II.

Characteristics

Astronomers were studying the Kuiper Belt long before the first KBO was detected. The orbits of objects within the belt fall into two major ranges: the inner belt and the scattered belt. Most known KBOs, called classical KBOs, orbit in the inner belt from 35 to 55 astronomical units (AU) from the Sun. (An astronomical unit is the average distance from Earth to the Sun, just under 150 million km [93 million mi].) Some KBOs have been discovered in the scattered belt, which stretches out beyond 1,000 AU. Although still largely unobserved, the scattered belt is thought to be as populous or more so than the inner belt.

The term plutino is used for large KBOs that are influenced by Neptune’s gravity and make two orbits around the Sun for every three orbits Neptune makes (a 2:3 orbital resonance). This group includes Pluto and smaller bodies such as Orcus, Ixion, Rhadamanthus, and Huya. KBOs with orbits that are not directly linked to the orbital period of Neptune are called classical Kuiper Belt Objects or “cubewanos” (after the temporary name QB₁ given the first object discovered). This group of KBOs currently includes the named objects Eris, Quaoar, Varuna, Chaos, and Deucalion. Also in this group are two objects that are about 75 percent the size of Pluto, with the temporary designations 2003 EL61 and 2005 FY9. Another recently discovered KBO, named Sedna, is more than half the diameter of Pluto and has an orbit that extends out of the Kuiper Belt to about 130 billion km (about 84 billion mi) from the Sun at its farthest point. Sedna is currently at the nearest point of its orbit, about 13 billion km (about 8 billion mi) from the Sun.

The largest known KBO is Eris with a diameter of about 2,400 km (1,490 mi), slightly larger than Pluto, which has a diameter of about 2,360 km (1,475 mi). The discoverers of Eris—Michael Brown of the California Institute of Technology, Chad Trujillo of Gemini Observatory, and David Rabinowitz of Yale University—originally called the object the “tenth planet” of the solar system because of its size. In 2006 the International Astronomical Union (IAU) classified Eris as a dwarf planet, a new category that included Ceres (formerly considered the largest asteroid) and Pluto, originally counted as the ninth planet. Many scientists, however, have not accepted the IAU’s new definition of a planet that changed the status of Pluto from a true planet to a dwarf planet that the IAU now lists with a number in the catalog of minor planets.

Eris completes one orbit in 560 years and is currently the most distant known body in the solar system at 14.5 billion km (9 billion mi) from the Sun. At the near point of its orbit, Eris comes to within 38 AU (5.7 billion km/3.5 billion mi)—inside the orbit of Pluto. Eris’s orbit is tilted 44° to the main plane of the solar system, much more inclined than Pluto’s orbit, which is tilted 17.2°. Because of its eccentric and tilted orbit, Eris is considered a scattered disk object that ranges outside the inner Kuiper Belt.

Small KBOs greatly outnumber large ones. Computer simulations indicate that the Kuiper Belt, like the asteroid belt, formed early in the history of the solar system. One theory holds that the Kuiper Belt was coalescing into one or more large planets when the growth process was interrupted. The formation of Neptune may have disturbed the region gravitationally and interrupted this growth.

The Kuiper Belt differs from the asteroid belt in two significant ways. First, KBOs formed more than ten times as far from the Sun as most asteroids. Second, they contain far more ice than asteroids, which are primarily rocky objects.

Based on their knowledge of the composition of Pluto and its moon, Charon, astronomers expect that KBOs consist of water-ice and rock, with some organic and other complex compounds as well. KBOs have a wide range of surface colors, varying from almost gray to very red. Their surfaces are usually quite dark, only reflecting from 3 percent to perhaps 25 percent of the light that falls on them. As a result, if one were to stand on a KBO, the surface would appear blacker than dirt. Based on the distance of KBOs from the Sun and their surface reflectivity, astronomers estimate that KBO surface temperatures are typically about -220°C (-360°F), not much warmer than absolute zero.

Collisions in the Kuiper Belt create craters on the surfaces of KBOs and reduce their mass. Small KBOs shrink over time because their gravity is so weak that material thrown up when impacts occur never falls back to their surfaces. Large KBOs, however, are not much affected by this process. Astronomers studying results from collision models believe that most KBOs smaller than about 50 km (30 mi) in diameter cannot have survived the collisional bombardment over the lifetime of the solar system. KBOs smaller than about 50 km in diameter, therefore, must be either remnants of larger KBOs or fragments created by collisions. It is now widely accepted by researchers that almost all Jupiter family comets, which originate in the Kuiper Belt, are bits of KBOs chipped off by collisions within the belt.

In 2001 astronomers learned that some KBOs have moons. Seven KBOs with moons were discovered in 2001 and 2002. As of 2005 about 20 KBOs are known to have satellites, suggesting that 10 to 20 percent of all KBOs have moons. In September 2005 astronomers at the Keck Observatory in Mauna Kea discovered that Eris (then called 2003 UB313) has a moon, which was later given the name Dysnomia. Surprisingly, the KBO satellites are much larger, relative to their parent KBOs, than planetary moons are relative to their planets. Whereas most moons have diameters just a few percent as large as that of the planet they circle, the KBO moons so far discovered are typically half as large as their parent KBO. One KBO has a companion that is apparently as large as the KBO itself. Such a pair is known as a binary KBO.