Trans-Neptunian object
A trans-Neptunian object, 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.
Typically, TNOs are further divided into the classical and resonant objects of the Kuiper belt, the scattered disc and detached objects with the sednoids being the most distant ones. As of February 2025, the catalog of minor planets contains 1006 numbered and more than 4000 unnumbered TNOs. However, nearly 5900 objects with semimajor axis over 30 AU are present in the MPC catalog, with 1009 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 the Sun directly, 15760 Albion. The most massive TNO known is Eris, followed by Pluto,,, and. More than 80 satellites have been discovered in orbit of trans-Neptunian objects. TNOs vary in [|color] and are either grey-blue or very red. 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 a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects.
History
Discovery of Pluto
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 and the scattered disc objects. 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 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, including, the distant detached objects, 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 and the plutinos, 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. These are non-resonant objects that come near enough to Neptune to have their orbits changed from time to time 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, 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, the objects in the scattered disc can be further divided into the "typical" scattered disc objects with a TN of less than 3, and into the detached objects 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 of all but the biggest trans-Neptunian objects, the physical studies are limited to the following:- thermal emissions for the largest objects
- colour indices, i.e. comparisons of the apparent magnitudes using different filters
- analysis of spectra, visual and infrared
Small TNOs are thought to be low-density mixtures of rock and ice with some organic 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. 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 . However, population comparisons between centaurs and TNOs are still controversial.
Color indices
are simple measures of the differences in the apparent magnitude of an object seen through blue, visible, i.e. green-yellow, and red 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 seem to be composed of two different colour populations: the so-called cold population, displaying only red colours, and the so-called hot population displaying the whole range of colours from blue to very red. A recent analysis based on the data from Deep Ecliptic Survey confirms this difference in colour between low-inclination and high-inclination objects. Red colours of the Core objects together with their unperturbed orbits suggest that these objects could be a relic of the original population of the belt.
- Scattered disc objects show colour resemblances with hot classical objects pointing to a common origin.
| Color | Plutinos | Cubewanos | Centaurs | SDOs | Comets | Jupiter trojans |
| B–V | ||||||
| V–R | ||||||
| V–I | ||||||
| R–I |