Neptune's orbit carries the planet from 29.8 to 30.3 AU from the Sun. Small bodies orbiting past Neptune are referred to as trans-Neptunian objects, and then are further subdivided into members of the Kuiper belt or the Oort cloud. Neptune marks the inner edge of the Kuiper belt. The Kuiper belt was originally thought to reach from 35 to 100 AU from the Sun, and then to merge into the Oort cloud of icy bodies. As study of the Kuiper belt has intensified and the orbits of more of its objects have been carefully calculated, there appears to be a gap between the edge of the Kuiper belt and the beginning of the comet-rich Oort cloud.The Kuiper belt begins around 30 AU and has a sharp outer edge at 49 AU.The reason for this gap is not understood; perhaps Kuiper belt bodies become fainter or smaller with distance and just cannot be seen as easily, or perhaps a sharp edge was formed by the disturbance of a passing planet or star, as unlikely as this event may be.
The Kuiper belt exists at extreme distances from the Sun. From the Kuiper belt the radius of the Sun appears 50 times smaller than it appears from Earth, which would make the Sun look more like a very bright star than something that dominates the day. Detecting objects at that distance from Earth is exceptionally difficult, and learning about their size and composition even more so. Only the most recent technology and largest telescopes allow the Kuiper belt to be explored.
The Kuiper belt had been postulated since 1943, but it remained a theory until 1992. Only the development of highly sensitive viewing instrument called a charge-coupled device has allowed astronomers to see the tiny bodies in the Kuiper belt. George Smith and Willard Boyle invented the charge-coupled device at Bell Laboratories in 1969, and once it was refined and put into mass production, it revolutionized cameras, fax machines, scanners, and, of course, telescopes. A chargecoupled device consists of many linked capacitors, which are electronic components that can store and transfer electrons. When a photon strikes the surface of the charge-coupled device, it can knock an electron off the atom in the surface it strikes. This electron is captured by the capacitors. In this case the capacitors are phosphorus-doped semiconductors, one for each pixel of the image. While photographic film records a paltry 2 percent of the light that strikes it, charge-coupled devices can record as much as 70 percent of incident light. Their extreme efficiency means that far dimmer objects can be detected. This sensitivity has made searching for small distant objects possible.
After a five-year search, David Jewitt and Jane Luu found the first Kuiper body other than Pluto in 1992 by examining series of photographs taken by the Mauna Kea telescope for moving bodies. The preliminary minor planet designation of this first body, about 150 miles (240 km) in diameter, was 1992 QBt (for more on these strange names, see the sidebar "Numbering and Naming Small Bodies" on page 128). The discoverers wished to name 1992 QB Smiley, after a character from John le Carre's novels, but an asteroid had already claimed that name. The scientists named the next body they found, 1993 FW, Karla, also from le Carre's novels. By
2003 there were about 350 Kuiper belt objects known, and by
2004 more than 1,000 objects had been found. Now there are thought to be thousands of bodies in the Kuiper belt with diameters of at least 620 miles (1,000 km), about 70,000 with diameters larger than 60 miles (100 km), and at least 450,000 bodies with diameters larger than 30 miles (50 km).
Kuiper belt bodies are divided into three classes according to their orbits; classical (or cubewano), resonance (or plutino), or scattered disk. Classical Kuiper belt objects have orbits with low eccentricity and low inclinations, indicating that they formed from the solar nebula in place and have not been further perturbed. These objects are sometimes called cubewanos and include any large Kuiper belt object orbiting between about 41 AU and 48 AU but not controlled by orbital resonances with Neptune.The odd name is derived from 1992 QB, the first Kuiper belt object found. Subsequent objects were called "que-be-one-os," or cubewanos. There are about 524 cube-wanos known as of 2004, including Varuna and Quaoar, described in more detail below.
Resonance Kuiper belt objects are protected from gravitational perturbation by integral ratios between their orbital periods and Neptune's. Like Pluto, many Kuiper belt bodies have orbits in periods of 3:2 with Neptune, which allows them to orbit without being disturbed by Neptune's gravity. Because they share their resonance with Pluto, this subclass of objects are called plutinos. As of 2005 there were about 150 plutinos and 22 other resonance objects known. Models indicate that only between 10 and 20 percent of Kuiper belt objects are plutinos, meaning there are likely more than 30,000 plutinos larger than 60 miles (100 km) in diameter.Though the Kuiper belt strictly begins at around 30 AU, the region between Neptune and about 42 AU is largely empty, with the exception of the plutinos, a large population of bodies that orbit at about 39 AU, a few bodies in the 4:3 resonance at 36.4 AU, and two objects, 1996 TR66 and 1997 SZ which seem to be in a 2:1 resonance with Neptune at 47.8 AU. These bodies in the 2:1 resonance have perihelia close to Neptune's orbit. A few more Kuiper belt bodies have been found at the 5:3 resonance near 42 AU.
The large number of bodies in resonant orbits is another paradox of the Kuiper belt. How have so many bodies fallen into these orbits? Renu Malhotra, a scientist at the University of Arizona, suggests that interactions with the gas giants early in solar system formation can explain these highly populated orbits. Computer modeling efforts as early as the 1980s indicated that gas giant planets are likely to migrate outward in the solar system early in formation. The early solar system certainly had more material in the orbits of the planets than it does now, probably including multiple bodies as large as the Earth in the orbits of the outer planets. The gas giant planets would collide with these planetesimals and scatter them either inward, toward the Sun, or outward. Planetesimals scattered outward by Neptune, Uranus, and Saturn almost certainly returned inward through the force of the Sun's gravity to be scattered again, until at last they were scattered inward toward the Sun. During each collision and scattering event the giant planet in question has its orbit altered. Scattering a planetesimal inward toward the Sun drives the giant planet outward. Saturn, Uranus, and Neptune scattered more planetesimals inward than out-
Numbering and Naming Small Bodies The first small bodies to require special naming conventions were the asteroids. The first asteroid, 1 Ceres, was discovered in 1801, just a few decades after the discovery of Uranus. Ceres was the first object smaller than a planet that had been discovered orbiting the Sun. By approximately 1850 about 50 asteroids had been discovered, necessitating a system of temporary numbers called "provisional designations." Provisional designations are assigned to each new possible asteroid discovery and kept until the asteroid is confirmed as a new body. It was thought at the time that there would be no more than 26 new discoveries per half month, and so each half month of the year is assigned a letter: The first half of January is called A, the second half B, the first half of February C, and so on. Within each half month, new discoveries are given letter designations as well, with the first asteroid of each half month called A. For example, the first asteroid discovered in the second half of February of the year 2004 has the provisional designation 2004 DA (D for the second half of February, A for the first asteroid of that month).
Unfortunately, this system quickly became too constraining, as the rate of asteroid discoveries accelerated. By the 1890s, photographic film could be used to search for asteroids: If a camera's shutter is left open for some period of time, an asteroid moves fast enough to appear as a streak, as shown in the lower color insert on page C-6, while stars in the background are more stable. If more than 26 new asteroids are discovered in a half month, then the next asteroid gets the designation A, and the next B, and so on through the next alphabet, until it is used up and a third alphabet begins, with designation A , and so on. The last asteroid discovered in one especially productive half month was designated 1998 SL , meaning the namers had gone halfway through their 166th alphabet! This represents 4,136 objects discovered in that half month.
To be issued a final number and have its provisional designation taken away, a new asteroid's orbit must be determined closely, and also it must be confirmed that this is not a new sighting of a previously known object. The new asteroid must be observed at opposition (see figure on page 129) four times to make it an official part of the permanent record. In the year 2000, 25,320 minor planets were confirmed and numbered. That year was the high point, however; in 2001, 13,295 were numbered; in 2002, 5,595 were numbered; in 2003, 1,050 were numbered; and in 2004, fewer than 300 were numbered. By February 2005 there were 99,906 confirmed and numbered minor planets in total, 108,000 unnumbered objects with fairly well-determined orbits, and 68,000 unnumbered objects with poorly known orbits. There are thought to be millions of asteroids in the solar system, so searchers have a long way to go. Confirmed minor planets are num-
Opposition and Conjunction
Opposition and Conjunction
Opposition and conjunction are the two cases when the Earth, the Sun, and the body in question form a line in space.
bered in the order of their confirmation. Thus, Ceres is number 1 and is formally notated 1 Ceres. Most (about 63 percent in 2005) still have names in addition to numbers, but only one has a name but no number: Hermes, whose provisional designation was 1937 UB.
Now, of course, scientists are concerned with more than just asteroids. Since the discovery of the first Kuiper belt object in 1992, the system of provisional designations, final numbers, and names has been expanded to include all small objects orbiting the Sun,
Numbering and Naming Small Bodies (continued) including Kuiper belt objects and others found still farther out in the solar system. Final numbers are still assigned in order as the object's orbit is characterized and it is proven not to be a repeat discovery. The discoverer of the asteroid has a decade after the assignment of the final number to suggest a name, which must be approved by the 11-member Small Bodies Names Committee.
About the first 400 asteroids were named after figures from classical mythology, but since that time, many other categories of names have been used. While discoverers of asteroids can choose names from almost any category, other objects must have names from a particular subject. All the objects that share Pluto's orbit, for example, must be named for underworld deities. Asteroids have been named after famous or accomplished people of all stripes, family members, committees, plants, and even machines. Asteroids with special numbers often get special names, as in 1000 Piazzi (the discoverer of the first asteroid), 2000 William Herschel, 3000 Leonardo da Vinci, 4000 Hipparchus, 5000 IAU (the International Astronomical Union), 6000 United Nations, 7000 Marie and Pierre Curie, 8000 Isaac Newton, and 9000 Hal (named for the computer, Hal 9000, in the 1968 movie 2001: A Space Odyssey). The asteroid 6765 Fibonacci is named because 6765 is a number in the Fibonacci sequence, a mathematical sequence discovered and investigated by Leonardo Fibonacci.
ward because those scattered outward returned to be scattered inward, and thus those three giant planets gradually migrated outward from the Sun. Jupiter, on the other hand, is massive enough that the planetesimals it scatters outward do not return under the Sun's gravity. Jupiter scattered slightly more planetesimals outward than inward, and so its orbit decayed slightly toward the Sun.
As Neptune, the most distant planet from the Sun, moved even further out, the locations of its resonant orbits moved outward ahead of it.These stable orbital positions thus could sweep up and capture small bodies that otherwise would never have encountered those resonant positions. Objects captured in Neptune's resonances also have their orbital eccentricities increased in a way predictable by theory. Pluto was ostensibly captured in this way, and to reach its current orbital eccentricity of 0.25, Neptune must have captured Pluto when
Neptune was at 25 AU from the Sun and Pluto at 33 AU, in comparison to their current 30 and 39 AU respective distances from the Sun. Neptune continues to change the orbits of Kuiper belt bodies that are not in resonant orbits, and over the age of the solar system, Neptune is thought to have removed 40 percent of the Kuiper belt through gravitational interactions.
Before the kinds of careful and sophisticated modeling that made the preceding description possible, both classical and resonance Kuiper belt bodies were thought to be orbiting at the points of their formation in the solar system, largely undisturbed since the beginning of the solar system. Unlike the randomly oriented orbits of the Oort cloud objects, Kuiper belt objects have orbits closer to the ecliptic plane. Most of the major planets' orbital planes form angles of less than a few degrees with the ecliptic plane. Neptune's orbital plane lies at a little less than two degrees from the ecliptic plane, but Pluto's orbit is the most highly inclined of the planets, at just over 17 degrees. Pluto's high inclination also marks it as a typical Kuiper belt object. A number of Kuiper belt objects have inclinations larger than 25 degrees, and the highest inclination is 31.6 degrees for 1996 RQ20, a mid-range cubewano. Because the Kuiper belt objects have nonzero orbital inclinations, the Kuiper belt itself has a thickness, in contrast to the planets out to Neptune, which almost define a plane. The Kuiper belt's thickness is about 10 degrees. In addition to the range of orbital inclinations for classical and resonance Kuiper belt bodies, their orbits have eccentricities up to 0.4. These ranges indicate that these objects must have been disturbed from their original orbits, which should have been almost circular and close to the ecliptic plane. They may have been disturbed by larger bodies that existed in the early Kuiper belt but have now been destroyed through collisions, or the disturbance may have been caused by Neptune moving its orbit outward, but the dynamics of this process are not well construed.
The third class, scattered disk Kuiper belt objects, has large, eccentric orbits, perhaps created by gravitational interactions with the giant planets.There are about 100 known scattered disk objects.The Kuiper belt object 1996 TL66 is a good example of this class, with an orbital eccentricity of 0.59 that carries it to 130 AU at aphelion. There are thought to be as many as 10,000 scattered Kuiper belt objects.The figure on page 132 shows the nearly circular orbits of the plutinos and the widely eccentric orbits of a few of the scattered objects.
Orbits of Plutinos and Some Scattered Kuiper Belt Objects
Orbits of Plutinos and Some Scattered Kuiper Belt Objects
This sketch of the range of orbits of the plutinos and a few selected scattered Kuiper belt objects shows not only their immense distance from the Sun (compare to Jupiter's orbit) but also the wide range of orbital shapes and sizes in the Kuiper belt.
The inner planets are thought to have accreted from collisions between smaller bodies that finally led to one large body in a given orbit. The Kuiper belt bodies are clearly not in this state, having survived to this point in the evolution of the solar system as a population of tens of thousands of small bodies. Collisions among current Kuiper belt bodies are infrequent enough that they cannot be building now into a larger body, and in fact, the high eccentricities and obliquities of their orbits make their collisions more likely to break bodies up into smaller pieces than to join them together into larger bodies. Earlier in solar system history, before Neptune had sufficiently perturbed the Kuiper belt bodies into their current range of orbits, collisions may have led to accretion and allowed the building of the larger bodies now seen.
Though scientists believe short-period comets originate in the Kuiper belt, they do not know which population of Kuiper belt objects is most likely to provide the comets see the lower color insert on page C-4 and the upper color insert on page C-7. Neptune, however, is thought to be the main provider of the gravitational perturbations needed to throw a Kuiper belt object into the inner solar system. Kuiper belt objects that are not in stable resonant orbits are thought to experience a close encounter with Neptune on the average of once every few tens of millions of years. This low but constant probability of Neptune encounters provides the inner solar system with a small, constant supply of short-term comets. A Kuiper belt object that comes close to Neptune has about a one-third probability of being moved into a short-period comet orbit, and it will otherwise continue in a new Kuiper belt orbit, be ejected from the solar system, or collide with a planet.
In general a Kuiper belt object should have enough volatiles to continue producing a tail as a comet for about 10,000 years, shown in the upper color insert on page C-8, and short-period comets have average lifetimes of about 100,000 years before colliding with a larger body or being expelled from the solar system by gravitational forces. Some orbiting objects, then, should be intermediate between comets and asteroids as they lose their volatiles. Other bodies should be in the process of moving from the Kuiper belt into short-period comet orbits. The Centaurs, bodies that orbit neat Saturn and Jupiter, are thought to be these bodies in transition.The Centaur 2060 Chiron has a cometary coma (a cloud of gas and dust encircling the solid body), supporting the theory that it originated in the Kuiper belt and may be perturbed into a cometary orbit.
The albedo of a body, the percent of sunlight it reflects, allows the body's size to be calculated. If the albedo is known and the sunlight intensity reflecting from the body is measured, then its size can be calculated. Most Kuiper belt objects are thought to have an albedo of about 4 percent, making them about as dark as charcoal. Based on a 4 percent albedo, the majority of Kuiper belt objects detected so far are judged to be typically about 60 miles (100 km) in radius.
The value of 4 percent albedo is a guess, however, and albedo is certain to differ among the objects. Albedo can be calculated by comparing the amount of sunlight reflected from a body with the amount of infrared radiation it emits. The infrared radiation is created by absorbed sunlight; it and the reflected sunlight should make up the total of sunlight striking the body. With a few assumptions about the material the body consists of (which controls how the sunlight heats the body and therefore its infrared emissions), the albedo of the body can be calculated.
The Kuiper belt body Varuna, for example, is thought to have an albedo of about 7 percent. For the same intensity of reflected light, a body with 7 percent albedo would be estimated to be smaller than a body with 4 percent albedo, which needs more surface area to reflect the same amount of sunlight. Pluto is exceptionally bright, with an albedo of about 60 percent. Its high albedo is thought to be created by constant cycles of ice sublimation and subsequent crystallization of fresh ice on the planet's surface as its atmosphere rises and freezes with its seasons.Young ice is highly reflective. A similar process may explain how dark Kuiper belt objects become bright short-period comets if they are perturbed into the inner solar system: Solar heating burns off the body's dark, weathered rind, and fresh icy and gassy material emerge from the interior.
Because of the extreme faintness of Kuiper belt objects, spectra are difficult to obtain. Most inferences about Kuiper belt objects have come from broadband colors, that is, the colors that the objects appear to the eye. The objects have a wide range of colors that may indicate either a range of compositions or that collisions have created fresh surfaces on some, while others retain surfaces that have been weathered by millennia in space. Many of them are exceptionally red. When solar system bodies are referred to as reddened, their color appears reddish to the unaided eye, but more important, the bodies have increased albedo (reflectance) at low wavelengths (the "red" end of the spectrum).
The unusual redness of Kuiper belt objects is not well understood, though laboratory tests show that the red spectrum of 5145 Pholus, a Centaur that orbits in the vicinity of Saturn and Uranus, can be reproduced by irradiating ices that contain nitrogen and methane. Nitrogen and methane are known constituents for Kuiper belt objects, and in fact, Centaurs are likely to have been Kuiper belt objects that were perturbed from their original orbits. The complex organic molecules produced by irradiating simple organic molecules are called tholins (named by the famed Cornell University astronomer Carl Sagan after the Greek word tholos, meaning "mud").Though tholins are definitely red, they unfortunately have no specific spectral absorptions and so cannot be definitively recognized remotely on Kuiper belt objects. Irradiation is certainly a significant factor in the development of sur faces in Kuiper belt objects, which have no inherent magnetic fields to shield them from cosmic rays and solar wind. When these ices are struck by high-energy particles, they lose the relatively light element hydrogen and form additional carbon-carbon bonds. Cosmic rays are highly energetic and can penetrate ices to a distance of several yards, but weak solar radiation can penetrate only a few microns.
The red color may also be caused by weathered silicate minerals, since spectral analysis of a few Kuiper belt bodies indicates that the mineral olivine may be present. Kuiper belt objects are also proven by spectral analysis to contain water ice, though it would be hard as rock at the temperatures in the Kuiper belt. These objects should also contain a large proportion of dust in addition to their ices. Comets near the Sun eject more dust than gas, and their ratios may indicate something about the bulk compositions of Kuiper belt objects.The non-ice dust in the outer solar system is likely to be rich in radiogenic elements such as potassium, thorium, and uranium.These elements emit heat when they decay, and this heat may alter the compositions and structures of the Kuiper belt bodies.
Small Kuiper belt bodies can conduct their internal heat into space efficiently because of their small radii: Heat has a shorter distance to travel before escaping into space. Larger bodies can retain internal heat over the age of the solar system (as the Earth has done) and so may experience internal alterations because of this heat. They may partially differentiate, and internal gases may sublimate and move in response to the heat, and these gases may leave the body or even lead to cryovolcanism. Heat is transferred through a body in a characteristic time, T, that depends on the thermal diffusivity of the material, K. Thermal diffusivity has units of area per second and measures the ease with which heat moves through the material in question.The thermal diffusivity of ice is 10-5 square feet per second (10-6 m2/sec).The radius through which heat moves with time is given as r = VkT.
Over the age of the solar system (T = 4.56 billion years, or 1.43 x 1017 seconds), heat can move from the center to the surface of a sphere with radius (r) equal to 310 miles (500 km). There are thousands of Kuiper belt bodies larger than this critical radius, and all these larger bodies are expected to show some internal changes from radiogenic heat. In particular, radiogenic heat should be sufficient to sublimate carbon monoxide and nitrogen, which might move toward the surface and then freeze again as they reach cooler, shallower depths.The Kuiper belt objects may thus acquire a layered structure.
It is a strange coincidence that Pluto and Charon are two of the largest Kuiper belt objects (see table on page 137), and they happen to be in a binary system with each other. At least a few percent of large Kuiper belt bodies are binaries, with orbits that have radii hundreds of times the radii of the bodies involved. There are eight binaries known now, including 1997 CQ29 and 2000 CF105, systems orbiting about 3,800 miles (6,100 km) and 6,300 miles (10,200 km) apart, respectively.The distance between these bodies is about 1/50 of the distance from the Earth to the Moon.
Binaries can be created by collision, by tidal capture, or by three-body interaction, in which three bodies pull toward each other and swing around each other, and then one body escapes, leaving the other two in a binary. The only way to make the binaries with objects of very similar mass, at great radii, is the third method. Peter Goldriech, at the Institute for Advanced Study at Princeton believes that almost all Kuiper belt objects are binaries. Observers have trouble discriminating them from each other:The binaries look like single objects from the distance of Earth.
Despite their surface irradiation, Kuiper belt objects are probably the least-altered objects in the solar system. Computer simulations by Matt Holman at the Harvard-Smithsonian Center for Astrophysics, Jack Wisdom at the Massachusetts Institute of Technology, and their colleagues show that Kuiper belt bodies can survive for the age of the solar system in a selection of their current, stable orbits. This study implies that Kuiper belt bodies are the remnants of the solar nebula that have stayed frozen and unaltered in the outer solar system for the last 4.56 billion years. Observations of the Kuiper belt, then, are literally observations of the original solar nebula itself. The Kuiper belt is thus a critical region for understanding solar system development, and it is a region targeted with ongoing and active research. Since 1992 a number of large bodies have been found in the Kuiper belt. Some of the largest Kuiper belt bodies are listed in the table, opposite.
David Jewitt and his colleagues discovered one of the first very large Kuiper belt bodies, 20000 Varuna, in November 2000 using the Spacewatch telescope in Arizona. Originally known as 2000 WR ,
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