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| ‡ Trans–Neptunian dwarf planets are "plutoids" |
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.
The first trans-Neptunian object to be discovered was Pluto in 1930. It took more than 60 years to discover, in 1992, a second trans-Neptunian object, (15760) 1992 QB1, with only the discovery of Pluto's moon Charon in 1978 before that. Now over 1200 trans-Neptunian objects appear on the Minor Planet Center's List Of Transneptunian Objects.[1] As of November 2009, two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.[2][3]
The largest known trans-Neptunian objects are Pluto and Eris, followed by Makemake and Haumea. The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space,[4] though treatments vary and a few objects such as Sedna do not fit easily into any division.[nb 1]
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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 1930. However, Pluto was too small to explain the discrepancies, and revised estimates of Neptune's mass 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 a long time, no one searched for other TNOs as it was generally believed that Pluto was the only major object of the Kuiper belt. Only after the discovery of a second TNO, (15760) 1992 QB1, in 1992, systematic searches for further such objects began. 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 2500 kilometers.
Eris, at the time thought to be the largest 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 orbit parameters, TNOs are classified in two large groups:
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 disk 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).
A fuller list of objects is being compiled in the List of trans-Neptunian objects.
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: 2.0 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.[13]
Like Centaurs, TNOs display a wide range of colours from blue-grey to very red, but unlike the centaurs, clearly re-grouped into two classes, the distribution appears to be uniform.[13]
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).[14] For reference, two moons: Triton and Phoebe, the centaur Pholus and planet Mars are plotted (yellow labels, size not to scale).
Correlations between the colors and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes.
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.[15]
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.[16]
Scattered disk objects show colour resemblances with hot classical objects pointing to a common origin.
Characteristically, big (bright) objects are typically on inclined orbits, while the invariable plane re-groups mostly small and dim objects. With the exception of Sedna, all big TNOs (Eris, Makemake, Haumea, Charon, and Orcus) display neutral colour (infrared index V-I < 0.2), while the relatively dimmer bodies (50000 Quaoar, Ixion, 2002 AW197, and Varuna), as well as the population as the whole, are reddish (V-I in 0.3 to 0.6 range). This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.[11]
The diagram illustrates the relative sizes, albedos and colours of the biggest TNOs. Also shown, are the known satellites and the exceptional shape of Haumea (2003 EL61) resulting from its rapid rotation. The arc around Makemake (2005 FY9) represents uncertainty given its unknown albedo. The size of Eris follows Michael Brown’s measure (2400 km) based on HST point spread model.[17] The arc around it represents the thermal measure (3000 km) by Bertoldi (see the related section of the article for the references).
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.[18] 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:
As an illustration of the two extreme classes BB and RR, the following compositions have been suggested
It is difficult to estimate the diameter of TNOs. For very large objects, with very well known orbital elements (namely, Pluto and Charon), 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 the 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 produce 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, as example for magnitude of 1.0, in uncertainty from 1200 – 3700 km!.[19]
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