Trans-Neptunian object

A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space.

The first trans-Neptunian object to be discovered was Pluto in 1930 (though it was not called as such until recently; it was considered to be the ninth planet until 2006).

It took more than 60 years to discover a second trans-Neptunian object, (15760) 1992 QB₁, in 1992 (with only the discovery of Pluto's moon Charon in 1978 in between). Since then however, 1075 objects have been discovered, differing in sizes, orbits and surface composition (though only 142 of these have their orbit well enough determined that they are given a minor planet number).^[1]

The largest known trans-Neptunian object is Eris (discovered in 2005), followed by Pluto.

History

Discovery of Pluto

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 Uranus and Neptune suggested that there were one or more additional planets beyond Neptune (see Planet X). 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 TNO's.

Discovery of other trans-Neptunian objects

After Pluto's discovery, noone searched for further TNOs for a long time. Indeed, it was generally believed that Pluto was the only major object of the Kuiper belt. Only after the discovery of the second TNO, (15760) 1992 QB₁, in 1992, systematic searches for further such objects began. A broad strip of the sky around the ecliptic was fotographed and digitally evaluated for slowly-moving objects. Hundreds of TNOs were found, with diameters in the range of 100 to 2500 kilometers.

Distribution and classification

Distribution 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 represented 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 1000 AU ( (87269) 2000 OO₆₇).

Notable trans-Neptunian objects

136472 Makemake compared to Eris, Pluto, Haumea (2003 EL61), Sedna, Orcus, Quaoar, Varuna, and Earth. Click the objects to go to their articles.

• 134340 Pluto, a dwarf planet.

• Charon, the largest moon of Pluto.

• (15760) 1992 QB₁, the prototype cubewano, the first Kuiper belt object discovered after Pluto and Charon.
• 1998 WW₃₁, the first binary Kuiper belt object discovered after Pluto and Charon.
• (15874) 1996 TL₆₆, the first object to categorized as a scattered disk object.
• (48639) 1995 TL₈ has a very large satellite and is the earliest discovered scattered disc object.
• 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),^[2] detached objects,^[3] Distant Detached Objects (DDO)^[4] or Scattered-Extended in the formal classification by DES^[5]
• 136108 Haumea^[6], a dwarf planet, cubewano, the fourth largest known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period (3.9 h).^[7]
• 136199 Eris, dwarf planet, a scattered disk object, currently the largest known trans-Neptunian object. One known satellite, Dysnomia.
• 136472 Makemake^[8], dwarf planet, 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 (148209) 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.

Physical characteristics

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 (See 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 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 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.^[9]

Colors

Colours of the Transneptunians.

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.^[9]

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).^[10] 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

Classical objects seem to be composed of two different color populations: so called cold (inclination <5°) displaying only red colors and hot (higher inclination) population displaying the whole range of colors from blue to very red.^[11]

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.^[12]

Scattered disk objects

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

The largest objects

Illustration of the relative sizes, albedos and colours of the largest TNOs.

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.^[7]

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.^[13] The arc around it represents the thermal measure (3000 km) by Bertoldi (see the related section of the article for the references).

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.^[14] 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

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![1].

Largest discoveries

Size comparison between Earth's Moon, Neptune's moon Triton, and several large TNOs

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

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.

Sources:^{[19][20][21][22]}

External links

• Minor Planet Center List of Transneptunian Objects
• TNO Italian website An Italian website about TNO
• Nine planets, University of Arizona
• David Jewitt's Kuiper Belt site
  • Large KBO page
• A list of the estimates of the diameters from jonhnstonarchive with references to the original papers
• Trans-Neptunian Object Orbital Database TNO Orbital Database

See also

• List of trans-Neptunian objects
• Triton
• Nemesis
• Dwarf planet
• Mesoplanet
• Small solar system body

References

The Solar System The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Haumean · Eridian Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt, Centaurs, TNOs: Kuiper belt/Scattered disc) · Comets (Oort cloud) See also astronomical objects, the Solar System's list of objects, sorted by radius or mass, and the Solar System Portal