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Space Topics: Trans-Neptunian ObjectsTypes of Trans-Neptunian ObjectsTrans-Neptunian objects are grouped according to the characteristics of their orbits. Within the gravitational influence of Neptune are the Kuiper belt objects, which have near-circular orbits that lie close to the plane of the solar system. There is a small population of scattered disk objects, which are also within the gravitational influence of Neptune, but have eccentric and inclined orbits. Beyond that lies a large region of nearly empty space. Finally, a thousand times farther from the Sun than the Kuiper belt, lies the hypothesized (but never observed) Oort Cloud. Kuiper Belt Objects (KBOs)The existence of a belt of objects orbiting beyond the giant planets has been theorized since 1930. These objects would be primitive bodies, leftovers from the formation of the solar system, in a region too cold and sedate for planetary formation to proceed. For a long time Pluto and Charon were the only bodies known to inhabit this region of the solar system. But beginning in 1992 with the discovery of 1992 QB1, the observed population of this belt has grown almost to a thousand objects. The Kuiper belt spans a region of the solar system outside the orbit of Neptune, from about 30 to 50 Astronomical Units (AU). This region is close enough to Neptune that all of the Kuiper belt objects are considered to be under Neptune’s gravitational influence. Almost no objects have been observed beyond 50 AU, though astronomers should be able to detect them if they exist. The 50-AU boundary is referred to as the “Kuiper cliff.” Whether the Kuiper cliff represents the outer boundary of the original planetary nebula, or whether it is merely the inner edge of a large “Kuiper gap” extending at least to 70 or 80 AU, is not known. (The most distant known trans-Neptunian object, Sedna, has a perihelion of 76 AU, outside Neptune’s gravitational influence.) Estimating the size of Kuiper belt objects is difficult, because in most cases Earth-based instruments cannot resolve them as disks. They can only measure the location of the objects and the visual magnitude -- how bright they appear. Estimates for the sizes of these objects are usually based on the assumption of a reflectivity, or albedo. A range of assumed albedoes gives a range of possible sizes. Another method to estimate the objects’ size is to measure the heat that they radiate. For any given temperature, at a known distance, a large object would radiate more heat than a small one. In practice, however, the heat radiated by these distant objects is so faint that it most often cannot be detected; many attempts at these measurements can only place an upper limit on the size. Most Kuiper belt objects have roughly circular orbits close to the plane of the ecliptic. These are referred to as cubewanos after the first such object that was discovered, 1992 QB1. Cubewanos include Chaos, Deucalion, Quaoar, and Varuna. Some objects have more elliptical orbits that travel in a 2:3 orbital resonance with Neptune, so that they orbit the Sun twice for every three times Neptune does. Pluto is the first known of these objects, so they are referred to as plutinos. Plutinos include Huya, Ixion, Orcus, and Rhadamanthus. A few known objects are in 1:2 orbital resonance with Neptune, traveling around the Sun once for every two Neptune orbits, and these are sometimes called twotinos. There is even one known object in a 1:3 orbital resonance with Neptune. Scattered Disk Objects (SDOs)Scattered disk objects have eccentric orbits that take them well above and below the plane of the ecliptic. Their closest approach to the Sun may be within the distance of the Kuiper belt, but they may travel far beyond the Kuiper belt as they travel outward. Prevailing theory states that scattered disk objects began as Kuiper belt objects, which were “scattered” through gravitational interactions with the giant planets. Recently discovered 2003 UB313 is a scattered disk object. Sedna, which has a very elliptical orbit that is inclined to the plane of the solar system, is sometimes classified as a scattered disk object. However, its perihelion at 76 AU is too far from Neptune (or any of the other giant planets) for an interaction with one of them to have produced Sedna’s current orbit. The Oort CloudIn 1950, Dutch astronomer Jan Oort developed a theory that explained many of the mysteries surrounding the origin of long-period comets. Oort theorized that the Sun is surrounded at a very great distance by a hollow sphere of cometary nuclei. From time to time, some of the objects are perturbed by a passing star or a collision, and some of the perturbed objects wind up passing through the inner solar system. The Oort cloud is exceedingly distant from the Sun. Theory states that it extends from roughly 50,000 to 100,000 AU from the Sun, or roughly one light year, or roughly a quarter of the distance from the Sun to the nearest star. If another star were to pass within two light years of the Sun, the Oort clouds of the two stars could interact, scattering some of the objects in different directions. Because the Oort cloud is so distant, no Oort cloud objects have ever been observed. The most distant known trans-Neptunian object, Sedna (formerly known as 2003 VB12), travels only to 928 AU -- only two percent of the distance to the hypothesized inner edge of the Oort cloud. Sedna’s existence may prove that the Oort cloud extends closer to the Sun than was originally thought, or Sedna could be an exception to the rule. |
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