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 names for three divisions of this volume of space. Pluto and its moon Charon are trans-Neptunian objects, and if Pluto had been discovered today, it might not have been called a planet. (See the definition of planet.)

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

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. Objects in orbital resonance with Neptune are plotted in red: (Neptune Trojans, plutinos, twotinos 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 ( ).

Notable trans-Neptunian objects

Pluto, also a planet, though its status as the latter is controversial.
Charon, the largest moon of Pluto
, the prototype cubewano, the first Kuiper belt object discovered after Pluto and Charon
, the first scattered disk object to be recognized
, the earliest discovered scattered disc object, and a binary
1993 RO, the next plutino discovered after Pluto
(20000) Varuna, a cubewano
(50000) Quaoar, a cubewano
(90482) Orcus, the largest known plutino
(38628) Huya, a plutino
(90377) Sedna, as yet unclassified¹, possibly an inner Oort cloud object
(a cubewano, the fourth largest known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period (3.9 h) David L. Rabinowitz, K. M. Barkume, Michael Brown, H. G. Roe, M. Schwartz, S. W. Tourtellotte, C. A. Trujillo (2005), Photometric Observations Constraining the Size, Shape, and Albedo of 2003 El₆₁, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt, Astrophysical Journal, submitted [http://www.arxiv.org/ftp/astro-ph/papers/0509/0509401.pdf Preprint on arXiv

, a scattered disk object, currently the largest known trans-Neptunian object. One known satellite.
(a cubewano David C. Jewitt, Audrey C. Delsanti The Solar System Beyond The Planets,to appear in the book Solar System Update, Springer-Praxis Ed., Horwood, Blondel and Mason, 2006. [http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf Preprint version (pdf) , the third largest known trans-Neptunian object
, a scattered disk object following unusual, highly inclined but circular orbit.

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

Some TNOs are thought to be lumps of ice with some organic (carbon-containing) material such as tholin, detected using spectroscopy. They are of the same composition as comets and many astronomers believe them to be just comets. The distinction between comet and asteroid is not yet clear and there is a substantial uncertainty, nutured by objects like 2060 Chiron and 133P/Elst-Pizarro. On the other hand, the recently confirmed high density of (2.6-3.3 g/cm³) suggests a very high non-ice content (compare with Pluto's density: 2.0 g/cm³).

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),
colour indices i.e. comparisons of the apparent magnitudes using different filters
analysis of spectra, visual and infrared

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.

Colours

Like Centaurs, TNO 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. N. Peixinho, A. Doressoundiram, A. Delsanti, H. Boehnhardt, M. A. Barucci, and I. Belskaya Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality Astronomy and Astrophysics, 410, L29-L32 (2003). Preprint on arXiv(pdf)

Colour 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 colour indices for all but the biggest objects (in slightly enhanced colour). O. R. Hainaut & A. C. Delsanti (2002) Color of Minor Bodies in the Outer Solar System Astronomy & Astrophysics, 389, 641 datasource For reference, two moons: Triton and Phoebe, the centaur Pholus and 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: so called cold (inclination <5°) displaying only red colours and hot (higher inclination) population displaying the whole range of colours from blue to very red. A. Doressoundiram, N. Peixinho, C. de Bergh, S. Fornasier, Ph. Thébault, M. A. Barucci and C. Veillet The color distribution in the Edgeworth-Kuiper Belt The Astronomical Journal, 124, pp. 2279-2296. Preprint on arXiv

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 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. Gulbis, Amanda A. S.; Elliot, J. L.; Kane, Julia F. The color of the Kuiper belt Core Icarus, 183 (july 2006), Issue 1, p. 168-178.

Scattered disk objects

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

The biggest 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: , , , Charon, and Orcus display neutral colour (infrared index V-I < 0.2), while the relatively dimmer bodies (50000 Quaoar, Ixion, , 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 resulting from its rapid rotation. The arc around represents uncertainty given its unknown albedo. The size of follows Brown’s measure (2400 km) based on HST point spread model. 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. A. Barucci Trans Neptunian Objects’ surface properties, IAU Symposium #229, Asteroids, Comets, Meteors, Aug 2005, Rio de Janeiro 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 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

Currently lying at 97 AU away, the celestial body designated is the farthest known object in the solar system, and the third brightest of the TNOs. It was first imaged by Michael Brown of the California Institute of Technology on October 31, 2003 with the Samuel Oschin Telescope at Palomar Observatory near San Diego, California. It is classified as a Scattered Disc Object, and recently it has been argued that its sheer size in relation to the nine known planets mean that it can only be classified as a planet. The discovering astronomer conceded he and his team did not know the exact size of the new object, but its brightness and distance tell them that it is at least as large as Pluto, which measures 2,302 kilometres in diameter. Scientists later estimated that the object was at least 1 1/2 times as large as Pluto. If confirmed, the discovery would be the first of a planet-mass object since Pluto was identified in 1930. is 15 terametres (15 billion kilometres) from the Sun, which it orbits every 560 years at an unusual 45-degree angle.

In July, 2005, the American scientists submitted a name for the "new planet" to the International Astronomical Union, re-igniting the debate about whether or not Pluto should be considered a planet at all.

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

Permanent Designation	Provisional Designation	Absolute magnitude	Albedo	Equatorial diameter (km)	Semimajor axis (AU)	Class	Discovery date	Discoverer(s)	Diameter method
		−1.2	~0.55 ± 0.15(thermal)	3000 ± 400	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
		−0.3	0.8 ± 0.2 (assumed)	1800 ± 200	45.7	KBO	2005	M. Brown, C. Trujillo & D. Rabinowitz	assumed albedo
		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		1.6	>0.2 (assumed)	<1800, >1180	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		2.6	0.10 ± 0.03	1260 ± 190	43.5	KBO	2002	C. Trujillo & M. Brown	disk resolved
(28978) Ixion		3.2	0.25 – 0.50	400 – 550	39.6	KBO	2001	Deep Ecliptic Survey	thermal
55636		3.3	> 0.19	< 709	43.1	KBO	2002	NEAT	thermal
55565		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		3.6	0.08?	~910	42.5	KBO	2002	A. Descour / Spacewatch	assumed albedo
(20000) Varuna		3.7	0.12 – 0.30	1060	43.0	KBO	2000	R. McMillan	thermal
		3.8	0.1 (assumed)	730?	41.8	KBO			assumed albedo
		3.8	0.1 (assumed)	730?	45.5	KBO			assumed albedo
		3.9	0.1 (assumed)	700?	39.6	KBO			assumed albedo
84522		3.9	> 0.03	< 1211	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.

Sources: Grundy et al. Diverse Albedos of Small Trans-Neptunian Objects Icarus Notes. Preprint on arXiv (pdf) Dale P. Cruikshank et al. Albedos, Diameters (and a Density) of Kuiper Belt and Centaur Objects from a session of the 37th meeting of the Division for Planetary Sciences of the American Astronomical Society and the Royal Astronomical Society (September 2005, Cambridge, UK) Abstract The original press release announcing the measuring of the albedo of 2003 UB₃₁₃ by Bertoldi et al. MPC Circular 2006-A28 for 2003 MW₁₂ data

External links

Minor Planet Center List of Transneptunian Objects
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.

References

Trans-Neptunian objects

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