(Redirected from Kuiper Belt)
Known objects in the Kuiper belt, derived from data from the
Minor Planet Center. Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few
Trojan asteroids are yellow, while Jupiter's are pink. The scale is in
astronomical units.
The 'Kuiper belt' (
pronounced , to rhyme with "viper"),
[1] sometimes called the 'Edgeworth-Kuiper belt', is a region of the
Solar System beyond the planets extending from the
orbit of
Neptune (at 30
AU)
[2] to approximately
55 AU from the
Sun.
[3] It is similar to the
asteroid belt, although it is far larger; 20 times as wide and 20–200 times as massive.
[ The Solar System Beyond The Planets Audrey Delsanti and David Jewitt ][4] Like the asteroid belt, it consists mainly of
small bodies (remnants from the Solar System's formation) and at least one
dwarf planet –
Pluto. But while the asteroid belt is composed primarily of
rock and
metal, the Kuiper belt is composed largely of
ices, such as
methane,
ammonia, and
water.
Since discovery, the number of known
Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70,000 KBOs over 1 km in diameter are believed to reside there.
[5] The Kuiper belt is believed to be the main repository for
periodic comets, those with orbits lasting less than 200 years.
The
centaurs, comet-like bodies that orbit among the
gas giants, are also believed to originate there, as are the
scattered disc objects such as
Eris—KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun.
Neptune's moon
Triton is believed to be a captured KBO.
[6] Pluto, a
dwarf planet, is the largest known member of the Kuiper belt. Originally considered a planet, it has many physical properties in common with the objects of the Kuiper belt, and has been known since the early 1990s to share its orbit with a number of similarly sized KBOs, now called
Plutinos.
The Kuiper belt should not be confused with the hypothesized
Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the
scattered disc and any potential
Hills cloud or
Oort cloud objects, are collectively referred to as
trans-Neptunian objects (TNOs).
[7]
History
Since the discovery of Pluto, many have speculated that it might not be alone. The region now called the Kuiper belt had been hypothesized in various forms for decades. It was only in 1992 that the first direct evidence for its existence was found. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it.
The first
astronomer to suggest the existence of a trans-Neptunian population was
Frederick C. Leonard in 1930, soon after Pluto's discovery. He pondered whether it was "not likely that in Pluto there has come to light the ''first'' of a ''series'' of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected".
[8]
Subsequent hypotheses
In 1943, in the ''Journal of the British Astronomical Association'',
Kenneth Edgeworth hypothesised that, in the region beyond
Neptune, the material within the primordial
solar nebula was too widely spaced to condense into planets, and so rather condensed into a myriad of smaller bodies. From this he concluded that “the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies"
[9] and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system,â€
[10] becoming what we call a
comet.
In 1951, in an article for the journal ''Astrophysics'',
Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, however, he did not believe that such a belt still existed today. Kuiper was operating on the assumption common in his time, that
Pluto was the size of the Earth, and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. By Kuiper's formulation, there wouldn't be a Kuiper belt where we now see it.
[ WHY "KUIPER" BELT? David Jewitt ]
The hypothesis took many other forms in the following decades: in 1962, physicist
Al G.W. Cameron postulated the existence of “a tremendous mass of small material on the outskirts of the solar system,â€
[Davies, p. 14] while in 1964,
Fred Whipple, who popularised the famous "
dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of
Uranus that had sparked the search for
Planet X, or at the very least, to affect the orbits of known comets.
[11] Observation, however, ruled out this hypothesis.
In 1977,
Charles Kowal discovered
2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a
blink comparator; the same device that had allowed
Clyde Tombaugh to discover
Pluto nearly 50 years before.
[12] In 1992, another object
5145 Pholus, was discovered in a similar orbit.
[13] Today, an entire population, the
centaurs, is known to exist in that region. The centaurs' orbits are unstable over periods longer than roughly 100 million years, a relatively short span when compared to the age of the Solar System. From the time of Chiron's discovery, astronomers speculated that they therefore must be frequently replenished by some outer reservoir.
[14]
Further evidence for the belt's existence later emerged from the study of comets. That comets have finite lifespans has been known for some time. As they approach the Sun, its heat causes their
volatile surfaces to sublimate into space, eating them gradually away. In order to still be visible over the age of the Solar System, they must be frequently replenished.
[ FROM KUIPER BELT OBJECT TO COMETARY NUCLEUS: THE MISSING ULTRARED MATTER DAVID C. JEWITT ] One such area of replenishment is the
Oort cloud; the spherical swarm of comets extending beyond 50,000
AU from the Sun first hypothesised by astronomer
Jan Oort in 1950.
[15] It is believed to be the point of origin for
long period comets, those, like
Hale-Bopp, with orbits lasting thousands of years.
There is however another comet population, known as
short period or
periodic comets; those with orbits lasting less than 200 years. By the 1970s, the rate at which short-period comets were being discovered was becoming increasingly inconsistent with them having emerged solely from the
Oort cloud.
[16] For an Oort cloud object to become a short-period comet, it would first have to be captured by the giant planets. In 1980, in the monthly notice of the
Royal Astronomical Society,
Julio Fernandez stated that for every short period comet to be sent into the inner solar system from the Oort cloud, 600 would have to be ejected into interstellar space. He speculated that a comet belt from between 35 and 50
AU would be required to account for the observed number of comets.
[17] Following up on Fernandez's work, in 1988 the Canadian team of Martin Duncan, Tom Quinn and
Scott Tremaine ran a number of computer simulations to determine if all observed comets could have arrived from the Oort cloud. They found that the Oort cloud could not account for short-period comets, particularly as short-period comets are clustered near the plane of the Solar System, whereas Oort cloud comets tend to arrive from any point in the sky. With a belt as Fernandez described it added to the formulations, the simulations matched observations.
[18] Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named this region the "Kuiper belt."
[19]
Discovery
The array of telescopes atop
Mauna Kea, with which the Kuiper belt was discovered
In 1987, astronomer
David Jewitt, then at
MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System."
[ Discovery of the candidate Kuiper belt object 1992 QB1 David Jewitt, Jane Luu ] He encouraged then-graduate student
Jane Luu to aid him in his endeavour to locate another object beyond
Pluto's orbit, because, as he told her, "If we don't, nobody will."
[Davies p. 50] Using telescopes at the
Kitt Peak National Observatory in
Arizona and the
Cerro Tololo Inter-American Observatory in
Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator.
Initially, examination of each pair of plates took about eight hours,
[20] but the process was speeded up with the arrival of electronic
Charge-coupled devices or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90 percent of the light that hit them, rather than the ten percent achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for all astronomical detectors.
[21] In 1988, Jewitt moved to the Institute of Astronomy at the
University of Hawaii. He was later joined by Jane Luu to work at the University of Hawaii’s 2.24 m telescope at Mauna Kea.
[22] Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly.
[23] Finally, after five years of searching, on
August 30,
1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" ;
Six months later, they discovered a second object in the region, 1993 FW.
[24]
Name
Astronomers sometimes use alternative name 'Edgeworth-Kuiper belt' to credit Edgeworth, and KBOs are occasionally referred to as EKOs. However,
Brian Marsden claims neither deserve true credit; "Neither Edgeworth or Kuiper wrote about anything remotely like what we are now seeing, but
Fred Whipple did."
[25] Conversely, David Jewitt comments that, "If anything . . . Fernandez most nearly deserves the credit for predicting the Kuiper Belt."
The term '
trans-Neptunian object' (TNO) is recommended for objects in the belt by several scientific groups because the term is less controversial than all others — it is not a
synonym though, as TNOs include all objects orbiting the Sun at the outer edge of the solar system, not just those in the Kuiper belt.
Largest KBOs
Main articles: List of the brightest KBOs
Image:EightTNOs.png|thumb|300 px|The relative sizes of the largest trans-Neptunian objects as compared to Earth.
#Earth
rect 646 1714 2142 1994 The Earth
#Eris and Dysnomia
circle 226 412 16 Dysnomia
circle 350 626 197 (136199) Eris
#Pluto and Charon
circle 1252 684 86 Charon
circle 1038 632 188 (134340) Pluto
#2005 FY9
circle 1786 614 142 (136472) 2005 FY9
#2003 EL61
circle 2438 616 155 (136108) 2003 EL61
#Sedna
circle 342 1305 137 (90377) Sedna
#Orcus
circle 1088 1305 114 (90482) Orcus
#Quaoar
circle 1784 1305 97 (50000) Quaoar
#Varuna
circle 2420 1305 58 (20000) Varuna
desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
#Notes:
#Details on the new coding for clickable images is here:
#While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
Since the year 2000, a number of KBOs with diameters of between 500 and 1200 km (about half that of Pluto) have been discovered.
50000 Quaoar, a classical KBO discovered in 2002, is over 1200 km across. (nicknamed "Easterbunny") and (nicknamed "Santa"), both announced on
29 July 2005, are larger still. Other objects, such as
28978 Ixion (discovered in 2001) and
20000 Varuna (discovered in 2000) measure roughly 500 km across.
Pluto
The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size,
Pluto was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs
).
Ceres was considered a planet before the discovery of its fellow
asteroids, and, based on this precedent, many astronomers concluded that Pluto should also be reclassified.
The issue was brought to a head by the discovery of
Eris, an object in the
scattered disc far beyond the Kuiper belt, that is now known to be 27 percent more massive than Pluto.
[26] In response, the
International Astronomical Union (IAU), was forced to
define a planet for the first time, and in so doing included in their definition that a planet must have "
cleared the neighbourhood around its orbit."
[27] As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.
Though Pluto is the largest KBO, a number of objects outside the Kuiper belt which may have begun their lives as KBOs are larger. Eris is the most obvious example, but Neptune's moon
Triton, which, as explained above, is probably a captured KBO, is also larger than Pluto.
Origins
Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter
The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of the
Pan-STARRS survey telescope, which should reveal many currently unknown KBOs, to determine more about this.
The Kuiper belt is believed to consist of
planetesimals; fragments from the original
protoplanetary disc around the
Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than 3000 km in diameter.
Modern
computer simulations show the Kuiper belt to have been strongly influenced by
Jupiter and
Neptune, and also suggest that neither
Uranus nor
Neptune could have formed ''in situ'' beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are believed to have formed closer to Jupiter, but to have been flung outwards during the course of the Solar System's early evolution. Work in 1984 by Fernandez and Ip suggests that exchange of
angular momentum with a the scattered objects can cause the planets to drift.
[28] Eventually, the orbits shifted to the point where Jupiter and Saturn existed in an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational pull from such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing them to switch places and for Neptune to travel outward into the proto-Kuiper belt, sending it into temporary chaos.
[29] As Neptune travelled outward, it excited and scattered many TNOs into higher and more eccentric orbits.
[30]
However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles,
[31] the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
Structure
At its fullest extent, including its outlying regions, the Kuiper belt stretches from roughly 30 to 55 AU. However, the main body of the belt is generally accepted to extend from the 2:3 resonance (
see below) at 39.5 AU to the 1:2 resonance at roughly 48 AU. The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the
ecliptic plane and a more diffuse distribution of objects extending several times farther. Overall it more resembles a
torus or doughnut than a belt.
[32] Its mean position is inclined to the ecliptic by 1.86 degrees.
[33]
The presence of
Neptune has a profound effect on the Kuiper belt's structure due to
orbital resonances. Over a timescale comparable to the age of the Solar System, Neptune's gravity destabilises the orbits of any objects which happen to lie in certain regions, and either sends them into the inner Solar System or out into the
Scattered disc or interstellar space. This causes the Kuiper belt to possess pronounced gaps in its current layout, similar to the
Kirkwood gaps in the
Asteroid belt. In the region between 40 and 42 AU, for instance, no objects can retain a stable orbit over such times, and any observed in that region must have migrated there relatively recently.
[34]
Classical belt
Main articles: Classical Kuiper belt object
Between ~42 ~48 AU, however, the gravitational influence of Neptune is negligible, and objects can exist with their orbits pretty much unmolested. This region is known as the
classical Kuiper belt, and its members comprise roughly two thirds of KBOs observed to date.
[35][36] Because the first modern KBO discovered,
1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as
cubewanos ("Q-B-1-os").
[37][38]
The classical Kuiper belt appears to be a composite of two separate populations. The first, known as "dynamically cold" population, has orbits much like the planets; nearly circular, with an
orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). The second, the "dynamically hot" population, has orbits much more inclined to the ecliptic, by up to 30°. The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up.
[39] The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, is believed to have formed more or less in its current position although it may also have been later swept outwards by Neptune during its migration.
Resonances
Main articles: Resonant trans-Neptunian object
When an object's orbital period is an exact ratio of Neptune's (a situation called a
mean motion resonance), then it can become locked in a synchronised motion with Neptune and avoid being perturbed away if their relative alignments are appropriate. If, for instance, an object is in just the right kind of orbit so that it orbits the Sun two times for every three Neptune orbits, then whenever it returns to its original position, Neptune will always be half an orbit away from it, since it will have completed 1½ orbits in the same time. This is known as the 2:3 (or 3:2) resonance, and it corresponds to a characteristic
semi-major axis of ~39.4AU. This 2:3 resonance is populated by about 200 known objects,
[41] including
Pluto together with its moons. In recognition of this, the other members of this family are known as
Plutinos. Many Plutinos, including Pluto, often have orbits which cross that of Neptune, though their resonance means they can never collide. Many others, such as
90482 Orcus and
28978 Ixion, are over half of Pluto's size.
[42][ Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope John Stansberry, Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot ] Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune.
[ RESONANCE OCCUPATION IN THE KUIPER BELT: CASE EXAMPLES OF THE 5 : 2 AND TROJAN RESONANCES E. I. Chiang, A. B. Jordan, R. L. Millis, M. W. Buie, L. H. Wasserman, J. L. Elliot, S. D. Kern, D. E. Trilling, K. J. Meech, and R. M. Wagner ] The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7AU, and is sparsely populated.
[43] Its residents are sometimes referred to as
twotinos. Minor resonances also exist at 3:4, 3:5, 4:7 and 2:5.
[44] Neptune possesses a number of
trojan objects, which occupy its
L4 and L5 points; gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are often described as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits and are unlikely to have been captured by Neptune, but rather formed alongside it.
Additionally, there is a relative absence of objects with semi-major axes below 39 AU which cannot apparently be explained by the present resonances. The currently accepted hypothesis for the cause of this is that as Neptune migrated outward, unstable orbital resonances moved gradually through this region, and thus any objects within it were swept up, or gravitationally ejected from it.
[45]
The "Kuiper cliff"
Graph showing the numbers of KBOs for a given distance from the Sun
The 1:2 resonance appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the Classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.
[46]
Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU;
[47] so this sudden drastic falloff, known as the "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown.
Alan Stern of the
Southwest Research Institute has claimed that the gravitiational attraction of an unseen large planetary object, perhaps the size of Earth or Mars, might be responsible.
[48] Bernstein and Trilling et al. have found evidence that the observed rapid decline in objects of 100 km or more in radius beyond 50 AU is a real decline in the number of objects, and not just an observational effect.
[49]
Former Kuiper belt objects
A number of objects in the Solar System, while not technically being KBOs themselves, are believed to have originated in the Kuiper belt.
Scattered objects
Main articles: Scattered disc,
Centaur (planetoid)
The orbits of objects in the scattered disc; the classical KBOs are blue, while the 2:5 resonant objects are green.
The scattered disc is a sparsely populated region beyond the Kuiper belt, extending as far as 100 AU and farther.
Scattered disc objects (SDOs) travel in highly elliptical orbits, usually also highly inclined to the ecliptic. Most models of solar system formation show icy planetoids first forming in the Kuiper belt, while later gravitational interactions, particularly with Neptune, displaced some of them outwards into the scattered disc.
According to the
Minor Planet Center, which officially catalogues all trans-Neptunian objects, a KBO, strictly speaking, is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. Objects found outside the belt are classed as scattered objects.
[ List Of Centaurs and Scattered-Disk Objects ] However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy planetoid native to the outer solar system believed to have been part of that initial class, even if its orbit during the bulk of solar system history has been beyond the Kuiper belt (e.g. in the scattered disk region). They often describe scattered disc objects as "scattered Kuiper belt objects."
[50] Other trans-Neptunian researchers have been more cautious in applying the KBO label to objects clearly outside the belt in the current epoch.
Eris, the recently discovered object now known to be larger than Pluto, is often referred to as a KBO, but is technically an SDO.
The centaurs, which are not normally considered part of the Kuiper belt, are also believed to be scattered Kuiper belt objects, the only difference being that they were scattered inward, rather than outward. The
Minor Planet Center groups the centaurs and the SDOs together as scatterd KBOs.
Triton
Main articles: Triton (moon)
.jpg)
Neptune's moon Triton
During its period of migration, Neptune is thought to have captured one of the larger KBOs and set it in orbit around itself. This is its moon
Triton, which is the only large moon in the Solar System to have a
retrograde orbit; it orbits in the opposite direction to Neptune's rotation. This suggests that, unlike the large moons of Jupiter and Saturn, which are thought to have coalesced from spinning discs of material encircling their young parent planets, Triton was a fully formed body that was captured from surrounding space. Gravitational capture of an object is not easy; it requires that some force act upon the object to slow it down enough to be snared by the larger object's gravity. How this happened to Triton is not well understood, though it does suggest that Triton formed as part of a large population of similar objects whose gravity could impede its motion enough to be captured. Spectral analysis of both Triton and Pluto shows that they are largely composed of similar materials, such as
methane and
carbon monoxide. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its outward migration.
[51]
Comets
Main articles: Comet
Comets in the solar system can be loosely divided into two categories: short-period and long period. Long period comets are believed to originate in the
Oort cloud. There are two recognised categories of short-period comets: Jupiter-family comets and Halley-family comets. The latter group, which is named for its prototype,
Halley's Comet, are believed to have emerged from the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets.
It is the former type, the Jupiter family, that are believed to have originated from the Kuiper belt. The centaurs are thought to be a dynamically intermediate stage between the Kuiper belt and the Jupiter family.
[52]
Despite the fact that many are universally thought to have hailed from the Kuiper belt, there exist a wide array of differences between KBOs and Jupiter-family comets. Although the centaurs share a reddish colouration with many KBOs, the nuclei of comets are far bluer, indicating a fundamental chemical or physical difference.
The current hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older reddish material.
Satellites
Satellites are markedly common among trans-Neptunian objects. Of the four largest TNOs, three (Eris, Pluto, and
2003 EL61) possess satellites, and two have more than one. A higher percentage of the largest KBOs possess satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible.
[53] There are also a high number of binaries (two objects close enough in mass to be orbiting "each other") in the Kuiper belt. The most notable example is the Pluto-Charon binary, but it is estimated that over 1 percent of KBOs (a high percentage) exist in binaries.
[ Binary Kuiper Belt Objects Dave Jewitt ]
Composition
The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines
Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as
methane),
ammonia, and water
ice, a composition they share with
comets.
[54] The temperature of the belt is only about 50K,
[ Crystalline water ice on the Kuiper belt object (50000) Quaoar David C. Jewitt & Jane Luu ] so many compounds that would remain gaseous closer to the Sun are solid.
Due to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine. The principal method by which astronomers determine the composition of a celestial object is
spectroscopy. When an object's light is broken into its component colours, an image akin to a rainbow is formed. This image is called a
spectrum. Different substances absorb light at different wavelengths, and when the spectrum for a specific object is unravelled, dark lines (called
absorption lines) appear where the substances within it have absorbed that particular wavelength of light. Every
element or
compound has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine what it is made of.
Initially, such detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their colour.
[ Surfaces of Kuiper Belt Objects Dave Jewitt ] These first data showed a broad range of colours among KBOs, ranging from neutral grey to deep red.
[ OPTICAL-INFRARED SPECTRAL DIVERSITY IN THE KUIPER BELT DAVID JEWITT, JANE LUU ] This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons.
This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of their volatile ices to the effects of cosmic rays.
[55] Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing.
However, Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in colour was too extreme to be easily explained by random impacts.
[56]
Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition.
In 1996, Robert H. Brown ''et al'' obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of
Pluto, as well as Neptune's moon
Triton, possessing large amounts of
methane ice.
[ Surface Composition of Kuiper Belt Object 1993SC Robert H. Brown, Dale P. Cruikshank, Yvonne Pendleton, Glenn J. Veeder ]
Water ice has been detected in several KBOs, including
1996 TO66,
[57] 2000 EB173 and 2000 WR106.
[58] In 2004, Mike Brown ''et al'' determined the existence of crystalline water ice and
ammonia hydrate on one of the largest known KBOs,
50000 Quaoar. Both of these substances would have been destroyed over the age of the solar system, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.
Mass and size distribution
Despite its vast extent, the collective mass of the Kuiper belt is relatively low; estimated at roughly a tenth the mass of the Earth.
Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses.
This missing >99% of the mass can hardly be dismissed, as it is required for the accretion of any KBOs larger than 100 km in diameter. At the current low density, these objects simply should not exist. Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent," resulting in destruction rather than accretion.
It appears that either the current residents of the Kuiper belt have been created closer to the Sun or some mechanism dispersed the original mass. Neptune’s influence is too weak to explain such a massive "vacuuming". While the question remains open, the conjectures vary from a passing star scenario to grinding of smaller objects, via collisions, into dust small enough to be affected by solar radiation.
[59]
Bright objects are rare compared with the dominant dim population, as expected from accretion models of origin, given that only some objects of a given size would have grown further. This relationship N(D), the population expressed as a function of the diameter, referred to as brightness slope, has been confirmed by observations. The slope is inversely proportional to some power of the diameter D.
:
where the current measures
[60] give q = 4 ±0.5.
Less formally, there is for instance 8 (=2³) times more objects in 100–200 km range than objects in 200–400 km range. In other words, for a single object with the diameter of 1000 km it should be there around 1000 (=10³) objects with diameter of 100 km.
The law is expressed in this differential form rather than as a cumulative cubic relationship, because only the middle part of the slope can be measured; the law must break at smaller sizes, beyond the current measure.
Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects)
Exploration
Main articles: New Horizons
Artist's conception of ''New Horizons'' at Pluto
On
January 19,
2006, the first spacecraft mission to explore the Kuiper belt, ''
New Horizons,'' was launched. The mission, headed by
Alan Stern of the
Southwest Research Institute, will arrive at
Pluto on
July 14 2015 and, circumstances permitting, will continue on to study another as-yet undetermined KBO. Any KBO chosen will be between 25 and 55 miles (40 to 90 km) in diameter and, ideally, white or grey, to contrast with Pluto's reddish colour.
[61] John Spencer, an astronomer on the ''New Horizons'' mission team, says that no target for a post-Pluto Kuiper belt encounter has yet been selected, as they are awaiting data from the
Pan-STARRS survey project to ensure as wide a field of options as possible.
[62] The Pan-STARRS project, due to come fully online by 2009,
[63] will survey the entire sky with four 1.4 gigapixel digital cameras to detect any moving objects, from
near-earth objects to KBOs.
[64]
Other Kuiper belts
The debris disks around two remote stars
As of 2006, nine stars other than the Sun are known to be circled by Kuiper belt-like structures. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (like our own Kuiper belt) with diameters of between 20 and 30 AU and relatively sharp boundaries. Most debris discs around other stars are fairly young, but the two imaged at right, taken by the Hubble Space Telescope in January, 2006, are old enough (roughly 300 million years) to have settled into stable configurations. The left image is a "top view" of a wide belt, and the right image is an "edge view" of a narrow belt. The black central circle is produced by the camera's
coronagraph which hides the central star to allow the much fainter disks to be seen.
[65]
References
1. Dutch requests
2. One AU, or "astronomical unit", is the average distance between the Earth and the Sun, or roughly 149 597 870 691 metres. It is the standard unit of measurement for interplanetary distances.
3. Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 AU Kuiper Gap S. ALAN STERN
4. Hidden Mass in the Asteroid Belt, , G. A., Krasinsky, Icarus, 2002
5. Pluto, perception & planetary politics David Jewitt, Jane Luu
6. Neptune’s capture of its moon Triton in a binary-planet gravitational encounter Craig B. Agnor & Douglas P. Hamilton
7. DESCRIPTION OF THE SYSTEM OF ASTEROIDS AS OF MAY 20, 2004 Gérard FAURE
8. What is improper about the term "Kuiper belt"? (or, Why name a thing after a man who didn't believe its existence?)
9. Beyond Pluto: Exploring the outer limits of the solar system, John Davies, , , Cambridge University Press, 2001,
10. Davies, p. 2
11. EVIDENCE FOR A COMET BELT BEYOND NEPTUNE FOR A COMET BELT BEYOND NEPTUNE
BY FRED L. WHIPPLE
12. The discovery and orbit of /2060/ Chiron CT Kowal, W Liller, BG Marsden
13. 1992 AD JV Scotti, DL Rabinowitz, CS Shoemaker, EM Shoemaker, DH Levy, TM King, EF Helin, J Alu, K Lawrence, RH McNaught, L Frederick, D Tholen, BEA Mueller
14. Davies p. 38
15. Oort, J. H., ''The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin'', Bull. Astron. Inst. Neth., ''11'', p. 91–110 (1950) Text at Harvard server (PDF)
16. Davies p. 39
17. On the existence of a comet belt beyond Neptune JA Fernandez
18. The origin of short-period comets M. Duncan, T. Quinn, and S. Tremaine
19. Davies p. 191
20. Davies p. 51
21. Davies pp. 52, 54, 56
22. Davies pp. 57, 62
23. Davies p. 65
24. 1993 FW BS Marsden
25. Davies p. 199
26. Dysnomia, the moon of Eris Mike Brown
27. IAU 2006 General Assembly: Resolutions 5 and 6
28. Neptune’s Migration into a Stirred–Up Kuiper Belt: A Detailed Comparison of Simulations to Observations Joseph M. Hahn
29. Orbital shuffle for early solar system Kathryn Hansen
30. THE FORMATION OF URANUS AND NEPTUNE AMONG JUPITER AND SATURN E. W. THOMMES, M. J. DUNCAN, H. F. LEVISON
31. Nonlinear Resonances in the Solar System
32. Discovering the Edge of the Solar System
33. THE PLANE OF THE KUIPER BELT Michael E. Brown, Margaret Pan
34. Large Scattered Planetesimals and the Excitation of the Small Body Belts Jean-Marc Petit, Alessandro Morbidelli, Giovanni B. Valsecchi
35. The Kuiper Belt Jonathan Lunine
36. CLASSICAL KUIPER BELT OBJECTS (CKBOs) Dave Jewitt
37. Cubewano P Murdin
38. THE DEEP ECLIPTIC SURVEY: A SEARCH FOR KUIPER BELT OBJECTS AND CENTAURS. II. DYNAMICAL CLASSIFICATION, THE KUIPER BELT PLANE, AND THE CORE POPULATION J. L. Elliot, S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie,
L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech
39. The formation of the Kuiper belt by the outward transport of bodies during Neptune’s migration Harold F. Levison, Alessandro Morbidelli
40. ORIGIN AND DYNAMICAL EVOLUTION OF COMETS AND THEIR RESERVOIRS Alessandro Morbidelli
41. List Of Transneptunian Objects
42. Ixion
43. Trans-Neptunian Objects Wm. Robert Johnston
44. Davies p. 104
45. Davies p. 107
46. RESONANCE OCCUPATION IN THE KUIPER BELT: CASE EXAMPLES OF THE 5 : 2 AND TROJAN RESONANCES E. I. Chiang, A. B. Jordan, R. L. Millis, M. W. Buie, L. H. Wasserman, J. L. Elliot,
S. D. Kern, D. E. Trilling,5 K. J. Meech, and R. M. Wagner
47. KECK PENCIL-BEAM SURVEY FOR FAINT KUIPER BELT OBJECTS E. I. Chiang and M. E. Brown
48. 13 Things that do not make sense Michael Brooks
49. The Size Distribution of Trans-Neptunian Bodies, G.M. Bernstein, D.E. Trilling, R.L. Allen, M.E. Brown, M. Holman and R. Malhotra, , , The Astrophysical Journal, 2004
50. The 1000 km Scale KBOs David Jewitt
51. TRITON, PLUTO, CENTAURS, AND TRANS-NEPTUNIAN BODIES DALE P. CRUIKSHANK
52. The Populations of Comet-like Bodies in the Solar System J Horner, NW Evans, ME Bailey, DJ Asher
53. Satellites of the Largest Kuiper Belt Objects M. E. Brown, M. A. van Dam, A. H. Bouchez, D. Le Mignant, R. D. Campbell, J. C. Y. Chin, A. Conrad, S. K. Hartman, E. M. Johansson, R. E. Lafon, D. L. Rabinowitz, P. J. Stomski, Jr., D. M. Summers, C. A. Trujillo, and P. L. Wizinowich
54. COMPOSITION OF THE VOLATILE MATERIAL IN HALLEY’S COMA FROM IN SITU MEASUREMENTS K. ALTWEGG and H. BALSIGER and J. GEISS
55. Davies p. 118
56. COLORS AND SPECTRA OF KUIPER BELT OBJECTS David C. Jewitt, Jane X. Luu
57. NEAR-INFRARED SPECTROSCOPY OF THE BRIGHT KUIPER BELT OBJECT 2000 EB173 Michael E. Brown, Geoffrey A. Blake, Jacqueline E. Kessler
58. NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106 J. Licandro, E. Oliva and M. Di Martino
59.
Morbidelli A. ''Origin and dynamical evolution of comets and their reservoirs.''
Preprint on arXiv (pdf)
60. Bernstein G.M., Trilling D.E., Allen R.L., Brown K.E, Holman M., Malhotra R. ''The size Distribution of transneptunian bodies.'' The Astronomical Journal, '128', 1364–1390.
preprint on arXiv (pdf)
61. New Horizons mission timeline
62. The Man Who Finds Planets Cal Fussman
63. Calibration of the Pan-STARRS 3Ï€ Survey E. Magnier
64. Pan-Starrs: University of Hawaii
65. Dusty Planetary Disks Around Two Nearby Stars Resemble Our Kuiper Belt
External links and data sources
★
Dave Jewitt's page @ University of Hawaii
★
★
The belt's name
★
List of short period comets by family
★
Kuiper Belt Profile by
NASA's Solar System Exploration
★
The Kuiper Belt Electronic Newsletter
★
Wm. Robert Johnston's TNO page
★
Minor Planet Center: Plot of the Outer Solar System, illustrating Kuiper gap
★
Website of the International Astronomical Union (debating the status of TNOs)
★
XXVIth General Assembly 2006
★
nature.com article: diagram displaying inner solar system, Kuiper Belt, and Oort Cloud
★ SPACE.com:
Discovery Hints at a Quadrillion Space Rocks Beyond Neptune (Sara Goudarzi) 15 August 2006 06:13 a.m. ET
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