Trans-Neptunian object

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 (semi-major axis) than Neptune, 30 astronomical units (AU). Twelve minor planets[nb 1] with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs).[1]

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. As of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Center's List of Transneptunian Objects.[2][3] Of these TNOs, 2,000 have a perihelion farther out than Neptune (30.1 AU).[4] As of November 2016, 242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation.[2][5]

The most massive known trans-Neptunian object is Eris, followed by Pluto, 2007 OR10, Makemake and Haumea. The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space,[6] though treatments vary and a few objects such as Sedna do not fit easily into any division.[nb 2]

Euler diagram of solar system bodies
Euler diagram showing the types of bodies in the Solar System.


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.[8]

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.

Discovery of other trans-Neptunian objects

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) 1992 QB1, 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.

Distribution and classification

TheTransneptunians 73AU
Distribution of trans-Neptunian objects

According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:

Kuiper belt objects (KBOs)

Scattered disc objects (SDOs)

  • The scattered disc contains objects farther from the Sun, usually with very irregular orbits (i.e. very elliptical and having a large inclination from the ecliptic). A typical example is the most massive known TNO, Eris. The scattered disc objects are further classified as follows[9]
    • Scattered-near (typical SDOs)—Scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a Tisserand parameter (relative to Neptune) less than 3.
    • Scattered-extended (detached objects)—Scattered-extended objects have a Tisserand parameter (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2

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, cubewanos in blue. The scattered disc extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU (Sedna) and aphelia beyond 1000 AU ((87269) 2000 OO67).


EarthMoonDysnomiaDysnomiaErisErisCharonCharonNixNixKerberosKerberosStyxStyxHydraPlutoPlutoMakemakeMakemakeNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaSalaciaSalaciaActaea2002 MS42002 MS4SednaSedna2007 OR102007 OR10WeywotWeywotQuaoarQuaoarVanthVanthOrcusOrcusFile:EightTNOs.png
Artistic comparison of Pluto, Eris, Makemake, Haumea, Sedna, 2002 MS4, 2007 OR10, Quaoar, Salacia, Orcus, and Earth along with the Moon.

This is a list of some notable trans-Neptunian objects. For a more extensive list, see list of trans-Neptunian objects.

Putative trans-Neptunian objects of planetary size

The existence of trans-Neptunian rock–ice bodies of planetary size, ranging from less than an Earth mass up to a brown dwarf has been often postulated[22][23] 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.[24]

Physical characteristics

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 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 tholin, detected in their spectra. On the other hand, the high density of Haumea, 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 133P/Elst–Pizarro). However, population comparisons between centaurs and TNOs are still controversial.[25]


TheTransneptunians Color Distribution
Colours of trans-Neptunian objects. Mars and Triton are not to scale. Phoebe and Pholus are not trans-Neptunian.

Like among centaurs, among TNOs there is a wide range of colours from blue-grey (neutral) to very red, but unlike the centaurs, clearly regrouped into two classes, the distribution appears to be uniform.[25]

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).[26] For reference, two moons: Triton and Phoebe, 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.

Classical objects

Classical objects seem to be composed of two different colour populations: the so-called cold (inclination <5°) population, displaying only red colours, and the so-called hot (higher inclination) population displaying the whole range of colours from blue to very red.[27]

A recent analysis based on the data from Deep Ecliptic Survey confirms this difference in colour between low-inclination (named Core) and high-inclination (named Halo) 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.[28]

Scattered disk objects

Scattered disk objects show colour resemblances with hot classical objects pointing to a common origin.

Largest objects

Selected Planemos
Size comparison between the Moon, Neptune's moon Triton, Pluto, several large TNOs, and the asteroid Ceres
TheTransneptunians Size Albedo Color
Illustration of the relative sizes, albedos and colours of some large TNOs

Characteristically, big (bright) objects are typically on inclined orbits, whereas the invariable plane regroups mostly small and dim objects. Although 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 the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.[16]

The third diagram on the right illustrates the relative sizes, albedos and colours of the biggest TNOs.


The objects present wide range of spectra, differing in reflectivity in visible red and near infrared. Neutral objects present a flat spectrum, reflecting as much red and infrared as visible spectrum.[29] 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, 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 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% CH4 (gaseous 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% H2O and 14% C2H6 (ethane)
  • (methanol) Ice tholin II, 80% H2O, 16% CH3OH (methanol) and 3% CO2

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

Size determination

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 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 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).

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 (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 1200–3700 km for an object of magnitude of 1.0.[30]

Missions to TNOs

The only mission to date that primarily targeted a trans-Neptunian object was NASA's New Horizons, which was launched in January of 2006 and flew by the Pluto system in July 2015.[31] Other missions have recently been proposed, including orbital capture and multi-target scenarios.[32]

See also


  1. ^ 2003 SS422 is excluded from the count because it has an observation arc of only 76 days and hence its semi-major axis is not securely known.
  2. ^ a b The literature is inconsistent in the use of the phrases "scattered disc" and "Kuiper belt". For some, they are distinct populations; for others, the scattered disk is part of the Kuiper belt, in which case the low-eccentricity population is called the "classical Kuiper belt". Authors may even switch between these two uses in a single publication.[7] In this article, the scattered disk will be considered a separate population from the Kuiper belt.


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