The Kuiper Belt and Dwarf Planets:
The Kuiper Belt
The Kuiper belt, also called the Edgeworth-Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger – 20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake. Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, may have originated in the region. Overlapping the outer edge of the main part of the Kuiper Belt is a second region called the scattered disk, which continues outward to nearly 1,000 AU, with some bodies in orbits that go even farther beyond. So far, more than 2,000 Kuiper Belt objects (KBOs) have been cataloged by observers, but they represent only a tiny fraction of the total number of objects scientists think are out there. It is estimated that there may be hundreds of thousands of icy bodies larger than 100 km, and an estimated trillion or more comets within the Kuiper Belt. Kuiper Belt objects are also called trans-Neptunian objects (TNOs)
Astronomers think the icy objects of the Kuiper Belt are remnants left over from the formation of the solar system. Similar to the relationship between the main asteroid belt and Jupiter, it is a region of objects that might have come together to form a planet had Neptune not been there. Neptune’s strong gravity stirred up this region of space so much that the small, icy objects there were not able to coalesce into a large planet. The amount of material presently found in the Kuiper Belt might be just a small fraction of what was originally there – the Kuiper Belt is slowly eroding away. According to one well-supported theory (known as the Nice Model, as in Nice, France), the shifting orbits of the four giant planets (Jupiter, Saturn, Uranus and Neptune) could have caused most of the original material – likely 7 to 10 times the mass of Earth – to be lost. Objects in the Kuiper Belt occasionally collide, with collisional fragments producing smaller KBOs (some of which may become comets), as well as dust that is blown out of the solar system by the solar wind. The total mass of all the material in the Kuiper Belt today is estimated to be no more than about 10 percent of the mass of Earth. Eris and Pluto are the largest-known members of the Kuiper belt. A fairly large number of KBOs either have moons or are binary objects. Pluto, Eris, Haumea and Quaoar are all large Kuiper Belt objects that have moons. Some dwarf planets within the Kuiper Belt have thin atmospheres that collapse when their orbit carries them farthest from the Sun.
The Kuiper Belt is also a source of comets, as it very slowly erodes itself away. Pieces produced by colliding KBOs can be pushed by Neptune's gravity into orbits that send them sunward, where Jupiter's gravity forces them into short loops around the sun lasting 20 years or less. These are called short-period Jupiter-family comets. Given their frequent trips into the inner solar system, most of them tend to exhaust their volatile ices fairly quickly and eventually become dormant, or dead comets with little or no detectable activity. Researchers have found that some near-Earth asteroids are actually burned-out comets, most of which started out in the Kuiper Belt. (The other source of comets is the Oort Cloud, where most long-period comets on highly tilted orbits originate.)
The first mission to explore the Kuiper Belt is the New Horizons, an interplanetary space probe that was launched as a part of NASA's New Frontiers program in January 2006. It flew past Pluto in 2015 and is on its way to explore another Kuiper Belt world. Astronomers are searching for a possible planet (nicknamed Planet 9) that might explain the strange orbits of several Kuiper Belt Objects.
The Dwarf Planets
Since the time of its discovery in 1930, Pluto had been considered the solar system’s ninth planet. However, in 2006, the International Astronomical Union defined a planet as an object that orbits the sun, is massive enough for its gravity to pull it into a spherical or nearly spherical shape, and must have cleared the neighborhood around its orbit of debris. Since Pluto did not fulfil the third criterion, it was expelled from the family of solar system planets, leaving only the eight others as planets. Consequently Pluto was classed as a dwarf planet. Dwarf planets are defined as objects that are in direct orbit of a star, are massive enough for their gravity to compress them into a shape that is in hydrostatic equilibrium, but have not cleared their neighborhood of other material around them.
Although hundreds, or perhaps thousands, more solar system bodies may eventually join the list, the IAU officially recognizes just five dwarf planets at the moment – Pluto, Eris, Haumea, Makemake and Ceres. Whereas the first four named are KBOs, Ceres is an asteroid.
Pluto and Eris are the largest known members of the Kuiper belt, while Ceres, the largest asteroid, orbits from within the main asteroid belt. For practical purposes, objects classified as dwarf planets are smaller than the planet Mercury, which has a diameter of about 4,879 km. Eris is thought to have a diameter of roughly 2,370 km. Pluto is slightly smaller, with a diameter of about 2,344 km. The diameter of Ceres is about 940 km.
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