Unlike the orbits of the planets and the Kuiper Belt, which lie mostly in the same flat disk around the Sun, the Oort Cloud is believed to be a giant spherical shell surrounding the rest of the solar system. It is like a big, thick-walled bubble made of icy pieces of space debris the sizes of mountains and sometimes larger. The Oort Cloud might contain billions, or even trillions, of objects.
Because the orbits of long-period comets are so extremely long, scientists suspect that the Oort Cloud is the source of most of those comets. For example, comet C/2013 A1 Siding Spring, which made a very close pass by Mars in 2014, will not return to the inner solar system for about 740,000 years.
The distance from the Sun to the Oort Cloud is so enormous that it’s useful to describe it not in the more common units of miles or kilometers, but astronomical units. One astronomical unit (or AU) is the distance between Earth and the Sun. Pluto’s elliptical orbit carries it as close as 30 AU from the Sun, and as far as 50 AU. The inner edge of the Oort Cloud, however, is thought to be between 2,000 and 5,000 AU from the Sun. The outer edge might be 10,000 or even 100,000 AU from the Sun — that's one-quarter to halfway between the Sun and the nearest neighboring star.
Though long-period comets observed among the planets are thought to originate in the Oort Cloud, no object has been observed in the distant Oort Cloud itself, leaving it a theoretical concept for the time being. But it remains the most widely-accepted explanation for the origin of long-period comets.
- Predicted Realm - The Oort Cloud is a predicted collection of icy objects farther away than everything else in the solar system. It fits with observations of comets in the planetary region of the solar system, but scientists have yet to observe any object in the Oort Cloud itself.
- Far, Far Away - The Oort Cloud is a spherical layer of icy objects surrounding our Sun, a star, and likely occupies space at a distance between about 2,000 and 100,000 astronomical units (AU) from the Sun.
- Long Way Round - Long-period comets (which take more than 200 years to orbit the Sun) probably come from the Oort Cloud, which is sometimes described as a “cometary reservoir."
- Big Numbers - Predictions show the Oort Cloud may contain more than a trillion icy objects.
- Closer and Bigger - When comets from the Oort Cloud approach the Sun, their surface ices vaporize, producing a cometary atmosphere (a coma) and often two tails (one dust, one gas) that can reach hundreds or even millions of miles (or kilometers) in length. The activity subsides, and the coma collapses, when the comet’s orbit carries it far enough away from the Sun.
- Primitive - Some of the molecules found on comets formed before the Sun was born. They could not survive at temperatures and pressures found on or around Earth. By studying the conditions under which primitive cometary molecules can form, scientists can better understand what the environment of our solar system was like at its birth, which provides clues about how it formed and evolved.
- Exo-comets - Astronomers have seen evidence for comets disintegrating around other stars, the same way Comet ISON did when it grazed our Sun in 2013. By using spectrometry to study those comets’ chemical composition, scientists can compare the birth of our solar system to those of other planetary systems. (A planetary system is the collection of planets, asteroids, etc. orbiting a star or stars. Our planetary system orbits the Sun, which is “Sol” in Latin. So we call our planetary system the solar system.)
- A Long Trip - No missions have been sent to explore the Oort Cloud yet, but five spacecraft will eventually get there. They are Voyager 1 and 2, New Horizons, and Pioneer 10 and 11. The Oort Cloud is so distant, however, that the power sources for all five spacecraft will be dead centuries before they reach its inner edge.
- Cold and Dark - The frozen, comet-like bodies of the Oort Cloud are not capable of supporting life as we know it.
- Deep Thinker - The Oort Cloud is named for Jan Oort, the Dutch astronomer who predicted its existence in the 1950s.
Even though Voyager 1 travels about a million miles per day, the spacecraft will take about 300 years to reach the inner boundary of the Oort Cloud and probably another 30,000 years to exit the far side.