Beyond the Earth, Mars, the asteroid belt, Saturn, and Jupiter, lies another belt of celestial objects. Found 50 astronomical units (AU) from the Sun, the Kuiper belt is a circumstellar disc that extends from Neptune's orbit. It resembles the asteroid belt, although it is 20 times wider and 20–200 times more massive than the asteroid belt. Asteroid belt-like leftovers of the Solar System's formation can be found here. Unlike asteroids, which are mostly made of rock and metal, Kuiper belt objects are mostly made of frozen volatiles (also known as "ices"), such as methane, ammonia, and water, as opposed to asteroids, which are primarily made of rock and metal. Dwarf planets like Orcus, Pluto, Haumea, Quaoar, and Makemake have their home in the Kuiper belt. Triton on Neptune and Phoebe on Saturn are two moons that may have formed in this region of the Solar System.
It was named after Dutch astronomer Gerard Kuiper, even though he didn't foresee its existence. It was in 1992 when the first Kuiper belt object (KBO) since Pluto and Charon was identified was Albion, a minor planet (15760). (in 1978). KBOs have grown since their discovery, and it is presently believed that there are more than 100,000 KBOs with a diameter of more than 100 kilometers (62 miles). Initially, the Kuiper belt was assumed to be the primary location for comets with orbits lasting fewer than 200 years, but this is no longer the case. A dynamically active zone created by Neptune's outward motion 4.5 billion years ago, the scattered disc, is the true origin of comets, according to studies conducted since the mid-1990s; scattered disc objects such as Eris have highly eccentric orbits that take them up to 100 AU away from the Sun.
Kuiper belts are not to be confused with the fictitious Oort cloud, which is thought to be a thousand times farther away and primarily spherical. Trans-Neptunian objects include the objects in the Kuiper belt, the dispersed disc, and any possible Hills cloud or Oort cloud objects (TNOs). A member of the Kuiper belt, Pluto is the largest and heaviest known TNO, second only to Eris in the dispersed disc in terms of mass. Pluto was downgraded from planet rank to dwarf planet in 2006 due to its inclusion in the Kuiper belt. It shares a 2:3 resonance with Neptune and is classified as a "plutino" type of Kuiper Belt Objects (KBOs) because of its comparable composition and orbital period.
The heliopause (a barrier between the Sun's solar wind and interstellar wind) and the distance at which the Sun's gravitational pull is equal to that of other stars can also be used as indicators of the size of the Solar System, as can the Kuiper belt and Neptune (approximately 50000 AU and 125000 AU).
Pluto's discovery in 1930 sparked speculation that it was not the only planet in the solar system. For many years, scientists have speculated about what is now known as the Kuiper belt. For the first time, scientists discovered proof of its existence only in 1992. Because of the wide range of previous hypotheses about the nature of the Kuiper Belt, it's unclear who deserves credit for the idea's inception.
Frederick C. Leonard was the first astronomer to propose the presence of a trans-Neptunian population. Early in the decade following Clyde Tombaugh's discovery of Pluto in 1930, Leonard wondered whether or not the first of several ultra-Neptunian bodies, which are yet to be discovered but would indeed be found, had been located in Pluto. Astronomer Armin O. Leuschner speculated that Pluto "may be one of the numerous long-period planetary objects still to be identified" in 2000.
Kenneth Edgeworth proposed in 1943 in the Journal of the British Astronomical Association that the initial solar nebula material beyond Neptune was too widely spread to condense into planets. Hence, it condensed into a multitude of more minor things. This hypothesis was later disproved. A comet is formed when a small body "wanders from its sphere and appears as a temporary guest to the inner solar system," based on his conclusion that "a very large number of comparably small bodies" occupy the outer region of the solar system beyond the orbits of the planets.
He postulated in a 1951 study published in Astrophysics: A Topical Symposium on the formation of such an object, although Gerard Kuiper did not believe such a belt still existed. To Kuiper, Pluto was the size of Earth, and as a result, these bodies had been dispersed into the Oort cloud or beyond the Solar System. The Kuiper belt would not exist now if Kuiper's hypothesis were correct.
In the subsequent decades, the hypothesis evolved into various versions. When scientist Al G.W. Cameron proposed in 1962 that there might be "a massive mass of tiny material outside the solar system," no one believed him. A "comet belt," according to Fred Whipple, who popularized the "dirty snowball" idea for cometary structure in 1964, would be large enough to account for the apparent anomalies in Uranus' orbit that inspired the search for Planet X. This idea was ruled out by simple observation.
Charles Kowal identified an ice planetoid with an orbit between Saturn and Uranus in 1977. Clyde Tombaugh used the same blink comparator to discover Pluto nearly 50 years earlier. In 1992, a second object, 5145 Pholus, was revealed in the same orbit. Between Jupiter and Neptune, there is a large population of comet-like bodies known as centaurs. The centaur orbits are unstable and only last for a few million years in dynamic terms. It has been suggested since the discovery of Chiron in 1977 that some external source must constantly replenish the centaurs.
The study of comets provided additional evidence for the presence of the Kuiper belt. For a long time, scientists have known that comets had a limited life span. Their flammable surfaces sublimate into space as they reach the Sun, eventually scattering. Comets must be recharged periodically if they remain observable for the duration of the Solar System. The Oort cloud, first hypothesized by Dutch astronomer Jan Oort in 1950 as a possible spherical swarm of comets reaching beyond 50,000 AU from the Sun, is one such suggestion. Long-period comets like Hale–Bopp, with orbits spanning thousands of years, are likely to have formed in the Oort cloud.
Those comets with orbital periods less than 200 years, such as Halley's Comet, are classified as part of the short-period or periodic comet population. During the 1970s, the discovery rate of short-period comets was becoming increasingly inconsistent with the Oort cloud's primary origin. To become a short-period comet, an Oort cloud object must first be grabbed by the planets. Julio Fernández, an Uruguayan astronomer, argued in a 1980 paper published in Monthly Notices of the Royal Astronomical Society that for every short-period comet to be brought into the inner Solar System, 600 would have to be ejected into interstellar space. He estimated that a belt of comets between 35 and 50 AU would be necessary to explain the observed number of comets. This was followed up by the Canadian team of Martin Duncan, Tom Quinn, and Scott Tremaine in 1988, who ran several computer simulations to assess whether or not all observed comets might have come from the Oort cloud. While the Oort cloud could not account for all short-period comets, they observed that they cluster around the solar system plane, while Oort-cloud comets tend to arrive from any place in the sky. The simulations matched the observations after Fernández added a "belt" to the formulae. Tremaine is said to have termed this hypothetical location the "Kuiper belt" because the phrases "Kuiper" and "comet belt" occurred in the opening sentence of Fernández's article.
When David Jewitt, an astronomer at MIT, got increasingly perplexed by "the apparent nothingness of the outer Solar System," he decided to investigate. Jane Luu, a Ph.D. student at the time, was enlisted in his search for a second object beyond Pluto's orbit because he told her, "If we don't, nobody else will." They used blink comparator telescopes at Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, just like Clyde Tombaugh and Charles Kowal had. Each plate pair was initially examined for about eight hours. These devices had a narrower view, but they were more efficient at collecting and retaining light (they retained the majority of the light that hit them, rather than 10% achieved by photographs) and allowed the blinking process virtually on the computer screen. Most astronomy detectors today use CCDs. The University of Hawaii's Institute of Astronomy welcomed Jewitt in 1988. At Mauna Kea's 2.24 m telescopes, Luu later joined him in his work with the University of Hawaii. Finally, the range of vision for CCDs expanded to 1024 by 1024 pixels, allowing for much faster searches. Jewitt and Luu announced on August 30, 1992, the "discovery of a candidate Kuiper belt object 1992 QB1" after five years of hunting. 15760 Albion was the name given to this object in the future. Six months later, they found a second object in the area: 181708) 1993 FW. There have been about 2000 Kuiper Belt objects found by the year 2018.
After discovering 1992 QB1 (named in 2018, 15760 Albion) 1992, more than a thousand bodies were found in a belt, suggesting a massive belt of bodies other than Pluto and Albion. To date, little is known about Kuiper belt bodies as of the year 2010. An uncrewed spacecraft flew past two KBOs in the late 2010s, affording far closer views of the Plutonian system and a second KBO.
For some time now, astronomers have suspected that the area currently known as the Kuiper belt is not where short-period comets originate but rather a loosely connected population now known as the dispersed disc. Neptune left behind a population of dynamically stable objects (the Kuiper belt proper) as it migrated into the proto-Kuiper belt, which had not yet drifted away from the Sun, and a population of objects that Neptune could still disturb as it traveled around the Sun (the scattered disc). The dispersed disc is now the most credible source for periodic comets because it is dynamically active, and the Kuiper belt is relatively stable.
To give credit where it is due, astronomers sometimes refer to the Kuiper Belt as the Edgeworth–Kuiper belt and KBOs as the EKOs. According to Brian G. Marsden, "Neither Edgeworth nor Kuiper wrote about anything slightly like what we are now experiencing, but Fred Whipple did." "Fernández "very nearly deserves" the credit for foreseeing the Kuiper Belt, according to David Jewitt.
Clyde Tombaugh coined the term "kuiperoids" to describe KBOs. For Kuiper Belt objects, various scientific organizations support the name "trans-Neptunian object" (TNO), which has fewer detractors than any other term. However, TNOs do not include only Kuiper Belt bodies; they also include all other solar system bodies that extend beyond Neptune's orbit.
Its outer sections, including the scattered disc, extend from around 30–55 AU in length when the Kuiper belt is at its most extensive (but not including it). The 2:3 mean-motion resonance (see below) is usually regarded to stretch the main belt from 39.5 AU out to the 1:2 resonance, which is around 48 AU away. The Kuiper belt's main concentration extends to ten degrees outside the ecliptic plane. In contrast, a more diffuse dispersion of objects extends many times farther than the primary concentration. There is a distinct lack of uniformity in the design, and its average location is 1.86 degrees out of alignment with the ecliptic.
Due to orbital resonances, Neptune's presence has a significant impact on the Kuiper Belt's structure. Gravitational destabilization of orbits due to Neptune's gravity occurs on a timescale roughly equivalent to the Solar System's age. This results in objects being flung into the inner Solar System or interstellar space. In its current configuration, the Kuiper belt has prominent gaps, like the Kirkwood gaps in the asteroid belt. An object can't remain in a stable orbit over such an extended period, and any observed in that zone must have recently arrived there.
The gravitational interactions with Neptune occur over an extended duration between the 2:3 and 1:2 resonances, at around 42–48 AU, and objects can survive with their orbits virtually unchanged. Around two-thirds of the known KBOs in this region are known as the classical Kuiper belt. Classical KBOs are often referred to as cubewanos since the first contemporary KBO discovered (Albion, but long-named (15760) 1992 QB1), is regarded as the prototype of this group ("Q-B-1-os"). According to IAU rules, classical KBOs must be named after legendary entities involved with the universe's origin.
There appear to be two distinct populations of Kuiper belt objects. A population referred to as "dynamically cool" has orbits resembling planets, with eccentricities under 0.1 and inclinations up to around 10 degrees (they lie close to the solar system plane rather than at an angle). Clusters with semi-major axes of 44–44.5 AU can be found in the cold population, which includes a cluster of objects known as the kernel. On the other hand, the "dynamically hot" population has orbits up to 30 degrees out of alignment with the ecliptic. Particles in a gas are analogous to the two populations, which have been referred to as such because of their increased velocity as they heat up. As well as orbiting in different directions, the cold and hot populations differ in color and albedo, with the cold population being redder and brighter, containing more binary objects, and lacking massive objects. The dynamically cool population has a mass 30 times smaller than the hot population. Depending on the composition, the varying colors could indicate that the rocks were created in separate places. During the migration of the large planets, the hot population may have developed around Neptune's original orbit and was dispersed. However, it is hypothesized that Neptune's contacts with the loose binaries of the cold population caused it to develop at its current location. It has been claimed that the color difference may represent changes in surface evolution, even though the Nice model can explain at least part of a compositional difference.
Objects can avoid being disturbed away from Neptune if their orbital periods are an exact ratio of Neptune's (referred to as a mean-motion resonance) and their relative alignments are correct. Whenever an object returns to perihelion after completing 1+12 orbits around Neptune, it will always be in the same relative position as it was when it first left perihelion; for example, if an object orbits the Sun twice for every three Neptune orbits and reaches perihelion with another object a quarter of an orbit away from it, then it is in the 2:3 (or 3:2) resonance, wherein a characteristic semi-major axis of approximately 39.4 AU can be found. Pluto and its moons are part of a 2:3 resonance that includes around 200 known objects. These family members are called "plutinos" due to their ancestry. Pluto, like many plutinos, has an orbit that crosses over with Neptune's, but because of their resonance, the two bodies cannot crash. With their extreme orbital eccentricities, the Plutinos appear to have been flung into their current orbits by Neptune's migration rather than being native to their recent locations. All Plutonos, like Pluto, must be named after gods of the underworld, according to IAU criteria. The semi-major axes of 47.7 AU correspond to the 1:2 resonance, where objects complete half an orbit for each of Neptune's. Twotinos are the locals' nicknames for themselves. In addition to the 3:4 and 3:5, there are other resonances at 4:7 and 2:5. Several trojan objects occupy Neptune's Lagrangian points, which are regions of gravitationally stable orbit around Neptune. Neptune trojans can maintain stable orbits in a 1:1 mean-motion resonance with Neptune.
Aside from that, there aren't many objects with semi-major axes less than 39 AU that doesn't fit neatly into the existing resonances. Neptune's slow outward migration is thought to have resulted in unstable orbital resonances moving across this region, which swept up and ejected any objects in it.
Objects beyond the 47.8 AU 1:2 resonance appear to be few and far between. It's unclear if this is the end of the classical belt or just the beginning of a vast chasm. Objects have been identified at the 2:5 resonance at around 55 AU, well outside the classical belt. Still, the predictions of a large number of planets in classical orbits between these resonances have not been confirmed by observation.
Earlier models of the Kuiper belt predicted a two-fold increase in the number of large objects beyond 50 AU based on estimates of the primordial mass needed to form Uranus and Neptune, as well as other bodies as large as Pluto (see mass and size distribution), so the sudden drastic falloff, known as the Kuiper cliff, was unexpected and its cause is unknown to date. There is strong evidence that the dramatic drop in objects with a radius of 100 km or greater beyond 50 AU is real and not due to observational bias. It's possible that the material was too rare or scattered to accumulate into huge items at that distance or that subsequent processes removed or destroyed those that did. Patryk Lykawka of Kobe University suggested that an undiscovered huge planetary object, maybe the size of Earth or Mars, might be to blame for this phenomenon.
Protoplanetary disc fragments that failed to merge into planetary bodies and instead coalesced into smaller bodies smaller than 3,000 kilometers (1,900 miles) in diameter make up the Kuiper belt, according to the theory of planetesimal formation. Findings on Pluto and Charon's crater counts show that these objects were formed as large, tens of kilometers in diameter objects, rather than being accreted from much smaller, about kilometers in diameter entities. The gravitational collapse of pebble clouds focused between eddies in a chaotic protoplanetary disk or streaming instabilities are two possible pathways for developing these bigger entities. These collapsing clouds may form binary systems.
It has been shown that Jupiter and Neptune significantly impacted the Kuiper belt in modern computer simulations. These simulations also show that Uranus and Neptune could not have formed in their current positions due to the lack of primordial matter in that range needed to produce objects of that mass. On the other hand, these planets are thought to have formed near Jupiter. The large planets' orbits migrated due to the dispersal of planetesimals early in the Solar System's history: Saturn, Uranus, and Neptune drifted outwards, while Jupiter drifted in. When the orbits of Jupiter and Saturn eventually moved to a precise 1:2 resonance, Jupiter orbited the Sun twice for every single Saturn orbit. Such a resonance eventually destabilized the Uranus and Neptune orbits, forcing them to be dispersed on high-eccentricity orbits that traversed the primordial disc.
A dynamically cold band of low-inclination planetesimals was formed when Neptune had a very eccentric orbit. Its mean-motion resonances coincided with those of the planetesimals, resulting in a chaotic evolution of the orbits of the planetesimals. To get to its current location, Neptune had to get rid of its eccentricity. During this movement, many planetesimals were captured, but others developed into higher-inclination and lower-eccentricity orbits that allowed them to escape the resonances. Some of the smaller fragments of planetesimals that had been flung outward ended up in the form of Jupiter trojan, planetary satellites, and asteroid belt outposts. In most cases, Jupiter ejected the remaining Kuiper belt objects from the Solar System, lowering the population of the primordial Kuiper belt by 99 percent or more.
"Nice" is a standard model that reproduces many of the Kuiper belt's properties like the "cold" and "hot' populations of objects, resonant objects, and a scattered disc. Still, it fails to account for sure of their distributions. Eccentricity is larger than observed (0.10–0.13 versus 0.07), and the distribution of high-inclination KBOs anticipated by the model is excessively sparse. Many far-apart and loosely connected binary objects in the cold belt complicate the model's job even further. Sometimes, the cold disc is the sole local population of tiny bodies in our solar system, and it is believed to have been separated during interactions with Neptune.
Five giant planets, including an ice-giant, have been added to the Nice model in a series of mean-motion resonances. The resonance chain breaks about 400 million years after the formation of the Solar System. The ice giants first move many AU out of the disc before dispersing. The planets' orbits become unstable due to this divergent migration, resulting in a resonance crossing. The additional ice giant collides with Saturn and is dispersed into a Jupiter-crossing orbit before being expelled from the Solar System after several collisions. All the planets in the planetesimal disc continue to migrate until only a few fragments remain in diverse regions.
During Neptune's outward migration, like in the original Nice model, objects are captured into resonances with Neptune. Those that remain in the resonances evolve into orbits with lower eccentricity and higher inclination. At the same time, those released into the dynamically heated classical belt do so in a more stable manner. If Neptune moved from 24 AU to 30 AU over 30 Myr, the hot belt's inclination distribution could be replicated. When Neptune reaches 28 AU, it encounters the additional ice giant. Neptune's semi-major axis jumps outward due to this encounter, leaving behind a concentration of captured objects from the cold belt at 44 AU. In addition, some weakly bound 'blue' binaries originate closer to the cold belt's current position. A primordial cold band will be preserved if Neptune's eccentricity remains low throughout this encounter. Otherwise, the chaotic evolution of orbits in the original Nice model will occur. As Neptune proceeds through its latter stages, the cold belt's eccentricity distribution is trimmed by a steady sweeping of mean-motion resonances.
Kuiper belt objects are considered relatively undisturbed by processes that have formed and altered other Solar System objects; therefore, identifying their composition would yield important information on the composition of the early Solar System. The chemical composition of KBOs is challenging to ascertain because of their small size and great distance from Earth. Spectroscopy is the primary tool used by astronomers to determine the chemical composition of celestial objects. When the light emitted by an item is dispersed into its constituent colors, an image like a rainbow emerges. It's called a spectrum, and you can see it here. There are black lines (referred to as absorption lines) when the compounds within an object have absorbed a specific wavelength of light when its spectra are unraveled because different substances absorb light at different wavelengths. Astronomers can detect an object's composition by reading its whole spectral "fingerprint," which is unique to each element or compound.
Rock and ice, such as water, methane, and ammonia, have been found in the Kuiper belt objects studied. Many chemicals that would be gaseous closer to the Sun remain solid because of the low temperature of the belt. Only a tiny number of objects have diameters and masses that can be used to calculate the density and rock–ice proportions. A high-resolution telescope like the Hubble Space Telescope, the period of an occultation when an object passes in front of a star, or the most widely used albedo of an object computed from its infrared emissions, can all be used to measure the diameter. This method relies on only a handful of binary objects to provide semi-major axes and periods for determining masses. The densities range from as low as 0.4 to 2.6 g/cm3 (grams per cubic centimeter). These items are assumed to be mostly ice and have a high degree of porosity, making them the least dense. Thick layers of ice and rock undoubtedly make up the densest items. When it comes to small objects, there is a trend between low density and high density. The ice loss could explain this pattern from the surface layers due to collisions between larger, differentiated objects.
Astronomers could only determine the essential characteristics of KBOs, such as their hue, due to the impossibility of doing in-depth analyses on them. These initial findings revealed various KBO colors, from neutral gray to deep red. As a result, a wide variety of substances, including filthy ice and hydrocarbons, might be found on their skin and other surfaces. Since most KBOs have lost most of their volatile ices to cosmic rays, astronomers had expected them to be uniformly black. Resurfacing by impacts or outgassing has been postulated as possible solutions to this disparity. The spectral analysis of known Kuiper belt objects by Jewitt and Luu in 2001 concluded that the color variation was too high to be explained by random impacts. Tholins are hypothesized to have been produced due to chemical changes to methane on the surface of KBOs caused by solar radiation. Ethane, ethylene, and acetylene are among the hydrocarbons generated from methane radiation processing that Makemake possesses.
Even though most KBOs remains spectrally featureless due to their obscurity, researchers have successfully figured out what makes them up. According to spectroscopic data collected in 1996 by Robert H. Brown et al., the surface composition of KBO 1993 SC resembles that of Pluto and Neptune's moon Triton, which have significant amounts of methane ice. Only colors and, in some cases, albedos have been identified for the smaller objects. Most of these objects are gray with low albedos or reddish-orange with high albedos. H2S retention or loss on these objects' surfaces is thought to be the cause of the variations in color and albedo, with irradiation reddening the surfaces of those that formed far enough from the Sun to retain H2S.
Large KBOs like Pluto and Quaoar have surfaces rich in volatile chemicals like methane, nitrogen, and carbon monoxide; these molecules are likely due to moderate vapor pressures in the 30–50 K temperature range of the Kuiper Belt. Compounds with lower boiling points can boil off their surfaces and fall back to the ground as snow, whereas those with higher boiling points would remain solid. Regarding the largest KBOs, the relative abundance of these three chemicals is determined by their surface gravity and ambient temperature. KBOs, including members of the Haumea family-like 1996 TO66, medium-sized objects like 38628 Huya and 20000 Varuna, and minor things have been found to have water ice. For example, 50000 Quaoar's crystalline ice may indicate that ancient tectonic action was helped along by the presence of ammonia to lower the melting point.
Despite its enormous size, the Kuiper belt has a small mass, with the hot population accounting for the equivalent of less than 1% of the Earth's mass. However, it is considered that the dynamically cold population was produced at its current location. In contrast, the dynamically hot population is thought to be the remnant of a much bigger population distributed outward during the migration of the massive planets. Based on its influence on the orbits of the planets, the Kuiper belt is estimated to be (1.97–0.30)102 Earth masses.
Because KBOs larger than 100 km (62 miles) in diameter require a substantial mass for accretion, the dynamically cold population's overall mass offers some difficulties for models of the Solar System's creation. Because of the low density of the cold classical Kuiper belt, these big objects could not have been created by the collision and mergers of smaller planetesimals. Furthermore, due to the "violent" nature of present orbital interactions, destruction rather than accretion results from their eccentricity and inclination. It is considered improbable that a significant portion of the dynamically cold population will be exterminated. To explain such a vast "vacuuming," Neptune's current influence is too weak. Collisional grinding can only lose so much mass before it disrupts loosely bound binaries in the cold disk. The larger object may have formed due to the collapse of pebble clouds rather than colliding with tiny planetesimals. According to early estimates based on observations of apparent magnitude distribution, there are 8 (=23) times as many objects in the 100–200 km range as there are in the 200–400 km range.
Recent studies have discovered that the size distributions of hot classical and cold classical objects differ. Q = 5.3 for big diameter hot objects, and q = 2.0 for small diameter hot objects, with the change in slope occurring at 110 kilometers. There is a slope of 8.2 for large objects and 2.9 for smaller ones, which changes at a distance of 140 kilometers. Similar to other dynamically heated populations, the size distributions of scattering objects, Plutono satellites, and Neptune trojans have slopes. Still, the number of objects below some given size may have a divot. This divot is thought to have originated as a result of the population forming without any items smaller than this, with the smaller objects being fragments of the original objects, as a result of the population evolving through collisions.
A telescope like the Hubble Space Telescope would be unable to observe the tiniest known Kuiper belt objects with radii below one kilometer because they are too dark (magnitude 35) to be seen directly. In December 2009, Schlichting et al. announced the discovery of a tiny, sub-kilometer-radius Kuiper belt object in archival Hubble photometry from March 2007. One of the Hubble Space Telescope's star-tracking systems picked up on an object with an estimated radius of about 500 meters, or 1040 meters in diameter. According to Schlichting et al., following a study published in December 2012, they discovered an additional occultation event by an object in the Kuiper belt that measured 53070 m in radius or 1060140 m in diameter. To account for these two years of occultation data, Schlichting et al. used assumptions including a single power law and uniform ecliptic latitude distribution to derive a slope for the Kuiper belt object size distribution of about 3.66 +/- 0.2 or 3.78 +/- 0.2. Kuiper belt objects less than one kilometer in diameter are scarce compared to the population of bigger Kuiper belt objects with dimensions greater than 90 kilometers.
The Kuiper belt overlaps with the dispersed disc, which extends beyond 100 AU, but is sparsely populated. As a result of their highly eccentric orbits and often steep inclinations toward the ecliptic, scattered disc objects (SDOs) have extremely elliptical paths. Most models of the birth of the Solar System show the formation of both KBOs and SDOs in a primordial belt, with later gravitational interactions, particularly with Neptune, sending the objects outward, some into stable orbits (the KBOs) and some into unstable orbits (the scattered disc). The Solar System's short-period comets are likely to have their origins in its scattered disc. They can be forced into the inner Solar System from time to time by their dynamic orbits, causing them first to become centaurs and subsequently short-period comets.
Any object that orbits solely within the Kuiper belt region, regardless of origin or composition, is a KBO, according to the Minor Planet Center, the official cataloger of all trans-Neptunian objects. Scattered objects are those found outside of the belt. It has become commonplace in some scientific circles to use the term "Kuiper belt object" to refer to an icy minor planet from the outer Solar System, even if its orbit during the majority of Solar System history has been outside of the Kuiper belt (e.g., in the scattered-disc region). "scattered Kuiper belt objects" is frequently used to refer to scattered disc objects. Even though it's more massive than Pluto, Eris is an SDO, not a KBO. However, scientists have yet to agree on the precise definition of the Kuiper belt, and thus this debate continues.
One difference between centaurs and other scattered objects is that centaurs are thought to be dispersed inward rather than outward from the Kuiper belt. The Minor Planet Center groups the SDOs and centaurs as dispersed objects.
Neptune is assumed to have snagged a huge KBO, Triton, during its migration era, which is the only major moon in the Solar System with a retrograde orbit. In contrast to the large moons of Jupiter, Saturn, and Uranus, which are thought to have formed from rotating material discs around their young parent planets, this suggests that Triton was a fully developed body captured from the surrounding space by its parent planet. For an object to be captured by a larger object's gravity, it must first be slowed down enough. If Triton was part of a binary when encountering Neptune, this could be why. A lot of KBOs are part of binaries.) See below for more information.) A possible explanation for Triton's capture could be that Neptune ejected the other component of its pair from the system. According to the spectral analysis of both worlds (Triton is only 14 percent larger than Pluto), the surface of Triton and Pluto are made up primarily of methane and carbon monoxide. In light of this evidence, it appears that Triton was previously a KBO that Neptune captured as it was on its way out of the solar system.
Several KBOs with diameters ranging from 500 to 1,500 kilometers (932 miles), or about half the size of Pluto (2370 kilometers in diameter), have been identified since 2000. Known as 50000 Quaoar, this KBO was discovered in 2002 and is more than 1,200 kilometers in circumference. Makemake and Haumea disclosed on July 29, 2005, are far larger than before. These include 28978 Ixion (found in 2001) and 20000 Varuna (discovered in 2000), which are both approximately 600–700 km (373–435 miles) across.
Many people concluded that Pluto wasn't all that different from the other Kuiper belt objects after finding these massive KBOs in orbits similar to Pluto's. In addition to their size resemblance to Pluto's, many of these objects have satellites and share a chemical composition with Pluto (methane and carbon monoxide have been found on Pluto and the largest KBOs). In the same way, Pluto had once been deemed a planet before its asteroids were discovered, some scientists now believe that Pluto should be reclassified as an asteroid instead.
The discovery of Eris, a dwarf-planet in the scattered disc outside the Kuiper belt now known to be 27 percent more massive than Pluto, brought the issue to the prominence of the scientific community. Eris was discovered in the scattered disc. (The New Horizons mission found that Eris is not greater in volume than Pluto, contrary to popular belief.) When the International Astronomical Union (IAU) was compelled to define what a planet was for the first time, they added the requirement that a planet must "clean the neighborhood surrounding its orbit" into their definition. Because many other massive bodies share Pluto's orbit, it was determined that Pluto had not cleared its orbit and was reclassified as a dwarf planet, putting it in the Kuiper belt.
Even though Pluto is currently the largest known KBO, at least one even larger object outside the Kuiper belt is known to have originated in it: Neptune's moon Triton, which is probably a captured KBO.
No one knows how many KBOs are massive enough to qualify as dwarf planets in our classification system. Many dwarf-planet candidates have unexpectedly low densities, which suggests that not many of them are. All of these bodies have been recognized by the scientific community except for the additions of Salacia, 2002 MS4, 2002 AW197, and Ixion, which a small group of astronomers suggested.
One or more satellites have been discovered on all six of the largest TNOs (Eris, Pluto Gonggong Makemake Hauumea, and Quaoar). The presence of satellites on a greater proportion of the Kuiper belt's larger KBOs than on the belt's smaller objects suggests a different development model. As a result, the Kuiper belt has many binaries (two objects near enough in mass to orbit one another). Pluto- Charon is the most well-known binary KBO, but roughly 11% of KBOs are thought to occur in pairs.
Launched on January 19, 2006, New Horizons, the first spacecraft to investigate the Kuiper belt, passed Pluto on July 14, 2015. Further exploration of Kuiper belt Kuiper objects was the mission's purpose, beyond the flyby of Pluto.
The New Horizons team started on October 15, 2014, when Hubble discovered three prospective targets, provisionally dubbed PT1, PT2, and PT3, by Hubble. At 43–44 AU distances from the Sun, the objects are too small to be seen by ground telescopes, but the encounters are expected to occur in 2018–2019. The earliest estimates of the reachability of these objects within the fuel budget of New Horizons were 100%, 7%, and 97%, respectively. Because they belonged to the classical Kuiper belt's "cool" (low-inclination, low-eccentricity) components, each of these dwarf planets was distinct from Pluto. The most advantageously located object, PT1, was discovered in January 2019 at a magnitude of 26.8 and a diameter of 30–45 kilometers (hence the temporary designation "1110113Y" on the HST website). Minor Planet Center designated the three KBOs as 2014 MU69 (PT1), 2014 OS393 (PT2), and 2014 PN70 once sufficient orbital data was provided (PT3). Follow-up observations had by that time eliminated the possibility of a further target, 2014 MT69. Before the Pluto flyby, PT2 was out of the race.
The first target of New Horizons, 2014 MU69 (nicknamed "Ultima Thule" and later renamed 486958 Arrokoth), was selected on August 26, 2015. A course correction was made in late October and early November of 2015, resulting in a flyby in January 2019. Additional funds were approved by NASA on July 1, 2016, for New Horizons' visit to the object.
On December 2, 2015, the New Horizons spacecraft spotted the 1994 JR1 (later renamed 15810 Arawn).
New Horizons flew by Arrokoth on January 1, 2019, and returned data showing that Arrokoth is a 32 km long by 16 km wide contact binary. The Ralph instrument confirmed Arrokoth's red color on New Horizons. Over the next two decades, the fly-data by's will be downloaded continuously.
Currently, there are no plans for a follow-up mission to New Horizons. However, at least two concepts for missions that might return to orbit or land on Pluto have been studied. Makemake and Haumea, two of the largest KBOs beyond Pluto, cannot be visited by New Horizons. This would necessitate the creation of new missions to investigate these things in greater depth. Haumea is an important scientific target because it is the parent body of a collisional family of TNOs, including several others, in addition to its ring and two moons. Thales Alenia Space, an aerospace company, has studied the logistics of an orbiter mission to Haumea. According to the lead author, Joel Poncy, spacecraft should be able to reach and orbit KBOs in 10–20 years or less. For the first time since Voyager 2's flybys of Uranus and Neptune in the 1980s, New Horizons Principal Investigator Alan Stern has proposed missions that fly by these ice giant planets before visiting new KBO destinations.
It has been suggested that because it is so close to the heliotrope, Quaoar could serve as a flyby target for a probe sent to investigate the interstellar medium. Johns Hopkins Applied Physics Laboratory researcher Pontus Brandt and his colleagues studied the feasibility of sending such a probe to fly by Quaoar in the 2030s before sending it to the interstellar medium via the heliotrope. Its possibly evaporating methane atmosphere and cryovolcanism are just two of their reasons for being interested in Quaoar. Using a flyby of Jupiter, Brandt and his colleagues explored a mission that would launch from the Space Launch System (SLS) and reach 30 kilometers per second. Additionally, a study published in 2012 concluded that Ixion and Huya are among the most viable targets for an orbiter mission. For example, if an orbiter mission starts in 2039, it will arrive at Ixion after a 17-year cruise.
For Kuiper belt objects, a design study by Glen Costigan and his colleagues in the late 2010s considered the possibility of orbital capture or multiple-target scenarios. 2002 UX25, 1998 WW31, and 47171 Lempo were the Kuiper belt objects examined in that paper. Ryan McGranaghan and colleagues conducted another design study in 2011 to investigate sending a space probe to survey the large trans-Neptunian objects Quaoar, Sedna, Makemake, Haumea, and Eris.
Scientists were able to identify Kuiper belt-like structures around a total of nine stars other than the Sun by 2006. Belts with a radius of more than 50 AU appear large, while belts with radii between 20 and 30 AU appear narrow (similar to the Solar System) and have very sharp boundaries. There is also evidence for massive Kuiper-belt-like structures in the infrared emissions from 15–20% of solar-type stars. Most known debris disks around other stars are only a few years old.