Trans-Neptunian object
A trans-Neptunian object, also written transneptunian object, is any minor planet or dwarf planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a 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 October 2018, the catalog of minor planets contains 528 numbered and more than 2,000 unnumbered TNOs.
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 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.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 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 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. In December 2018, the discovery of, nicknamed "Farout", was announced. Farout is the most distant solar system object so-far observed and is about 120 AU away from the sun, likely taking more than 1,000 years to complete one orbit.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 to the right illustrates the distribution of known trans-Neptunian objects in relation to the orbits of the planets and the centaurs for reference. Different classes are represented in different colours. Resonant objects are plotted in red, classical Kuiper belt objects in blue. The scattered disc extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU and aphelia beyond 1,000 AU.KBOs
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 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 Edgeworth-Kuiper belt include 15760 Albion, 50000 Quaoar and Makemake.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.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 tholin, 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. The diagram illustrates known colour indices for all but the biggest objects.For reference, two moons: Triton and Phoebe, the centaur Pholus and the planet Mars are plotted . 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 object 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 |
Spectral type
Among TNOs, as among centaurs, there is a wide range of colors from blue-grey to very red, but unlike the centaurs, clearly regrouped into two classes, the distribution appears to be uniform. The wide range of spectra differ in reflectivity in visible red and near infrared. Neutral objects present 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 uses the total of four classes from BB to RR with BR and IR as intermediate classes. BR and IR differ mostly in 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:
- Titan tholin, believed to be produced from a mixture of 90% N2 and 10%
- Triton tholin, as above but with very low methane content
- Ice tholin I, believed to be produced from a mixture of 86% and 14% C2H6
- Ice tholin II, 80% H2O, 16% CH3OH and 3%
- for Sedna : 24% Triton tholin, 7% carbon, 10% N2, 26% methanol, and 33% methane
- for Orcus : 85% amorphous carbon, +4% Titan tholin, and 11% H2O ice
Size determination and distribution
It is difficult to estimate the diameter of TNOs. For very large objects, with very well known orbital elements, 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, and one assumes that most of its surface is in thermal equilibrium. 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 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.
Thus there are two unknowns, which can be determined by two independent measurements. Unfortunately, TNOs are so far from 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 the thermal method is only applicable to the largest KBOs. For the majority of 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 1200–3700 km for an object of magnitude of 1.0.
Notable objects
| Object | Description |
| Pluto | a dwarf planet and the first TNO discovered |
| 15760 Albion | the prototype cubewano, the first Kuiper belt object discovered after Pluto |
| the first binary Kuiper belt object discovered after Pluto | |
| 79360 Sila–Nunam | another binary Kuiper belt planet with bodies of similar size |
| 47171 Lempo | almost triple Kuiper belt planet with two bodies of similar size and a large third satellite |
| the first object to be identified as a scattered disc object | |
| has a relatively large satellite and is the earliest-discovered scattered disc object | |
| 1993 RO | the next plutino discovered after Pluto |
| 20000 Varuna | a large cubewano, known for its rapid rotation and elongated shape |
| 28978 Ixion | large plutino, was considered to be among the largest Kuiper belt objects upon discovery |
| 50000 Quaoar | large cubewano with a satellite; sixth-largest-known Kuiper belt object |
| 90482 Orcus | large plutino with a large satellite |
| 90377 Sedna | a distant object, proposed for a new category named extended scattered disc, detached objects, distant detached objects or scattered-extended in the formal classification by DES. |
| 120347 Salacia | large cubewano with a large satellite |
| 136108 Haumea | a dwarf planet, the third-largest-known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period. It is the most-massive-known member of a collisional family. |
| 136199 Eris | a dwarf planet, a scattered disc object, and currently the most-massive-known trans-Neptunian object. It has one known satellite, Dysnomia |
| 136472 Makemake | a dwarf planet, a cubewano, and the fourth-largest-known trans-Neptunian object |
| 225088 Gonggong | second-largest scattered-disc object with a satellite |
| a scattered disc object following a highly inclined but nearly circular orbit | |
| remarkable for its eccentric orbit and large aphelion | |
| remarkable for its eccentric orbit and large aphelion | |
| the first retrograde TNO, having an orbital inclination of i = 104° | |
| a likely dwarf planet with perihelion 80 AU from the Sun | |
| a TNO having an unusual 110°-tilted solar orbital plane | |
| 486958 Arrokoth | the target of the New Horizons spacecraft in its 2019 encounter |
| the first trans-Neptunian object discovered while beyond from the Sun |
Exploration
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,, and Lempo.
The existence of planets beyond Neptune, ranging from less than an Earth mass up to 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 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 visitation of minor planet 50000 Quaoar, in the 2030s.