by Wm. Robert Johnston
last updated 18 December 2009
Contents:
List of known trans-Neptunian objects and Centaurs
Classes of outer solar system objects, with numbers of known objects (1,432, as of 13 December 2009) 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).

Below is a histogram of outer solar system objects by distance (click here for a larger version) (Note: this graph is out of date.).

Three trans-Neptunian objects--(87269) 2000 OO67, (90377) Sedna, and 2007 TG422--have aphelion distances near 1000 AU; another (2006 SQ372) has aphelion distance near 2100 AU. (Four unusual asteroids have aphelia over 1000 AU: 2002 RN109, 2005 VX3, 2007 DA61, and 2009 UG89; these may be inactive comets.) A total of 69 TNOs (plus 10 more unusual objects) have aphelion distances exceeding 100 AU. Relatively few (14) 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 47.3 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 1,432 outer solar system objects counted above, 33 have inclinations from 40° to 90°, and 24 "unusual" asteroids plus three SDOs 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).

The 32 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 47 TNOs appear to be at least 500 km in diameter and 117 at least 300 km. The total volume of known TNOs (including Pluto) is estimated at 179% 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 albedo vs. diameter 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.

In 2001 the TNO 1998 WW31 was found to be double, and companions to other TNOs have since been found. The possibility of companions for four more TNOs has been suggested (for (20000) Varuna, (32929) 1995 QY9, (33128) 1998 BU48, and (47932) 2000 GN171). In 2005 two companions to (136108) Haumea were discovered (shortly before two additional satellites of Pluto were reported). Data for the 63 known TNO binaries or multiple systems are given below:
| TNO | companion(s) | component diameters (km) | separation (km) | orbital period (d) | year reported | |
|---|---|---|---|---|---|---|
| (134340) Pluto | Charon | 2340 | 1212 | 19,571 | 6.387 | 1978 |
| Nix | 90 | 48,680 | 24.856 | 2005 | ||
| Hydra | 110 | 64,780 | 38.206 | 2005 | ||
| (26308) 1998 SM165 | S/2001 (26308) 1 | 290 | 100 | 11,300 | 130.1 | 2002 |
| (42355) Typhon | Echidna | 130 | 80 | 1,630 | 18.971 | 2006 |
| (47171) 1999 TC36 | 280 | 260 | 900? | 1.9? | 2007 | |
| S/2001 (1999 TC36) 1 | 140 | 7,720 | 50.38 | 2002 | ||
| (48939) 1995 TL8 | 350 | 160 | 420? | 0.5? | 2005 | |
| (50000) Quaoar | Weywot | 1140 | 100 | 11,000? | 19? | 2007 |
| (55637) 2002 UX25 | 650 | 200 | 5,000? | 8? | 2007 | |
| (58534) Logos | Zoe | 80 | 70 | 8,010 | 312 | 2002 |
| (60458) 2000 CM114 | 150 | 120 | 2,200? | 18? | 2006 | |
| (60621) 2000 FE8 | 150 | 120 | 1,200? | 7? | 2007 | |
| (65489) Ceto | Phorcys | 170 | 130 | 1,840 | 9.554 | 2006 |
| (66652) Borasisi | 170 | 140 | 4,660 | 46.26 | 2003 | |
| (79360) 1997 CS29 | 300 | 290 | 2,300? | 6? | 2005 | |
| (80806) 2000 CM105 | 220 | 130 | 2,700? | 15? | 2005 | |
| (82075) 2000 YW134 | 430 | 240 | 1,900? | 3? | 2005 | |
| (88611) Teharonhiawako | Sawiskera | 180 | 120 | 27,300 | 825 | 2001 |
| (90482) Orcus | "Vanth" | 910 | 260 | 8,700? | 11? | 2007 |
| (119067) 2001 KP76 | 150 | 140 | 8,900? | 130? | 2008 | |
| (119979) 2002 WC19 | 400 | 130 | 2,800? | 7? | 2007 | |
| (120347) 2004 SB60 | 550 | 190 | 3,300? | 5? | 2006 | |
| (123509) 2000 WK183 | 170 | 140 | 2,500? | 16? | 2007 | |
| (134860) 2000 OJ67 | 250 | 180 | 2,360 | 22.04 | 2005 | |
| (136108) Haumea | Namaka | 1380 | 160 | 25,660 | 18.278 | 2005 |
| Hi'iaka | 320 | 49,900 | 49.46 | 2005 | ||
| (136199) Eris | Dysnomia | 2670 | 350 | 37,400 | 15.772 | 2005 |
| (139775) 2001 QG298 | 180 | 170 | 170? | 0.57? | 2004 | |
| (148780) Altjira | 340 | 250 | 5,800? | 29? | 2007 | |
| (160091) 2000 OL67 | 170 | 130 | 7,800? | 100? | 2008 | |
| (160256) 2002 PD149 | 210 | 180 | 24,000? | 380? | 2007 | |
| (182933) 2002 GZ31 | 190 | 120 | 2,100? | 11? | 2007 | |
| (208996) 2003 AZ84 | 690 | 70 | 7,200? | 13? | 2007 | |
| 1998 WV24 | 110 | 80 | 1,400? | 16? | 2008 | |
| 1998 WW31 | S/2000 (1998 WW31) 1 | 130 | 110 | 22,300 | 574 | 2001 |
| 1999 OJ4 | 170 | 90 | 3,270 | 84.12 | 2005 | |
| 1999 RT214 | 120 | 70 | 3,300? | 60? | 2006 | |
| 1999 RY214 | 120 | 80 | 1,800? | 20? | 2009 | |
| 1999 XY143 | 210 | 180 | 2,600? | 14? | 2009 | |
| 2000 CF105 | 170 | 120 | 23,000? | 600? | 2002 | |
| 2000 CQ114 | 160 | 130 | 5,900? | 60? | 2004 | |
| 2000 QL251 | 180 | 180 | 5,000 | 56.45 | 2006 | |
| 2000 WT169 | 200 | 170 | 2,800? | 15? | 2009 | |
| 2001 FL185 | 140 | 100 | 1,900? | 16? | 2007 | |
| 2001 QC298 | 240 | 220 | 3,690 | 19.23 | 2002 | |
| 2001 QQ322 | 200 | 190 | 4,000? | 25? | 2008 | |
| 2001 QW322 | 90 | 90 | 120,000 | 9,900 | 2001 | |
| 2001 QY297 | 280 | 230 | 2,800? | 12? | 2007 | |
| 2001 RZ143 | 200 | 190 | 1,400? | 7? | 2007 | |
| 2001 XR254 | 240 | 230 | 9,270 | 125.6 | 2007 | |
| 2002 VF130 | 130 | 110 | 22,000? | 700? | 2009 | |
| 2002 VT130 | 260 | 210 | 2,500? | 10? | 2009 | |
| 2002 XH91 | 280 | 180 | 20.000? | 200? | 2009 | |
| 2003 QA91 | 360 | 260 | 1,800? | 5? | 2007 | |
| 2003 QR91 | 190 | 170 | 1,700? | 9? | 2007 | |
| 2003 QW111 | 230 | 120 | 10,400? | 140? | 2006 | |
| 2003 QY90 | 140 | 100 | 14,200 | 444 | 2003 | |
| 2003 TJ58 | 100 | 80 | 3,770 | 137.3 | 2007 | |
| 2003 UN284 | 130 | 100 | 64,000? | 1,700? | 2003 | |
| 2003 WU188 | 250 | 180 | 1,300? | 4? | 2007 | |
| 2003 YU179 | 150 | 80 | 2,300? | 20? | 2009 | |
| 2004 PB108 | 210 | 120 | 10,480 | 97.05 | 2007 | |
| 2005 EO304 | 210 | 120 | 87,000? | 7,400? | 2005 | |
| 2005 PR21 | 240 | 140 | 3,700? | 22? | 2008 | |
| 2006 SF369 | 140 | 140 | 3,100? | 28? | 2008 | |
| 2007 TY430 | 140 | 140 | 13,000? | 360? | 2008 | |
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 63 binary/multiple TNOs (or SDOs/centaurs) (through December 2009) in a population of 1,357 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.
© 2001-2008, 2009 by Wm. Robert Johnston.
Last modified 18 December 2009.
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