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    Comets that take more than 200 years to make one revolution around the Sun are notoriously difficult to study. Because they spend most of their time far from our area of the solar system, many "long-period comets" will never approach the Sun in a person's lifetime. In fact, those that travel inward from the distant Oort Cloud can have periods of thousands or even millions of years.

    Solar System in Perspective
    This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU. Image Credit: NASA/JPL-Caltech

    NASA's WISE spacecraft, scanning the entire sky at infrared wavelengths, delivered new insights about these distant wanderers. Scientists found that there are about seven times more long-period comets measuring at least 0.6 miles (1 kilometer) across than had been predicted previously. They also found that long-period comets are on average up to twice as large as "Jupiter family comets," whose orbits are shaped by Jupiter's gravity and have periods of less than 20 years. Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago.

    The Oort Cloud is lies far beyond Pluto and the most distant edges of the Kuiper Belt. While the planets of our solar system orbit in a flat plane, the Oort Cloud is believed to be a giant spherical shell surrounding the Sun, planets, and Kuiper Belt Objects. It is like a big, thick bubble around our solar system. The Oort Cloud's icy bodies can be as large as mountains — and sometimes larger.

    Go farther. Explore the Oort Cloud in Depth ›

    10 Need-to-Know Things About the Oort Cloud


    Predicted Place

    The Oort Cloud is a predicted, but undiscovered  region of space.


    Far, Far Away

    The Oort Cloud surrounds our Sun, a star. The Oort Cloud is a spherical shell, occupying space at a distance between about 1,000 and 100,000 astronomical units (AU).


    Long Way Round

    Long-period comets (which take more than 200 years to orbit the Sun) come from the Oort Cloud.


    Big Numbers

    The Oort Cloud may contain more than a trillion icy bodies.


    Closer and Bigger

    Comets that originate from the Oort Cloud gain atmospheres (the coma) when they near the sun. This atmosphere collapses when the comet's orbit carries it farthest from the sun.


    Too Far to See

    There are no known moons of Oort Cloud objects.


    Another Unknown

    The are no known rings around objects in this region of space.


    A Long Trip

    There have been no missions sent to the Oort Cloud.


    Cold and Dark

    The Oort Cloud is not capable of supporting life as we know it.


    Deep Thinker

    The Oort Cloud is named for the astronomer who predicted its existence during the 1950s: Jan Oort.

    Jan Oort

    Fast Fact

    Fast Fact

    Voyager 1 travels at a million miles a day. At that speed, it will take the spacecraft about 300 years to reach the inner layer of the Oort Cloud and probably another 30,000 years to get to the far side. The Oort cloud is likely that thick.

    Kid-Friendly Oort Cloud

    Illustration of space debris.

    Kid-Friendly Oort Cloud

    Way past Neptune's orbit, even past the Kuiper belt, what is there out there? It's called the Oort Cloud.

    Unlike the orbits of the planets and the Kuiper Belt, which are pretty flat like a disk, the Oort Cloud is a spherical shell surrounding everything in our solar system. It's like a bubble with a thick shell.

    The Oort Cloud is made up of icy pieces of space debris. Sometimes those debris are jostled out of the cloud and fall toward the Sun, becoming comets. It can take thousands of years for an Oort Cloud comet to go around the Sun.

    Visit NASA SpacePlace for more kid-friendly facts.

    NASA Space Place: All About the Oort Cloud

    More Resources

    More Resources

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