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Trans-Neptunian objects orbit beyond the sun.

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Trans-Neptunian objects are any objects in the solar system that orbit the sun at a greater distance on average than Neptune. The Kuiper belt, Scattered disk, and Oort cloud are names for three divisions of this volume of space.

trans-Neptunian Objects.
Distribution of trans-Neptunian Objects.

The orbit of each of the planets is affected by the gravitational influences of all the other planets. Discrepancies in the early 1900s between the observed and expected orbits of the known planets suggested that there were one or more additional planets beyond Neptune (see Planet X). The search for these led to the discovery of Pluto. Pluto is too small to explain the discrepancies, however, and revised estimates of Neptune's mass showed that the problem was spurious.

It took more than 60 years to discover another TNO (with only the discovery of Pluto’s moon Charon in between). Since 1992 however, more than 1000 objects have been discovered, differing in sizes, orbits and surface composition.

Distribution and classification of Trans-Neptunian objects.

The diagram illustrates the distribution of known trans-Neptunian objects (up to 70 AU) in relation to the orbits of the planets together with Centaurs for reference. Different classes are repesented in different colours. resonant objects (i.e. objects in orbital resonance with Neptune) are plotted in red: (Neptune Trojans, plutinos, and a number of smaller families). The term Kuiper belt re-groups classical objects (cubewanos, in blue) with plutinos and Twotinos (in red).

The scattered disk extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU (Sedna) and aphelia beyond 1,000 AU (87269) 2000 OO67).

Notable trans-Neptunian objects.

  • Pluto, dwarf planet.
  • Charon, the largest moon of Pluto.
  • (15760) 1992 QB1, the prototype Cubewano, the first Kuiper belt object discovered after Pluto and Charon.
  • (15874) 1996 TL66, the first Scattered Disk Object to be recognized.
  • (48639) 1995 TL8, the earliest discovered scattered disc object, and a binary.
  • 1993 RO, the next Plutino discovered after Pluto.
  • (20000) Varuna and (50000) Quaoar, large cubewanos.
  • (90482) Orcus and (28978) Ixion, large plutinos.
  • (90377) Sedna, a distant object, classified in a new category named Extended scattered disc (E-SDO), Distant Detached Objects (DDO) or Scattered-Extended in the formal classsification by DES.
  • (136108) 2003 EL, a cubewano, the fourth largest known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period (3.9 h).
  • Eris, dwarf planet, a scattered disk object, currently the largest known trans-Neptunian object. One known satellite, Dysnomia.
  • (136472) 2005 FY, a cubewano, the third largest known trans-Neptunian object.
  • 2004 XR190, a scattered disk object following unusual, highly inclined but circular orbit.
  • (87269) 2000 OO67 and 2000 CR105, remarkable for their eccentric orbits and aphelia beyond 1000 AU.

A fuller list of objects is being compiled in the List of trans-Neptunian objects.

1Included in extended scattered disk by Jewitt (see References).

Physical characteristics 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:

  • thermal emissions for the largest objects (size determination),.
  • color indices i.e. comparisons of the apparent magnitudes using different filters.
  • analysis of spectra, visual and Infrared.

Studying colors 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 recently confirmed high density of 2003 EL61 (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 TNO 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 TNO are still object of controversy


Trans-Neptunian objects.
Colours of the Trans-Neptunian objects.

Like Centaurs, TNO display a wide range of colors from blue-grey to very red but unlike the centaurs, clearly re-grouped into two classes, the distribution appears to be uniform.

Color indices are simple measures of the differences of the apparent magnitude of an object seen through blue (B), visible (V) i.e. green-yellow and red (R) filters. The diagram illustrates known color indices for all but the biggest objects (in slightly enhanced color). 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 Trans-Neptunian objects.

Classical objects seem to be composed of two different color populations: so called cold (inclination <5º) displaying only red colors and hot (higher inclination) population displaying the whole range of colors from blue to very red.

A recent analysis based on the data from Deep Ecliptic Survey confirms this difference of colours between low inclination objects (named Core) and high inclination (named Halo). Red colors 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 disk objects.

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

The largest Trans-Neptunian objects.

Trans-Neptunian objects.
Illustration of the relative sizes, albedos and colours of the largest Trans-Neptunian objects.

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, 2005 FY9, 2003 EL61, 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.

The diagram illustrates the relative sizes, albedos and colours of the biggest TNOs. Also shown, are the known satellites and the exceptional shape of 2003 EL61 resulting from its rapid rotation. The arc around 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. The arc around it represents the thermal measure (3000 km) by Bertoldi.

Trans-Neptunian object: Spectra.

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. 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, 33% methane.
  • for Orcus (BB, grey/blue): 85% amorphous carbon +4% titan tholin, 11% H20 ice.

Size determination of Trans-Neptunian objects.

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 freqencies, 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: astronomers thus 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!.

Largest discoveries of Trans-Neptunian objects.

large Trans-Neptunian objects.
Size comparison between Earth's Moon and several large Trans-Neptunian objects.

Currently lying at 97 AU away, Eris is the farthest known object in the solar system, and the third brightest of the TNOs. Classified as a scattered disk object (SDO), Eris follows an orbit at 10 billion kilometres from the Sun, completing it in 560 years at an unusual 45-degree angle.

The size of Eris, currently estimated to be slightly larger than Pluto, re-ignited the debate about whether or not Pluto should be considered a planet at all (see 2006 redefinition of planet).

Eris is the most recent discovery in the race to discover TNO bigger that Pluto, as attested by the round’ numbers of 20000 Varuna and 50000 Quaoar considered at one time the biggest TNO.

The brightest known TNOs (with absolute magnitudes < 4.0), are:

Absolute magnitudeAlbedoEquatorial diameter
Semimajor axis
ClassDiscovery dateDiscoverer(s)Diameter method
Eris 2003 UB313 -1.2 ~0.86 0.07 2400 100 67.7 SDO 2005 M. Brown, C. Trujillo & D. Rabinowitz thermal
Pluto -1.0 0.49 to 0.66 2306 20 39.4 KBO 1930 C. Tombaugh occultation
2005 FY9 -0.3 0.8 0.2 1800 200 45.7 KBO 2005 M. Brown, C. Trujillo & D. Rabinowitz
2003 EL61 0.1 0.7 0.1 ~1500 43.3 KBO 2005 M. Brown, C. Trujillo & D. Rabinowitz density inferred from rotation & oblate shape
Charon S/1978 P 1 1 0.36 to 0.39 1205 2 39.4 KBO Satellite 1978 J. Christy occultation
(90377) Sedna 2003 VB12 1.6 >0.2? 1180 - 1800 502.0 SDO? 2003 M. Brown, C. Trujillo & D. Rabinowitz thermal
(90482) Orcus 2004 DW 2.3 0.1 (assumed) ~1500 39.4 KBO 2004 M. Brown, C. Trujillo & D. Rabinowitz assumed albedo
(50000) Quaoar 2002 LM60 2.6 0.10 0.03 1260 190 43.5 KBO 2002 C. Trujillo & M. Brown disk resolved
(28978) Ixion 2001 KX76 3.2 0.25 - 0.50 < 822 39.6 KBO 2001 Deep Ecliptic Survey thermal
55636 2002 TX300 3.3 > 0.19 < 709 43.1 KBO 2002 NEAT thermal
55565 2002 AW197 3.3 0.14 - 0.20 650 - 750 47.4 KBO 2002 C. Trujillo, M. Brown, E. Helin, S. Pravdo, K. Lawrence & M. Hicks / Palomar Observatory thermal
55637 2002 UX25 3.6 0.09? ~838 42.5 KBO 2002 A. Descour / Spacewatch assumed albedo
(20000) Varuna 2000 WR106 3.7 0.037 936+238-324 43.0 KBO 2000 R. McMillan thermal
2002 MS4 3.8 0.1 (assumed) 730? 41.8 KBO assumed albedo
2003 MW12 3.8 0.1 (assumed) 730? 45.5 KBO assumed albedo
2003 AZ84 3.9 0.1 (assumed) 700? 39.6 KBO assumed albedo
84522 2002 TC302 3.9 >0.051 450 - 1190 55.1 SDO 2002 NEAT thermal

The list has been sorted by increasing Absolute magnitude. Estimated diameter is greatly affected by surface Albedo which has often been assumed, not measured. Some potentially large Kuiper belt objects have not been included.

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