Trans-Neptunian Objects

by Wm. Robert Johnston
last updated 1 October 2007

Contents:

List of known trans-Neptunian objects and Centaurs


Dynamical classes:

Classes of outer solar system objects, with numbers of known objects (1,342, as of 1 October 2007) in parenthesis (see note below):

Note that these counts are approximate, since these classifications are in some cases not universally defined and since orbits are not well determined for some objects. In the case of resonance objects, accurate classification requires extended observations and dynamical studies. These counts include delineations based on preliminary orbital elements (listed as possible members), and classifications for objects in such cases are subject to revision. Consult the Minor Planet Center for authoritative classifications.

The graph below plots eccentricity versus semimajor axis for outer solar system objects by class (click here for a larger version). Black asterisks indicate values for Jupiter, Saturn, Uranus, Neptune, and Pluto (from left to right). The light blue curves indicate values corresponding to perihelion distances of 40 AU, 45 AU, and 50 AU (from left to right on the graph).


Distribution by distance:

Below is a histogram of outer solar system objects by distance (click here for a larger version) (Note: this graph is out of date.).

Two objects, the SDO (87269) 2000 OO67 and the inner Oort cloud object (90377) Sedna, have aphelion distances near 1000 AU; another (2006 SQ372) has aphelion distance near 1900 AU. (Three unusual asteroids have aphelia over 1000 AU: 2002 RN109, 2005 VX3, and 2007 DA61; these may be inactive comets.) A total of 55 TNOs (plus 8 more unusual objects and one Apollo asteroid) have aphelion distances exceeding 100 AU. Relatively few (21) have been discovered with perihelion distances greater than 46 AU, and only three with perihelion distances greater than 47 AU: Sedna at 76 AU, 2004 XR190 at 51 AU, and 2004 VN112 at 47.4 AU. Data thus far suggests than there is indeed a cutoff to the classical TNO population at 47 AU, although the discoveries of Sedna and 2004 XR190 suggest an unrecognized population of much more distant objects (see below).

Of the 1,342 outer solar system objects counted above, 25 have inclinations from 40° to 90°, and 16 "unusual" asteroids (mostly damocloids but including one Apollo) have retrograde orbits. These latter objects may actually be comets no longer showing cometary activity.

The graph below (click here for larger version) shows estimated TNO size versus perihelion distance. A line indicates the nature of the cutoff that would result from detectability limits; in other words, objects lying along this line would have the same apparent brightness from Earth (assuming the same albedo). Note the lack of detected objects with perihelions beyond 47 AU.

The graph below (click here for larger version and discussion) shows eccentricity versus semimajor axis for outer solar system objects. Green lines indicate the location of orbital resonances with Neptune where objects have been found. The second resonance from the left is the 2:3 resonance, location of a cluster of objects (the Plutinos).


Largest objects:

The 28 largest known TNOs (plus one satellite) are, with estimated diameters:

Diameters with question marks are estimated (see discussion here). For comparison, the largest asteroid, (1) Ceres, is 920 km in diameter. A total of 44 TNOs appear to be at least 500 km in diameter and 120 at least 300 km. The total volume of known TNOs (including Pluto) is estimated at 161% of that of the Moon (45% in (136199) 2003 UB313 and 35% in Pluto/Charon). (A note: these size-associated figures are very dependent on the assumed albedo value for these objects. The above figures assume an albedo of 9% for objects with unmeasured diameters/albedos. As an indication of the effect of this assumption, the figures on the January 2005 version of this page assumed a 4% albedo and counted 65 TNOs at least 500 km in diameter and 295 at least 300 km in diameter.) For more discussion of large TNOs see:

The graph below (click here for larger version and discussion) plots diameter vs. magnitude for TNOs and centaurs with measured diameters.

The chart below (click here for larger version) shows the orbits of some of the largest known TNOs with the outer planets.


Binary objects:

In 2001 the TNO 1998 WW31 was found to be double, and companions to other TNOs have since been found. Another TNO may be a contact binary. In 2005 two companions to (136108) 2003 EL61 were discovered (shortly before two additional satellites of Pluto were reported). Data for the 48 known TNO binaries or multiple systems are given below:

objectcomponent diameters (km)separation (km)orbital period (d)
(134340) Pluto
  Charon
  Nix
  Hydra
2,3401,212 19,5716.38723
9048,68024.8562
11064,78038.2065
(26308) 1998 SM16529010011,300130.1
(42355) Typhon
  Echidna
130801,300?11?
(47171) 1999 TC363801407,72050.38
(48939) 1995 TL8350160420?0.5?
(50000) Quaoar12609511,000?10?
(55637) 2002 UX256502055,000?8?
(58534) Logos
  Zoe
80708,010312
(60458) 2000 CM1141501202,200?18?
(60621) 2000 FE81501151,200?6?
(65489) Ceto
  Phorcys
1701301,8409.55
(66652) Borasisi1701404,66046.26
(79360) 1997 CS293002902,300?6?
(80806) 2000 CM1052201302,700?15?
(82075) 2000 YW1344302401,900?3?
(88611) Teharonhiawako
  Sawiskera
18012027,300825
(90482) Orcus9102608,700?11?
(119979) 2002 WC194001302,700?7?
(120347) 2004 SB605501903,500?5?
(123509) 2000 WK1831701402,400?17?
(134860) 2000 OJ672501802,300?9?
(136108) 2003 EL611,38031049,50049.12
17039,30034.7
(136199) Eris
  Dysnomia
2,66035036,000?14?
(139775) 2001 QG298170170240?0.5739?
(148780) 2001 UQ183402505,800?23?
(160256) 2002 PD149280???
1998 WW3113011022,300574
1999 OJ4170902,200?17?
1999 RT214100703,300?55?
2000 CF10517012023,000?500?
2000 CQ1141601305,900?70?
2000 QL2511801807,000?70?
2001 FL1851401001,900?16?
2001 QC2982402203,69019.23
2001 QW3229090130,000?1,500?
2001 QY2972802302,800?10?
2001 RZ1432001901,400?5?
2001 XR2542402303,400?15?
2002 GZ311901202,100?13?
2003 AZ84690707,200?13?
2003 QA912802601,800?5?
2003 QR911901701,700?8?
2003 QW11123012010,000?90?
2003 QY9014010014,000460
2003 TJ5895753,500?70?
2003 UN28413010060,000?3,300?
2003 WU1882501801,300?4?
2004 PB1082101205,400?45?
2005 EO30421012085,000?7,000?

Values of binary TNO separations or orbital periods if marked by "?" are approximate, assuming that observed (projected) separations are comparable to the semimajor axes and that both components have densities of order 1,000 kg/m3. For additional data see the following listings:

With 49 confirmed or suspected binary/multiple TNOs (or SDOs/centaurs) (through March 2007) in a population of 1,298 objects, this suggests that binary TNOs are quite common. Binary TNOs identified in 2005 include several close binaries, suggesting more binaries that cannot yet be resolved.

For more information on binary TNOs, see the Asteroids with Satellites section at this web site.


General links:


© 2001-2006, 2007 by Wm. Robert Johnston.
Last modified 1 October 2007.
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