A giant shell of icy bodies known as the Oort Cloud encircles the solar system. When its inhabitants interact with passing stars, molecular clouds, and gravity from the galaxy, they may find themselves spiraling inward toward the sun, or cast completely out of the solar system into distant regions of space. Although this shell was first proposed in 1950, its extreme distance makes it challenging for scientists to identify objects within it.
Identifying the Oort Cloud
In 1950, Dutch astronomer Jan Oort suggested that some of the comets entering the solar system come from a cloud of icy bodies that may lie as far as 100,000 times Earth's distance from the sun, a distance of up to 93 trillion miles (150 trillion kilometers).
Two types of comets travel through the solar system. Those with short periods, on the order of a few hundred years, stem from the Kuiper Belt, a pancake of icy particles near the orbit of Pluto. Longer period comets, with orbits of thousands of years, come from the more distant Oort Cloud.
The two regions vary primarily in terms of distance and location. The Kuiper Belt orbits in approximately the same plane as the planets, ranging from 30 to 50 times as far from the Sun as Earth. But the Oort Cloud is a shell that surrounds the entire solar system, and is a hundred times as distant.
Comets from the Oort Cloud can travel as far as three light-years from the sun. The farther they go, the weaker the sun's gravitational hold grows. Passing stars and clouds of molecular gas can easily change the orbit of these comets, stripping them from our star or casting them back toward it. The path of the comets is constantly shifting, depending on what factors influence it.
Oort Cloud inhabitants
The estimated two trillion objects in the Oort Cloud are primarily composed of ices such as ammonia, methane, and water. Formed in the beginning of the solar system, they remain pristine chunks of its early life, allowing comets to provide insight into the environment in which the early Earth evolved. While gravity drew other bits of dust and ice together into larger celestial bodies, the residents of the Oort Cloud weren't as fortunate. Gravity from the other planets—primarily gas giants such as Jupiter—kicked them into the outer solar system, where they remain. [PHOTOS: Spectacular Comet Views from Earth and Space]
The population of the Oort Cloud is in a constant state of flux. Not only are some of its residents permanently booted out of the system through interactions with passing neighbors, the sun may also capture the inhabitants from the shells surrounding other stars. Some of the bodies plunging toward the sun may have been kidnapped early in the sun's evolution, when it was part of a more closely-packed cluster of stars.
When the comet Hyakutake passed within 9 million miles (15 million kilometers) of Earth in 1996, it was completing a journey of about 17,000 years from the distant reaches of the Oort Cloud. Hale-Bopp was another long-period comet that traveled in from the Oort Cloud. Visible for nearly a year and a half, it passed within 122 million miles (197 million kilometers) of the Earth. Both of these Oort Cloud objects had their orbits drastically changed as a result of their pass through the solar system. Halley's Comet is also believed to have originally come from the Oort Cloud, although it is now a Kuiper Belt object.
Scientists have identified four other objects that they believe are part of this distant group. The largest of the four, Sedna, thought to be three-quarters the size of Pluto, lies 8 billion miles (13 billion kilometers) from Earth and orbits the sun approximately every 10,500 years. The other three objects are known as 2006 SQ372, 2008 KV42, and 2000 CR105, and range between 30 to 155 miles (50 to 250 km) in size.