Discover

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.
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 ^[1], differing in sizes, orbits and surface composition.

Contents

Distribution and classification

Notable trans-Neptunian objects

Physical characteristics

Colors

Spectra

Size determination

Largest discoveries

External links

See also

References

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 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 1,000 AU ( ).

Notable trans-Neptunian objects

Image:EightTNOs.png|thumb|250px|right|(136472) 2005 FY₉ compared to Eris, Pluto, (136108) 2003 EL₆₁, Sedna, Orcus, Quaoar, Varuna, and Earth. Click the objects to go to their articles.
#Earth
rect 646 1714 2142 1994 The Earth
#Eris and Dysnomia
circle 226 412 16 Dysnomia
circle 350 626 197 (136199) Eris
#Pluto and Charon
circle 1252 684 86 Charon
circle 1038 632 188 (134340) Pluto
#2005 FY9
circle 1786 614 142 (136472) 2005 FY9
#2003 EL61
circle 2438 616 155 (136108) 2003 EL61
#Sedna
circle 342 1305 137 (90377) Sedna
#Orcus
circle 1088 1305 114 (90482) Orcus
#Quaoar
circle 1784 1305 97 (50000) Quaoar
#Varuna
circle 2420 1305 58 (20000) Varuna
desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
#Notes:
#Details on the new coding for clickable images is here:
#While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.


★ Pluto, a dwarf planet.

★ Charon, the largest moon of Pluto.

★ , the prototype cubewano, the first Kuiper belt object discovered after Pluto and Charon.

★ , the first object to categorized as a scattered disk object.

★ 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)^[1], ''detached objects'' ^[2], ''Distant Detached Objects'' (DDO)
Rodney S. Gomes, John J. Matese, and Jack J. Lissauer
''A Distant Planetary-Mass Solar Companion May Have Produced Distant Detached Objects''
To appear in Icarus (2006). Preprint or ''Scattered-Extended'' in the formal classsification by DES
J. L. Elliot, S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie, L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech
''The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population.''
The Astronomical Journal, '129' (2006), pp.
preprint

★ ([2]), a cubewano, the fourth largest known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period (3.9 h)^[3].

★ Eris, dwarf planet, a scattered disk object, currently the largest known trans-Neptunian object. One known satellite, Dysnomia.

★ ([3]), a cubewano, the third largest known trans-Neptunian object

★ , a scattered disk object following unusual, highly inclined but circular orbit.

★ and , 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 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).
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 ^[4]

Colors

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).^[5]
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. 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 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.
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 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, , , 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 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 (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.^[6]
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, 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![4].

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 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).


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
(136199) Eris		−1.2	~0.86 ± 0.07	2400 ± 100	67.7	SDO	2005	M. Brown, C. Trujillo & D. Rabinowitz	thermal
(134340) Pluto		−1.0	0.49 to 0.66	2306 ± 20	39.4	KBO	1930	C. Tombaugh	occultation
		−0.3	0.8 ± 0.2	1800 ± 200	45.7	KBO	2005	M. Brown, C. Trujillo & D. Rabinowitz
		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?	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		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	< 822	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	0.09?	~838	42.5	KBO	2002	A. Descour / Spacewatch	assumed albedo
(20000) Varuna		3.7	0.037	936	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
		3.8	0.09 (assumed)	765	63.1	SDO	2006		assumed albedo
		3.9	0.1 (assumed)	700?	39.6	KBO			assumed albedo
84522		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.''
Sources: ^{[7]
[8]
[9]
[10]}

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

1. ''Evidence for an Extended Scattered Disk?''
2. D.Jewitt, A.Delsanti ''The Solar System Beyond The Planets'' in ''Solar System Update : Topical and Timely Reviews in Solar System Sciences '', Springer-Praxis Ed., ISBN 3-540-26056-0 (2006) Preprint of the article (pdf)
3. David L. Rabinowitz, K. M. Barkume, Michael E. 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 Preprint on arXiv

4.
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)
5.
O. R. Hainaut & A. C. Delsanti (2002) ''Color of Minor Bodies in the Outer Solar System'' Astronomy & Astrophysics, '389', 641 datasource

6.
A. Barucci ''Trans Neptunian Objects’ surface properties'', IAU Symposium #229, Asteroids, Comets, Meteors, Aug 2005, Rio de Janeiro
7. Grundy et al. ''Diverse Albedos of Small Trans-Neptunian Objects'' Icarus Notes. Preprint on arXiv (pdf)
8. 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
9. The original press release announcing the measuring of the albedo of by Bertoldi et al.
10. MPC Circular 2006-A28 for data