Kuiper belt

Kuiper belt, flat ring of icy small bodies that revolve around the Sun beyond the orbit of the planet Neptune. It was named for the Dutch American astronomer Gerard P. Kuiper and comprises hundreds of millions of objects—presumed to be leftovers from the formation of the outer planets—whose orbits lie close to the plane of the solar system. The Kuiper belt is thought to be the source of most of the observed short-period comets, particularly those that orbit the Sun in less than 20 years, and for the icy Centaur objects, which have orbits in the region of the giant planets. (Some of the Centaurs may represent the transition from Kuiper belt objects [KBOs] to short-period comets.) Although its existence had been assumed for decades, the Kuiper belt remained undetected until the 1990s, when the prerequisite large telescopes and sensitive light detectors became available.

KBOs orbit at a mean distance from the Sun larger than the mean orbital distance of Neptune (about 30 astronomical units [AU]; 4.5 billion km [2.8 billion miles]). The outer edge of the Kuiper belt is more poorly defined but nominally excludes objects that never go closer to the Sun than 47.2 AU (7.1 billion km [4.4 billion miles]), the location of the 2:1 Neptune resonance, where an object makes one orbit for every two of Neptune’s. The Kuiper belt contains the large objects Eris, Pluto, Makemake, Haumea, Quaoar, and many, likely millions, of other smaller bodies.

Discovery of the Kuiper belt

The Irish astronomer Kenneth E. Edgeworth speculated in 1943 that the distribution of the solar system’s small bodies was not bounded by the present distance of Pluto. Kuiper developed a stronger case in 1951. Working from an analysis of the mass distribution of bodies needed to accrete into planets during the formation of the solar system, Kuiper demonstrated that a large residual amount of small icy bodies—inactive comet nuclei—must lie beyond Neptune. A year earlier the Dutch astronomer Jan Oort had proposed the existence of a much-more-distant spherical reservoir of icy bodies, now called the Oort cloud, from which comets are continually replenished. This distant source adequately accounted for the origin of long-period comets—those having periods greater than 200 years. Kuiper noted, however, that comets with very short periods (20 years or less), which all orbit in the same direction as all the planets around the Sun and close to the plane of the solar system, require a nearer, more-flattened source. This explanation, clearly restated in 1988 by the American astronomer Martin Duncan and coworkers, became the best argument for the existence of the Kuiper belt until its direct detection.

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In 1992 American astronomer David Jewitt and graduate student Jane Luu discovered (15760) 1992 QB₁, which was considered the first KBO. The body is about 200–250 km (125–155 miles) in diameter, as estimated from its brightness. It moves in a nearly circular orbit in the plane of the planetary system at a distance from the Sun of about 44 AU (6.6 billion km [4.1 billion miles]). This is outside the orbit of Pluto, which has a mean radius of 39.5 AU (5.9 billion km [3.7 billion miles]). The discovery of 1992 QB₁ alerted astronomers to the feasibility of detecting other KBOs, and within 20 years about 1,500 had been discovered.

On the basis of brightness estimates, the sizes of the larger known KBOs approach or exceed that of Pluto’s largest moon, Charon, which has a diameter of 1,208 km (751 miles). One KBO, given the name Eris, appears to be twice that diameter—i.e., only slightly smaller than Pluto itself. Because of their location outside Neptune’s orbit (mean radius 30.1 AU; 4.5 billion km [2.8 billion miles]), they are also called trans-Neptunian objects (TNOs).

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Because several KBOs like Eris are nearly as large as Pluto, beginning in the 1990s, astronomers wondered if Pluto should really be considered as a planet or as one of the largest bodies in the Kuiper belt. Evidence mounted that Pluto was a KBO that just happened to have been discovered 62 years before 1992 QB₁, and in 2006 the International Astronomical Union voted to classify Pluto and Eris as dwarf planets.

Orbital subpopulations

KBOs are classified by their semimajor axis (the mean distance from the Sun), their perihelion distance (the closest approach to the Sun), and the inclination of their orbital plane to that formed by the planets of the solar system. Using these parameters, KBOs are often found in three distinct orbital substructures.

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• Resonant objects: KBOs in mean motion resonance (MMR) with Neptune. An estimated 55,000 KBOs larger than 100 km (60 miles) in diameter orbit the Sun in an integer ratio of Neptune orbital periods. For example, Pluto is in the 3:2 Neptune MMR, completing two orbits around the Sun in the time it takes Neptune to complete three. In fact, nearly one-quarter of all MMR objects are in the 3:2 resonance. In recognition of this kinship, these objects have been dubbed Plutinos.
• Hot classicals: KBOs with inclinations drawn from a wide distribution (about 16°) and with perihelion distances of between 35 and 40 AU (5.2 billion and 6 billion km [3.3 billion and 3.7 billion miles]). The hot classical population consists of approximately 120,000 objects with diameters larger than 100 km. This population is estimated to included 80,000 objects whose mean distance from the Sun exceeds 50 AU (7.5 billion km [4.6 billion miles]) and that are therefore sometimes referred to collectively as the “outer” or “detached” Kuiper belt.
• Cold classicals: KBOs drawn from a narrow distribution of orbit inclinations (about 2.6°), with mean orbital distances restricted to 42.5–47.2 AU (6.4 billion–7.1 billion km [4 billion–4.4 billion miles]) and perihelion distances smoothly distributed between 38 AU (5.7 billion km [3.5 billion miles]) and 47.2 AU. The cold classical population is approximately 75,000 objects with diameters of 100 km and larger. Within the cold classicals are a small subpopulation called “the kernel” of 25,000 objects with diameters larger than 100 km. The kernel objects have semimajor axes between 43.8 and 44.4 AU (6.55 billion and 6.64 billion km [4.07 billion and 4.13 billion miles]), orbital eccentricities of between 0.03 and 0.08, and a narrow inclination distribution like the rest of the cold classical component.

The above list contains the currently well-defined substructures of the orbital space of the Kuiper belt. These objects are in metastable orbits; that is, their orbits are stable over timescales of 100 million to 1 billion years. However, some will chaotically diffuse out of the stable region. As more KBOs are discovered, additional significant orbital populations are likely to be found.

KBOs that have significant gravitational interactions with Neptune are called “scattering KBOs.” Scattering KBOs are on orbits that are unstable on million-year timescales. These objects are thought to be in transition from being metastable KBOs to becoming Centaur objects and eventually short-period comets. The metastable region that supplies the scattering population is not known, but it may be the hot classicals or perhaps the resonant KBOs. Not all scattering orbits are equally unstable, and understanding how a KBO in a metastable orbit becomes a short-period comet is an area of active research. The estimated population of scattering sources (3,000–15,000 objects larger than 100 km in diameter) is significantly smaller than theoretical expectations.

Because of the small number of detected sources, the estimated numbers of KBOs are still quite uncertain. Particularly uncertain is the number of small (1–10-km) KBOs if this region of the solar system really is the reservoir for short-period comets. For comparison, there are estimated to be 250 asteroids larger than 100 km in diameter and perhaps 1 million larger than 1 km. If the relation between the number of objects and size for KBOs is similar to that of asteroids, that implies a total Kuiper belt population of more than 100 billion sources larger than 1 km in diameter. This extrapolation is derived from the few hundred sources for which precise detection circumstances are available. However, extrapolating from 300 objects to 100 billion is subject to considerable uncertainty.

As noted above, the planet Neptune has a strong gravitational influence over the orbital structure of the Kuiper belt. There are two prevailing models for the formation of structure in the orbital distribution of KBOs. In the “migration” model, Neptune’s mean orbital distance was initially smaller (around 23 AU; 3.4 billion km [2.1 billion miles[) and slowly migrated to its current location of about 30 AU (4.5 billion km [2.8 billion miles]). During this slow orbital growth many KBOs became trapped into orbital resonance with Neptune. However, this model does not produce the hot classical component, and some other process must therefore lead to more-inclined orbits for KBOs.

Alternatively, in the “Nice” model (named after the French city where it was first proposed), the giant planets of the solar system formed in a more-compact configuration than is seen today, and through gravitational interaction Neptune and Uranus were scattered to their current locations. The Nice model provides a reasonable representation of the hot component of the Kuiper belt but is less successful at producing the resonant objects and does not provide for a cold classical component. A complete explanation of the formation of structure in the outer solar system may be some combination of these two scenarios or some completely different model of evolution.

In addition to the nominal members of the Kuiper belt described above, there are some KBOs whose closest approach to the Sun leaves them well outside the influence of Neptune. Sedna, an object whose closest approach is 76.3 AU (11.4 billion km [7.1 billion miles]), is the most extreme example of these distant outliers. These rare objects (only two objects with closest approaches greater than 47.2 AU [7.1 billion km (4.4 billion miles)] and mean Sun distances larger than 200 AU (29.9 billion km (18.6 billion miles)] are currently known) may represent the very outer edge of the Kuiper belt region or the inner edge of an entirely new population of sources. Sedna is sometimes referred to as a member of the inner Oort cloud.