Trans-Neptunian object

Discovery of Pluto , the first known TNO, imaged by New Horizons in 2015. 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 the discovery of Pluto in February 1930, which was progressively determined to be too small to explain the discrepancies. Revised estimates of Neptune's mass from the Voyager 2 flyby in 1989 showed that there is no real discrepancy: The problem was an error in the expectations for the orbits. Pluto was easiest to find because it is the brightest of all known trans-Neptunian objects. It also has a lower inclination to the ecliptic than most other large TNOs, so its position in the sky is typically closer to the search zone in the disc of the Solar System. Subsequent discoveries After Pluto's discovery, American astronomer Clyde Tombaugh continued searching for some years for similar objects but found none. For 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 the range of 50 to 2,500 kilometers. Eris, the most massive known 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. In 2006, Pluto and Eris were classified as dwarf planets by the International Astronomical Union. == Classification ==

According to their distance from the Sun and their orbital parameters, TNOs are classified in two large groups: the Kuiper belt objects (KBOs) and the scattered disc objects (SDOs). The diagram below illustrates the distribution of known trans-Neptunian objects beyond the orbit of Neptune at 30.07 AU. Different classes of TNOs are represented in different colours. The main part of the Kuiper belt is shown in orange and blue between the 2:3 and 1:2 orbital resonances with Neptune. Plutinos (orange) are the objects in the 2:3 resonance, including the dwarf planets Pluto and Orcus. Classical Kuiper belt objects are shown in blue, with the largest of these, including Haumea, Makemake, and Quaoar in the dynamically 'hot' population in light blue, and the dynamically 'cold' population, including 486958 Arrokoth, in low-eccentricity orbits clustered near 44 AU in dark blue. The scattered disc can be found beyond the Kuiper belt, shown in grey and purple. These objects, including dwarf planets Eris and Gonggong have been excited into eccentric orbits due to gravitational perturbations by Neptune, resulting in a concentration of their perihelia in the horizontal band between 30 and 40 AU. Some detached objects, such as however have higher perihelia. Centaurs, shown in green, have been perturbed from the scattered disc onto orbits crossing the outer planets. Bodies in both of these groups may be found in mean-motion resonances with Neptune; these are plotted in red. Finally, extreme trans-Neptunian objects are shown at the right of the diagram, with many having orbits that extend over 1000 AU from the sun. These can be divided into the extended scattered disc (pink), including , the distant detached objects (brown), including , and the four known sednoids, including Sedna and 541132 Leleākūhonua. KBOs The EdgeworthKuiper 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. EdgeworthKuiper belt objects are further classified into 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 the classical EdgeworthKuiper 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 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 the source of the so-called Jupiter-family comets (JFCs), which have periods of less than 20 years. SDOs 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 (TN), the objects in the scattered disc can be further divided into the "typical" scattered disc objects (SDOs, Scattered-near) with a TN of less than 3, and into the detached objects (ESDOs, Scattered-extended) with a TN 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 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. == Physical characteristics ==

Given the apparent magnitude (>20) of all but the biggest trans-Neptunian objects, the physical studies are limited to the following: • thermal emissions for the largest objects (see size determination) • colour indices, i.e. comparisons of the apparent magnitudes using different filters • analysis of spectra, visual and infrared Studying colours and spectra provides insight into the objects' origin and 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, 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 , 2.6–3.3 g/cm3, suggests a very high non-ice content (compare with Pluto's density: 1.86 g/cm3). The composition of some small TNOs could be similar to that of comets. Indeed, some centaurs undergo seasonal changes when they approach the Sun, making the boundary blurred (see 2060 Chiron and 7968 Elst–Pizarro). However, population comparisons between centaurs and TNOs are still controversial. Color indices 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. Correlations between the colours and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes: • Classical Kuiper belt objects (cubewanos) seem to be composed of two different colour populations: the so-called cold (inclination 2 (nitrogen) and 10% (methane) • Triton tholin, as above but with very low (0.1%) methane content • (ethane) Ice tholin I, believed to be produced from a mixture of 86% and 14% C2H6 (ethane) • (methanol) Ice tholin II, 80% H2O, 16% CH3OH (methanol) and 3% As an illustration of the two extreme classes BB and RR, the following compositions have been suggested • for Sedna (RR very red): 24% Triton tholin, 7% carbon, 10% N2, 26% methanol, and 33% methane • for Orcus (BB, grey/blue): 85% amorphous carbon, +4% Titan tholin, and 11% H2O ice Spectral types after the James Webb Space Telescope Recent observations with the James Webb Space Telescope (JWST), and in particular its high sensitivity with the NIRSpec instrument in the 0.7–5.3 μm range, have led to a new spectral classification for trans-Neptunian objects (TNOs). This classification is based on the Discovering the Surface Composition of TNOs (DiSCo) Large program and, for the first time, incorporates not only spectral profiles but also how they relate to the surface composition of TNOs. Additionally to the discovery of species that had not been detected before, the most significant discoveries from the spectral study of TNOs with Webb is the prevalence of CO2 on the surface for TNOs, independently of the size, albedo, and color and the non-prevalence of water ice, clearly present only in 20% of the sample. == Notable objects ==

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 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 Uni, , and Lempo. 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 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. == Extreme trans-Neptunian objects ==

Among the extreme trans-Neptunian objects are high-perihelion objects classified as sednoids, four of which have been confirmed: 90377 Sedna, , 541132 Leleākūhonua, and . They are distant detached objects with perihelia greater than 70 AU. Their high perihelia keep them at 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. ==See also==