young gaseous planets, such as Jupiter, and formed into a cloud like mass of comets. Some astronomers believe that there may be bodies as large as Mars or Mercury in the Oort Cloud.
Within the Oort Cloud there could be as many as 100 billion comets and icy bodies orbiting the sun. These comets can take as long as 200 years to complete their orbit around the sun. The Oort Cloud is roughly 20,000 astronomical units to as far as 200,000 AU from the center of our solar system, the sun. The Oort Cloud is believed to be made up of two layers, the outer layer being more spherical in shape while the inner layer is more elliptical or disc-shaped.
It is believed that the different densities of stars and matter in our Milky Way galaxy create a variation in the gravitational pull that is exerted on the Oort Cloud. This may be the reason the outer layer of the Oort Cloud is more susceptible to losing comets into the vast reaches of our Milky Way galaxy, due to the much weaker gravitational pull of the sun at the extreme outer reaches of the solar system.
Scientists believe that the inner layer of the Oort Cloud contains more comets per volume than the outer core since there is a stronger gravitational pull from the sun. The inner layer, known as the Hills Cloud, may be source of long-period comets, such as Hale-Bopp, that enter the inner solar system and pass by the planets as they orbit the sun. As stellar and molecular disturbances occur in the Hills Cloud, comets may be lost to the outer layer of the Oort Cloud. It is believed that it is in this way that the Hills Cloud replenishes the more sparsely populated outer layer.
Everything about the Oort Cloud is still a mystery and is conjecture since scientists and astronomers do not possess the technology to study and verify the theory of the Opik-Oort Cloud at this time. Perhaps as technology advances scientists will be able to verify that the Oort Cloud does exist.