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86-561: A trans-Neptunian object ( TNO ), also written transneptunian object , is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune , which has an orbital semi-major axis of 30.1 astronomical units (AU). Typically, TNOs are further divided into the classical and resonant objects of the Kuiper belt , the scattered disc and detached objects with

172-528: A brown dwarf has been often postulated for different theoretical reasons to explain several observed or speculated features of the Kuiper belt and the Oort cloud . It was recently proposed to use ranging data from the New Horizons spacecraft to constrain the position of such a hypothesized body. NASA has been working towards a dedicated Interstellar Precursor in the 21st century, one intentionally designed to reach

258-478: A centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because of this; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela , which may be in a stable (though retrograde) orbit . Centaurs typically exhibit

344-509: A 'planet' at the time and an 'asteroid' soon after; the term minor planet was not introduced until 1841, and was considered a subcategory of 'planet' until 1932. The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planets since 2006. Historically, the terms asteroid , minor planet , and planetoid have been more or less synonymous. This terminology has become more complicated by

430-446: A Data Base of Physical and Dynamical Properties of Near Earth Asteroids. Environmental characteristics have three aspects: space environment, surface environment and internal environment, including geological, optical, thermal and radiological environmental properties, etc., which are the basis for understanding the basic properties of minor planets, carrying out scientific research, and are also an important reference basis for designing

516-442: A centaur by JPL, Hidalgo ( q = 1.95 AU ; T J = 2.07 ) would also change category to a Jupiter-family comet. Schwassmann-Wachmann 1 ( q = 5.72 AU ; T J = 2.99 ) has been categorized as both a centaur and a Jupiter-family comet depending on the definition used. Other objects caught between these differences in classification methods include (44594) 1999 OX 3 , which has a semi-major axis of 32 AU but crosses

602-547: A class of Kuiper belt object that display a similar bicoloured nature, and there are suggestions that not all plutinos' orbits are as stable as initially thought, due to perturbation by Pluto . Further developments are expected with more physical data on Kuiper belt objects. Some centaurs may have their origin in fragmentation episodes, perhaps triggered during close encounters with Jupiter. The orbits of centaurs 2020 MK4 , P/2008 CL94 (Lemmon), and P/2010 TO20 (LINEAR-Grauer) pass close to that of comet 29P/Schwassmann–Wachmann ,

688-479: A comet designation. Other centaurs, such as 52872 Okyrhoe , are suspected of having shown comas . Any centaur that is perturbed close enough to the Sun is expected to become a comet. A centaur has either a perihelion or a semi-major axis between those of the outer planets (between Jupiter and Neptune). Due to the inherent long-term instability of orbits in this region, even centaurs such as 2000 GM 137 and 2001 XZ 255 , which do not currently cross

774-408: A dozen known centaurs follow retrograde orbits. Their inclinations range from modest ( e.g ., 160° for Dioretsa ) to extreme ( i < 120° ; e.g . 105° for (342842) 2008 YB 3 ). Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin. Because the centaurs are not protected by orbital resonances , their orbits are unstable within

860-460: A false positive or become lost later on —called a provisionally designated minor planet . After the observation arc is accurate enough to predict its future location, a minor planet is formally designated and receives a number. It is then a numbered minor planet . Finally, in the third step, it may be named by its discoverers. However, only a small fraction of all minor planets have been named. The vast majority are either numbered or have still only

946-494: A flat spectrum, reflecting as much red and infrared as visible spectrum. Very red objects present a steep slope, reflecting much more in red and infrared. A recent attempt at classification (common with centaurs) uses the total of four classes from BB (blue, or neutral color, average B−V = 0.70, V−R = 0.39, e.g. Orcus ) to RR (very red, B−V = 1.08, V−R = 0.71, e.g. Sedna ) with BR and IR as intermediate classes. BR (intermediate blue-red) and IR (moderately red) differ mostly in

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1032-459: A long time, no one searched for other TNOs as it was generally believed that Pluto, which up to August 2006 was classified as a planet, was the only major object beyond Neptune. Only after the 1992 discovery of a second TNO, 15760 Albion , did systematic searches for further such objects begin. A broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in

1118-555: A name (e.g. 433 Eros ). The formal naming convention uses parentheses around the number, but dropping the parentheses is quite common. Informally, it is common to drop the number altogether or to drop it after the first mention when a name is repeated in running text. Minor planets that have been given a number but not a name keep their provisional designation, e.g. (29075) 1950 DA . Because modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The earliest discovered to be left unnamed

1204-406: A number of models of the surface. Water ice signatures have been confirmed on a number of centaurs (including 2060 Chiron , 10199 Chariklo and 5145 Pholus ). In addition to the water ice signature, a number of other models have been put forward: Chiron appears to be the most complex. The spectra observed vary depending on the period of the observation. Water ice signature was detected during

1290-517: A period of low activity and disappeared during high activity. Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma (a cloud of gas and dust evaporating from its surface). It is thus now officially classified as both a minor planet and a comet, although it is far larger than a typical comet and there is some lingering controversy. Other centaurs are being monitored for comet-like activity: so far two, 60558 Echeclus , and 166P/NEAT have shown such behavior. 166P/NEAT

1376-497: A potential correlation with other classes of objects, namely centaurs and some satellites of giant planets ( Triton , Phoebe ), suspected to originate in the Kuiper belt . However, the interpretations are typically ambiguous as the spectra can fit more than one model of the surface composition and depend on the unknown particle size. More significantly, the optical surfaces of small bodies are subject to modification by intense radiation, solar wind and micrometeorites . Consequently,

1462-427: A provisional designation. Example of the naming process: A newly discovered minor planet is given a provisional designation . For example, the provisional designation 2002 AT 4 consists of the year of discovery (2002) and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given

1548-465: A semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs). The orbit of each of the planets is slightly affected by the gravitational influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune . The search for these led to

1634-648: A somewhat larger surface soil layer size. Soil layers are inevitably subject to intense space weathering that alters their physical and chemical properties due to direct exposure to the surrounding space environment. In silicate-rich soils, the outer layers of Fe are reduced to nano-phase Fe (np-Fe), which is the main product of space weathering . For some small planets, their surfaces are more exposed as boulders of varying sizes, up to 100 metres in diameter, due to their weaker gravitational pull. These boulders are of high scientific interest, as they may be either deeply buried material excavated by impact action or fragments of

1720-1108: A sufficient distance to avoid significant gravitational perturbations from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an unknown planet on a distant orbit and a distant encounter with a random star or a member of the Sun's birth cluster that passed near the Solar System. Solar System   → Local Interstellar Cloud   → Local Bubble   → Gould Belt   → Orion Arm   → Milky Way   → Milky Way subgroup   → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster   → Local Hole   → Observable universe   → Universe Each arrow ( → ) may be read as "within" or "part of". Minor planet Minor planets include asteroids ( near-Earth objects , Earth trojans , Mars trojans , Mars-crossers , main-belt asteroids and Jupiter trojans ), as well as distant minor planets ( Uranus trojans , Neptune trojans , centaurs and trans-Neptunian objects ), most of which reside in

1806-509: A timescale of 10 –10  years. For example, 55576 Amycus is in an unstable orbit near the 3:4 resonance of Uranus. Dynamical studies of their orbits indicate that being a centaur is probably an intermediate orbital state of objects transitioning from the Kuiper belt to the Jupiter family of short-period comets . (679997) 2023 RB will have its orbit notably changed by a close approach to Saturn in 2201. Objects may be perturbed from

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1892-433: A variety of other rich geological effects on the surface of minor planets, such as mass wasting on slopes and impact crater walls, large-scale linear features associated with graben , and electrostatic transport of dust. By analysing the various geological processes on the surface of minor planets, it is possible to learn about the possible internal activity at this stage and some of the key evolutionary information about

1978-421: Is confirmed that their orbits cannot be explained by perturbations from the giant planets , nor by interaction with the galactic tides . However, a passing star could have moved them on their orbit. Given the apparent magnitude (>20) of all but the biggest trans-Neptunian objects, the physical studies are limited to the following: Studying colours and spectra provides insight into the objects' origin and

2064-446: Is currently inactive and was seen to be active only before it was perturbed into a centaur orbit by Jupiter in 1963. The faint comet 38P/Stephan–Oterma would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5 AU. By the year 2200, comet 78P/Gehrels will probably migrate outwards into a centaur-like orbit. A periodogram analysis of the light-curves of these Chiron and Chariklo gives respectively

2150-548: Is evidence that Saturn 's moon Phoebe , imaged by the Cassini probe in 2004, may be a captured centaur that originated in the Kuiper belt . In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus . Ceres may have originated in the region of the outer planets, and if so might be considered an ex-centaur, but the centaurs seen today all originated elsewhere. Of

2236-506: Is similar to that of carbon- or iron-bearing meteorites, the interaction between the minor planets and the solar wind is likely to be unipolar induction , resulting in an external magnetic field for the minor planet. In addition, the magnetic fields of minor planets are not static; impact events, weathering in space and changes in the thermal environment can alter the existing magnetic fields of minor planets. At present, there are not many direct observations of minor planet magnetic fields, and

2322-414: Is that the Sun emits almost all of its energy in visible light and at nearby frequencies, while at the cold temperatures of TNOs, the heat radiation is emitted at completely different wavelengths (the far infrared). Thus there are two unknowns (albedo and size), which can be determined by two independent measurements (of the amount of reflected light and emitted infrared heat radiation). TNOs are so far from

2408-795: The International Astronomical Union is dedicated to the Physical Study of Comets & Minor Planets. Archival data on the physical properties of comets and minor planets are found in the PDS Asteroid/Dust Archive. This includes standard asteroid physical characteristics such as the properties of binary systems, occultation timings and diameters, masses, densities, rotation periods, surface temperatures, albedoes, spin vectors, taxonomy, and absolute magnitudes and slopes. In addition, European Asteroid Research Node (E.A.R.N.), an association of asteroid research groups, maintains

2494-402: The Kuiper belt and the scattered disc . As of October 2024 , there are 1,392,085 known objects, divided into 740,000 numbered , with only one of them recognized as a dwarf planet (secured discoveries) and 652,085 unnumbered minor planets, with only five of those officially recognized as a dwarf planet . The first minor planet to be discovered was Ceres in 1801, though it was called

2580-450: The Kuiper belt objects (KBOs) and the scattered disc objects (SDOs). The diagram to the right illustrates the distribution of known trans-Neptunian objects (up to 70 au) in relation to the orbits of the planets and the centaurs for reference. Different classes are represented in different colours. Resonant objects (including Neptune trojans ) are plotted in red, classical Kuiper belt objects in blue. The scattered disc extends to

2666-631: The discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptune's mass from the Voyager 2 flyby in 1989 showed that the problem was spurious. Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has a lower inclination to the ecliptic than most other large TNOs. After Pluto's discovery, American astronomer Clyde Tombaugh continued searching for some years for similar objects but found none. For

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2752-519: The eccentricity of the orbits is represented by red segments (extending from perihelion to aphelion). The orbits of centaurs show a wide range of eccentricity, from highly eccentric ( Pholus , Asbolus , Amycus , Nessus ) to more circular ( Chariklo and the Saturn-crossers Thereus and Okyrhoe ). To illustrate the range of the orbits' parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow : Over

2838-408: The invariable plane regroups mostly small and dim objects. It is difficult to estimate the diameter of TNOs. For very large objects, with very well known orbital elements (like Pluto), diameters can be precisely measured by occultation of stars. For other large TNOs, diameters can be estimated by thermal measurements. The intensity of light illuminating the object is known (from its distance to

2924-440: The resonant trans-Neptunian object that are locked in an orbital resonance with Neptune , and the classical Kuiper belt objects , also called "cubewanos", that have no such resonance, moving on almost circular orbits, unperturbed by Neptune. There are a large number of resonant subgroups, the largest being the twotinos (1:2 resonance) and the plutinos (2:3 resonance), named after their most prominent member, Pluto . Members of

3010-463: The sednoids being the most distant ones. As of July 2024, the catalog of minor planets contains 901 numbered and more than 3,000 unnumbered TNOs . however, nearly 5000 objects with semimajor axis over 30 AU are present in the MPC catalog, with 1000 being numbered. The first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting

3096-454: The Kuiper belt, so that surface transformation processes have not yet taken place. Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions. The interpretation of spectra is often ambiguous, related to particle sizes and other factors, but the spectra offer an insight into surface composition. As with the colours, the observed spectra can fit

3182-416: The Kuiper belt, whereupon they become Neptune -crossing and interact gravitationally with that planet (see theories of origin ). They then become classed as centaurs, but their orbits are chaotic, evolving relatively rapidly as the centaur makes repeated close approaches to one or more of the outer planets. Some centaurs will evolve into Jupiter-crossing orbits whereupon their perihelia may become reduced into

3268-621: The Pluto system in July 2015 and 486958 Arrokoth in January 2019. In 2011, a design study explored a spacecraft survey of Quaoar, Sedna, Makemake, Haumea, and Eris. In 2019 one mission to TNOs included designs for orbital capture and multi-target scenarios. Some TNOs that were studied in a design study paper were 2002 UX 25 , 1998 WW 31 , and Lempo . The existence of planets beyond Neptune , ranging from less than an Earth mass ( Sub-Earth ) up to

3354-481: The Sun directly, 15760 Albion . The most massive TNO known is Eris , followed by Pluto , Haumea , Makemake , and Gonggong . More than 80 satellites have been discovered in orbit of trans-Neptunian objects. TNOs vary in color and are either grey-blue (BB) or very red (RR). They are thought to be composed of mixtures of rock, amorphous carbon and volatile ices such as water and methane , coated with tholins and other organic compounds. Twelve minor planets with

3440-512: The Sun that they are very cold, hence producing black-body radiation around 60 micrometres in wavelength . This wavelength of light is impossible to observe on the Earth's surface, but only from space using, e.g. the Spitzer Space Telescope . For ground-based observations, astronomers observe the tail of the black-body radiation in the far infrared. This far infrared radiation is so dim that

3526-469: The Sun), and one assumes that most of its surface is in thermal equilibrium (usually not a bad assumption for an airless body). For a known albedo , it is possible to estimate the surface temperature, and correspondingly the intensity of heat radiation. Further, if the size of the object is known, it is possible to predict both the amount of visible light and emitted heat radiation reaching Earth. A simplifying factor

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3612-498: The Sun, making the boundary blurred (see 2060 Chiron and 7968 Elst–Pizarro ) . However, population comparisons between centaurs and TNOs are still controversial. Colour indices are simple measures of the differences in the apparent magnitude of an object seen through blue (B), visible (V), i.e. green-yellow, and red (R) filters. The diagram illustrates known colour indices for all but the biggest objects (in slightly enhanced colour). For reference, two moons, Triton and Phoebe ,

3698-405: The binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy. Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown 's website include 10199 Chariklo , (523727) 2014 NW 65 and 2060 Chiron . The diagram illustrates the orbits of known centaurs in relation to the orbits of the planets. For selected objects,

3784-482: The centaur Pholus and the planet Mars are plotted (yellow labels, size not to scale) . Correlations between the colours and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes: While the relatively dimmer bodies, as well as the population as the whole, are reddish (V−I = 0.3–0.6), the bigger objects are often more neutral in colour (infrared index V−I < 0.2). This distinction leads to suggestion that

3870-461: The centaurs that become Jupiter-family comets. Four objects are known to occupy this region, including 29P/Schwassmann-Wachmann , P/2010 TO20 LINEAR-Grauer , P/2008 CL94 Lemmon , and 2016 LN8, but the simulations indicate that there may of order 1000 more objects >1 km in radius that have yet to be detected. Objects in this gateway region can display significant activity and are in an important evolutionary transition state that further blurs

3956-437: The characteristics of both asteroids and comets . They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000. The first centaur to be discovered, under

4042-493: The classical Edgeworth–Kuiper belt include 15760 Albion , Quaoar and Makemake . Another subclass of Kuiper belt objects is the so-called scattering objects (SO). These are non-resonant objects that come near enough to Neptune to have their orbits changed from time to time (such as causing changes in semi-major axis of at least 1.5 AU in 10 million years) and are thus undergoing gravitational scattering . Scattering objects are easier to detect than other trans-Neptunian objects of

4128-458: The convection of the conductive fluid will generate a large and strong magnetic field . However, the size of a minor planet is generally small and most of the minor planets have a "crushed stone pile" structure, and there is basically no "dynamo" structure inside, so it will not generate a self-generated dipole magnetic field like the Earth. But some minor planets do have magnetic fields—on the one hand, some minor planets have remanent magnetism : if

4214-539: The definition of the Jet Propulsion Laboratory and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo , which at 260 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings . It was discovered in 1997. No centaur has been photographed up close, although there

4300-533: The derived CO production rate was calculated to be sufficient to account for the observed coma. The calculated CO production rate from both 60558 Echeclus and Chiron is substantially lower than what is typically observed for 29P/Schwassmann–Wachmann , another distantly active comet often classified as a centaur. There is no clear orbital distinction between centaurs and comets. Both 29P/Schwassmann-Wachmann and 39P/Oterma have been referred to as centaurs since they have typical centaur orbits. The comet 39P/Oterma

4386-458: The discovery of numerous minor planets beyond the orbit of Jupiter , especially trans-Neptunian objects that are generally not considered asteroids. A minor planet seen releasing gas may be dually classified as a comet. Objects are called dwarf planets if their own gravity is sufficient to achieve hydrostatic equilibrium and form an ellipsoidal shape. All other minor planets and comets are called small Solar System bodies . The IAU stated that

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4472-559: The distinction between the centaur and Jupiter-family comet populations. The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on any side of the debate. Instead, it has adopted the following naming convention for such objects: Befitting their centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only

4558-399: The fact that most minor planets are rubble pile structures, which are loose and porous, gives the impact action on the surface of minor planets its unique characteristics. On highly porous minor planets, small impact events produce spatter blankets similar to common impact events: whereas large impact events are dominated by compaction and spatter blankets are difficult to form, and the longer

4644-703: The few existing planets detection projects generally carry magnetometers, with some targets such as Gaspra and Braille measured to have strong magnetic fields nearby, while others such as Lutetia have no magnetic field. Solar System   → Local Interstellar Cloud   → Local Bubble   → Gould Belt   → Orion Arm   → Milky Way   → Milky Way subgroup   → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster   → Local Hole   → Observable universe   → Universe Each arrow ( → ) may be read as "within" or "part of". Centaur (small Solar System body) In planetary astronomy ,

4730-452: The following rotational periods: 5.5±0.4~h and 7.0± 0.6~h. Centaurs can reach diameters up to hundreds of kilometers. The largest centaurs have diameters in excess of 300 km, and primarily reside beyond 20 AU . The study of centaurs’ origins is rich in recent developments, but any conclusions are still hampered by limited physical data. Different models have been put forward for possible origin of centaurs. Simulations indicate that

4816-437: The infrared bands I, J and H . Typical models of the surface include water ice, amorphous carbon , silicates and organic macromolecules, named tholins , created by intense radiation. Four major tholins are used to fit the reddening slope: As an illustration of the two extreme classes BB and RR, the following compositions have been suggested Characteristically, big (bright) objects are typically on inclined orbits, whereas

4902-596: The inner Solar System and they may be reclassified as active comets in the Jupiter family if they display cometary activity. Centaurs will thus ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly Jupiter . Compared to dwarf planets and asteroids, the relatively small size and distance of centaurs precludes remote observation of surfaces, but colour indices and spectra can provide clues about surface composition and insight into

4988-690: The interstellar medium, and as part of this the flyby of objects like Sedna are also considered. Overall this type of spacecraft studies have proposed a launch in the 2020s, and would try to go a little faster than the Voyagers using existing technology. One 2018 design study for an Interstellar Precursor, included a visit of minor planet 50000 Quaoar, in the 2030s. Among the extreme trans-Neptunian objects are three high-perihelion objects classified as sednoids : 90377 Sedna , 2012 VP 113 , and 541132 Leleākūhonua . They are distant detached objects with perihelia greater than 70 au. Their high perihelia keep them at

5074-469: The long-term interaction with the external environment, which may lead to some indication of the nature of the parent body's origin. Many of the larger planets are often covered by a layer of soil ( regolith ) of unknown thickness. Compared to other atmosphere-free bodies in the solar system (e.g. the Moon ), minor planets have weaker gravity fields and are less capable of retaining fine-grained material, resulting in

5160-431: The lowest-numbered unnamed minor planet is (4596) 1981 QB , and the highest-numbered named minor planet is 594913 ꞌAylóꞌchaxnim . There are various broad minor-planet populations: All astronomical bodies in the Solar System need a distinct designation. The naming of minor planets runs through a three-step process. First, a provisional designation is given upon discovery—because the object still may turn out to be

5246-420: The minor planet exploration mission, measuring the albedo and color changes of the planet surface is also the most basic method to directly know the difference in the material composition of the planet surface. The geological environment on the surface of minor planets is similar to that of other unprotected celestial bodies, with the most widespread geomorphological feature present being impact craters: however,

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5332-404: The objects known to occupy centaur-like orbits, approximately 30 have been found to display comet-like dust comas , with three, 2060 Chiron , 60558 Echeclus , and 29P/Schwassmann-Wachmann 1, having detectable levels of volatile production in orbits entirely beyond Jupiter. Chiron and Echeclus are therefore classified as both centaurs and comets, while Schwassmann-Wachmann 1 has always held

5418-806: The orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets. Some astronomers count only bodies with semimajor axes in the region of the outer planets to be centaurs; others accept any body with a perihelion in the region, as their orbits are similarly unstable. However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements : The Gladman & Marsden (2008) criteria would make some objects Jupiter-family comets: Both Echeclus ( q = 5.8 AU , T J = 3.03 ) and Okyrhoe ( q = 5.8 AU ; T J = 2.95 ) have traditionally been classified as centaurs. Traditionally considered an asteroid, but classified as

5504-416: The orbit of some Kuiper belt objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. Scattered disc objects would be dynamically the best candidates (For instance, the centaurs could be part of an "inner" scattered disc of objects perturbed inwards from the Kuiper belt.) for such expulsions, but their colours do not fit the bicoloured nature of the centaurs. Plutinos are

5590-533: The orbits of both Uranus and Neptune. It is listed as an outer centaur by the Deep Ecliptic Survey (DES). Among the inner centaurs, (434620) 2005 VD , with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES. A recent orbital simulation of the evolution of Kuiper Belt Objects through the centaur region has identified a short-lived " orbital gateway " between 5.4 and 7.8 AU through which 21% of all centaurs pass, including 72% of

5676-718: The origin of the bodies. The colours of centaurs are very diverse, which challenges any simple model of surface composition. In the side-diagram, the colour indices are measures of apparent magnitude of an object through blue (B), visible (V) (i.e. green-yellow) and red (R) filters. The diagram illustrates these differences (in exaggerated colours) for all centaurs with known colour indices. For reference, two moons: Triton and Phoebe , and planet Mars are plotted (yellow labels, size not to scale). Centaurs appear to be grouped into two classes: There are numerous theories to explain this colour difference, but they can be broadly divided into two categories: As examples of

5762-462: The parent body had a magnetic field or if the nearby planetary body has a strong magnetic field, the rocks on the parent body will be magnetised during the cooling process and the planet formed by the fission of the parent body will still retain remanence, which can also be detected in extraterrestrial meteorites from the minor planets; on the other hand, if the minor planets are composed of electrically conductive material and their internal conductivity

5848-428: The payload of exploration missions Without the protection of an atmosphere and its own strong magnetic field, the minor planet's surface is directly exposed to the surrounding radiation environment. In the cosmic space where minor planets are located, the radiation on the surface of the planets can be divided into two categories according to their sources: one comes from the sun, including electromagnetic radiation from

5934-444: The periodic change of the planet's light curve, which can be observed by ground-based equipment, so as to obtain the planet's magnitude , rotation period , rotation axis orientation, shape, albedo distribution, and scattering properties. Generally speaking, the albedo of minor planets is usually low, and the overall statistical distribution is bimodal, corresponding to C-type (average 0.035) and S-type (average 0.15) minor planets. In

6020-399: The planet's parent body that have survived. The rocks provide more direct and primitive information about the material inside the minor planet and the nature of its parent body than the soil layer, and the different colours and forms of the rocks indicate different sources of material on the surface of the minor planet or different evolutionary processes. Usually in the interior of the planet,

6106-564: The planets receive such large impacts, the greater the overall density. In addition, statistical analysis of impact craters is an important means of obtaining information on the age of a planet surface. Although the Crater Size-Frequency Distribution (CSFD) method of dating commonly used on minor planet surfaces does not allow absolute ages to be obtained, it can be used to determine the relative ages of different geological bodies for comparison. In addition to impact, there are

6192-487: The publication of the Minor Planet Circular (MPC) of October 19, 2005, which saw the highest-numbered minor planet jump from 99947 to 118161. The first few asteroids were named after figures from Greek and Roman mythology , but as such names started to dwindle the names of famous people, literary characters, discoverers' spouses, children, colleagues, and even television characters were used. Commission 15 of

6278-473: The range of 50 to 2,500 kilometers. Eris , the most massive TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, and whether objects like Pluto can be considered planets. Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union . According to their distance from the Sun and their orbital parameters , TNOs are classified in two large groups:

6364-413: The right, far beyond the diagram, with known objects at mean distances beyond 500 au ( Sedna ) and aphelia beyond 1,000  ( (87269) 2000 OO 67 ). The Edgeworth– Kuiper belt contains objects with an average distance to the Sun of 30 to about 55 au, usually having close-to-circular orbits with a small inclination from the ecliptic . Edgeworth–Kuiper belt objects are further classified into

6450-430: The same size because they come nearer to Earth, some having perihelia around 20 AU. Several are known with g-band absolute magnitude below 9, meaning that the estimated diameter is more than 100 km. It is estimated that there are between 240,000 and 830,000 scattering objects bigger than r-band absolute magnitude 12, corresponding to diameters greater than about 18 km. Scattering objects are hypothesized to be

6536-417: The scattered disc can be further divided into the "typical" scattered disc objects (SDOs, Scattered-near) with a T N of less than 3, and into the detached objects (ESDOs, Scattered-extended) with a T N greater than 3. In addition, detached objects have a time-averaged eccentricity greater than 0.2 The Sednoids are a further extreme sub-grouping of the detached objects with perihelia so distant that it

6622-449: The second category, the reddish colour of Pholus has been explained as a possible mantle of irradiated red organics, whereas Chiron has instead had its ice exposed due to its periodic cometary activity, giving it a blue/grey index. The correlation with activity and color is not certain, however, as the active centaurs span the range of colors from blue (Chiron) to red (166P/NEAT). Alternatively, Pholus may have been only recently expelled from

6708-456: The source of the so-called Jupiter-family comets (JFCs), which have periods of less than 20 years. The scattered disc contains objects farther from the Sun, with very eccentric and inclined orbits. These orbits are non-resonant and non-planetary-orbit-crossing. A typical example is the most-massive-known TNO, Eris . Based on the Tisserand parameter relative to Neptune (T N ), the objects in

6794-400: The sun, and ionizing radiation from the solar wind and solar energy particles; the other comes from the sun outside the solar system, that is, galactic cosmic rays , etc. Usually during one rotation period of a minor planet, the albedo of a minor planet will change slightly due to its irregular shape and uneven distribution of material composition. This small change will be reflected in

6880-431: The surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath. Among TNOs, as among centaurs , there is a wide range of colors from blue-grey (neutral) to very red, but unlike the centaurs, bimodally grouped into grey and red centaurs, the distribution for TNOs appears to be uniform. The wide range of spectra differ in reflectivity in visible red and near infrared. Neutral objects present

6966-619: The term minor planet may still be used, but the term small Solar System body will be preferred. However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still used. Hundreds of thousands of minor planets have been discovered within the Solar System and thousands more are discovered each month. The Minor Planet Center has documented over 213 million observations and 794,832 minor planets, of which 541,128 have orbits known well enough to be assigned permanent official numbers . Of these, 21,922 have official names. As of 8 November 2021 ,

7052-439: The thermal method is only applicable to the largest KBOs. For the majority of (small) objects, the diameter is estimated by assuming an albedo. However, the albedos found range from 0.50 down to 0.05, resulting in a size range of 1,200–3,700 km for an object of magnitude of 1.0. The only mission to date that primarily targeted a trans-Neptunian object was NASA's New Horizons , which was launched in January 2006 and flew by

7138-616: The thin optical surface layer could be quite different from the regolith underneath, and not representative of the bulk composition of the body. Small TNOs are thought to be low-density mixtures of rock and ice with some organic ( carbon -containing) surface material such as tholins , detected in their spectra. On the other hand, the high density of Haumea , 2.6–3.3 g/cm, suggests a very high non-ice content (compare with Pluto 's density: 1.86 g/cm). The composition of some small TNOs could be similar to that of comets . Indeed, some centaurs undergo seasonal changes when they approach

7224-509: Was discovered while it exhibited a coma, and so is classified as a comet, though its orbit is that of a centaur. 60558 Echeclus was discovered without a coma but recently became active, and so it too is now classified as both a comet and an asteroid. Overall, there are ~30 centaurs for which activity has been detected, with the active population biased toward objects with smaller perihelion distances. Carbon monoxide has been detected in 60558 Echeclus and Chiron in very small amounts, and

7310-411: Was finally named 15760 Albion in January 2018. A few objects are cross-listed as both comets and asteroids, such as 4015 Wilson–Harrington , which is also listed as 107P/Wilson–Harrington . Minor planets are awarded an official number once their orbits are confirmed. With the increasing rapidity of discovery, these are now six-figure numbers. The switch from five figures to six figures arrived with

7396-517: Was for a long time (3360) 1981 VA , now 3360 Syrinx . In November 2006 its position as the lowest-numbered unnamed asteroid passed to (3708) 1974 FV 1 (now 3708 Socus ), and in May 2021 to (4596) 1981 QB . On rare occasions, a small object's provisional designation may become used as a name in itself: the then-unnamed (15760) 1992 QB 1 gave its "name" to a group of objects that became known as classical Kuiper belt objects ("cubewanos") before it

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