Trans-Neptunian Objects (TNOs) are solid bodies orbiting the Sun beyond Neptune, including Pluto, Eris, and comets. Most are found in the Kuiper belt, consisting of methane, ammonia, and water ice. TNOs can have eccentric or inclined orbits, and some may be Mars-sized.
Trans-Neptunian Object (TNO) is a generic term that applies to any solid body orbiting the Sun beyond the orbit of Neptune. This includes Pluto and similar bodies called plutinos, the recently discovered Eris and other scattered disk objects, Kuiper belt objects, and many comets. The first trans-Neptunian object to be discovered was Pluto, in 1930. Between 1930 and 1992, no other trans-Neptunian objects were discovered other than Pluto’s moon Charon. Around 1992, telescope technology improved to the point where many more trans-Neptunian objects could be resolved, and now more than 1,000 are known.
Neptune is located about 30 AU (Earth-Sun distances) from us, while trans-Neptunian objects include anything that orbits at distances between 30 AU and 50,000 AU or more. Objects under the gravitational influence of the Sun extend out to a light-year, almost a quarter of the distance from the nearest star, Proxima Centauri. Most of these are comets orbiting in a hypothetical cluster called the Oort cloud. The Oort Cloud is so distant that it is impossible to probe its dark, rocky contents with current telescopes.
Most trans-Neptunian objects are found in the Kuiper belt, which lies from the orbit of Neptune at 30 AU to about 55 AU from the Sun. Unlike the asteroid belt which consists mostly of rock and metal, objects in the asteroid belt Kuiper consist of methane, ammonia and water ice. When distant Solar System bodies made mostly of ice enter orbits that carry them through the central Solar System, their ices melt and turn to vapor, producing comet tails.
Unlike planets, trans-Neptunian objects can have extremely eccentric (non-circular) or inclined (planet-of-elliptical) orbits. One object, (87269) 2000 OO67, approaches the Sun as close as 21 AU, but then orbits as close as 1,000 AU. It takes 12,705 years to orbit the Sun. Another object, 2004 XR190, is the most “tilted” object known, orbiting at an inclination of 47 degrees from the elliptical.
As our telescopes get more powerful, more trans-Neptunian objects are sure to be discovered. Some scientists believe there may be Mars-sized objects out there.
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