The Kuiper Belt. NASA
The Kuiper Belt. NASA

What is the Kuiper Belt?

The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune — billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of comets that orbit our sun.

What is the Oort Cloud?

In 1950, Dutch astronomer Jan Oort proposed that certain comets come from a vast, extremely distant, spherical shell of icy bodies surrounding the solar system. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the sun: about 150 million km or 93 million miles.) The outer extent of the Oort Cloud is believed to be in the region of space where the sun's gravitational influence is weaker than the influence of nearby stars.

The Oort Cloud probably contains 0.1 to 2 trillion icy bodies in solar orbit. Occasionally, giant molecular clouds, stars passing nearby, or tidal interactions with the Milky Way's disc disturb the orbits of some of these bodies in the outer region of the Oort Cloud, causing the object to fall into the inner solar system as a so-called long-period comet. These comets have very large, eccentric orbits and take thousands of years to circle the sun. In recorded history, they are observed in the inner solar system only once.

In contrast, short-period comets take less than 200 years to orbit the sun and they travel approximately in the plane in which most of the planets orbit. They are presumed to come from a disc-shaped region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper. (It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth.) The objects in the Oort Cloud and in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago.

The Kuiper Belt extends from about 30 to 55 AU and is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.

In 1992, astronomers detected a faint speck of light from an object about 42 AU from the sun — the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,300 KBOs have been identified since 1992. (They are sometimes called Edgeworth-Kuiper Belt objects, and they are sometimes called transneptunian objects or TNOs for short.)

Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.

One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons — Hi'iaka and Namaka — make up the family.

In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU — instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.

In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.

The discovery of Eris — orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) — forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.

While no spacecraft has yet traveled to the Kuiper Belt, NASA's New Horizons spacecraft is scheduled to arrive at Pluto in 2015. The New Horizons mission team hopes to study one or more KBOs after its Pluto mission is complete.

The presumed distance of the Oort cloud compared to the rest of the Solar System. Image courtesy of Wikipedia.
The presumed distance of the Oort cloud compared to the rest of the Solar System. Image courtesy of Wikipedia.