Trans-Neptunian objects

Trans-Neptunian Objects

by Wm. Robert Johnston
last updated 18 December 2009

Dynamical classes:

Classes of outer solar system objects, with numbers of known objects (1,432, as of 13 December 2009) in parenthesis (see note below):

Centaurs--asteroid-like objects with semimajor axes between those of Jupiter and Neptune. First one discovered in 1977. (86)
Neptune Trojans--objects orbiting in Neptune's L4 and L5 positions (similar semimajor axis to Neptune, but orbiting 60° ahead of or behind Neptune). All currently known objects are in the L4 position. First one discovered in 2001. (6)
Trans-Neptunian Objects (TNOs) or Kuiper Belt Objects (KBOs)--non-cometary objects with semimajor axes beyond Neptune's orbit. First one discovered in 1992. (1,265) Subgroups include:
- Inner KBO--TNOs orbiting near 38 AU. (15)
- Plutinos--TNOs near the 2:3 resonance with Neptune. Pluto, discovered in 1930, is the largest object near this resonance. First one other than Pluto discovered in 1993. (107+104 possible)
- Resonance objects--TNOs in resonances with Neptune (other than Plutinos and Neptune Trojans). Resonance locations with known objects include: 4:5 (5), 3:4 (8), 3:5 (11), 4:7 (23), 5:9 (3), 1:2 (17), 3:7 (4), 2:5 (16), 1:3 (2), 1:4 (1), and 1:5 (1). (91 total)
- Cubewanos--TNOs with eccentricities below 0.15 and with semimajor axes between 41.8 AU and 48 AU. First one discovered in 1992. (361+359 possible)
- Outer KBO--TNOs orbiting outside the main Cubewano belt. (2)
- Scattered disk objects (SDOs)--TNOs with aphelion distances near Neptune's orbit and semimajor axes greater than 50 AU. First one discovered in 1995. (Note: this count does not include the 18 objects in 2:5 and 1:3 resonances.) (140)
- Other TNOs not fitting in the above groups. (85)
- Inner Oort cloud objects?--objects orbiting beyond the classical Kuiper belt, with perihelia greater than 50 AU. (1)
Other asteroids with aphelion distances greater than 7.5 AU. (75) This includes a few members of the following groups:
- Damocloids--A loosely defined class of asteroids with cometary orbits, includes "unusual" minor planets with semimajor axes greater than 7.5 AU. First one discovered 1991. (31)
- Apollos--Asteroids with perihelion distances less than 1 AU (those included here are those with aphelion distances greater than 7.5 AU. (1)
- Other unusual objects--Minor planets identified by the MPC as unusual. (43)

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.).

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).

Largest objects:

The 32 largest known TNOs (plus one satellite) are, with estimated diameters:

(136199) Eris (scattered disk object), 2670 km
(134340) Pluto (plutino), 2340 km
(225088) 2007 OR10 (SDO), up to 1800 km?
(90377) Sedna (Inner Oort cloud object?), 1600 km
(136472) Makemake (cubewano), 1500 km
(136108) Haumea (other TNO), 1260 km
(84522) 2002 TC302 (2:5 resonance object), 1150 km
(50000) Quaoar (cubewano), 1140 km
Charon (satellite of Pluto), 1212 km
(202421) 2005 UQ513 (other TNO), 920 km?
2005 QU182 (SDO), 920 km?
(90482) Orcus (plutino), 910 km
(19308) 1996 TO66 (cubewano), 900 km
2007 UK126 (SDO), 880 km?
(174567) 2003 MW12 (cubewano), 840 km?
(20000) Varuna (cubewano), 800 km
(55565) 2002 AW197 (other TNO), 770 km
2006 QH181 (SDO), 760 km?
(19521) Chaos (cubewano), 740 km
(145452) 2005 RN43 (cubewano?), 730 km?
(28978) Ixion (plutino), 730 km
2002 MS4 (other TNO), 730 km
(84922) 2003 VS2 (plutino), 720 km
(55636) 2002 TX300 (cubewano), 710 km
(24835) 1995 SM55 (other TNO), 700 km
(145453) 2005 RR43 (cubewano?), 700 km?
2004 XA192 (other TNO), 700 km?
(208996) 2003 AZ84 (plutino), 690 km
(90568) 2004 GV9 (cubewano), 680 km
(120178) 2003 OP32 (cubewano), 670 km?
(55637) 2002 UX25 (cubewano), 650 km
(42301) 2001 UR163 (scattered disk object), 640 km?
(15874) 1996 TL66 (scattered disk object), 600 km

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.

Binary objects:

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	c
(134340)
	(26308) 1998 SM165
	(42355) Typhon
(47171)
(47171)	(48939) 1995 TL8
(50000) Quaoar
(55637) 2002 UX25
(58534) Logos
(60458) 2000 CM114
(60621) 2000 FE8
(65489) Ceto
(66652) Borasisi
(79360) 1997 CS29
(80806) 2000 CM105
(82075) 2000 YW134
(88611)
(90482) Orcus
(119067) 2001 KP76
(119979) 2002 WC19
(120347) 2004 SB60
(123509) 2000 WK183
(134860) 2000 OJ67
(136108)
(136108)	(136199) Eris
(139775) 2001 QG298
(148780) Altjira
(160091) 2000 OL67
(160256) 2002 PD149
(182933) 2002 GZ31
(208996) 2003 AZ84
1998 WV24
1998 WW31
1999 OJ4
1999 RT214
1999 RY214
1999 XY143
2000 CF105
2000 CQ114
2000 QL251
2000 WT169
2001 FL185
2001 QC298
2001 QQ322
2001 QW322
2001 QY297
2001 RZ143
2001 XR254
2002 VF130
2002 VT130
2002 XH91
2003 QA91
2003 QR91
2003 QW111
2003 QY90
2003 TJ58
2003 UN284
2003 WU188
2003 YU179
2004 PB108
2005 EO304
2005 PR21
2006 SF369
2007 TY430

ompanion(s)component diameters (km)separation (km)orbital period (d)year
reported align="left"> PlutoCharon 23401212 19,571 6.3871978 Nix 90 48,680 24.8562005 Hydra 110 64,780 38.2062005 S/2001 (26308) 1 290100 11,300 130.1 2002 Echidna 130 80 1,630 18.9712006 1999 TC36 280260 900? 1.9? 2007 S/2001 (1999 TC36) 1 140 7,720 50.38 2002 350160 420? 0.5? 2005 Weywot 1140100 11,000? 19? 2007 650200 5,000? 8? 2007 Zoe 80 70 8,010 312 2002 150120 2,200? 18? 2006 150120 1,200? 7? 2007 Phorcys 170130 1,840 9.5542006 170140 4,660 46.26 2003 300290 2,300? 6? 2005 220130 2,700? 15? 2005 430240 1,900? 3? 2005 TeharonhiawakoSawiskera 180120 27,300 825 2001 "Vanth" 910260 8,700? 11? 2007 150140 8,900? 130? 2008 400130 2,800? 7? 2007 550190 3,300? 5? 2006 170140 2,500? 16? 2007 250180 2,360 22.04 2005 HaumeaNamaka 1380160 25,660 18.2782005 Hi'iaka 320 49,900 49.46 2005 Dysnomia 2670350 37,400 15.7722005 180170 170? 0.57?2004 340250 5,800? 29? 2007 170130 7,800? 100? 2008 210180 24,000? 380? 2007 190120 2,100? 11? 2007 690 70 7,200? 13? 2007 110 80 1,400? 16? 2008 S/2000 (1998 WW31) 1 130110 22,300 574 2001 170 90 3,270 84.12 2005 120 70 3,300? 60? 2006 120 80 1,800? 20? 2009 210180 2,600? 14? 2009 170120 23,000? 600? 2002 160130 5,900? 60? 2004 180180 5,000 56.45 2006 200170 2,800? 15? 2009 140100 1,900? 16? 2007 240220 3,690 19.23 2002 200190 4,000? 25? 2008 90 90120,000 9,900 2001 280230 2,800? 12? 2007 200190 1,400? 7? 2007 240230 9,270 125.6 2007 130110 22,000? 700? 2009 260210 2,500? 10? 2009 280180 20.000? 200? 2009 360260 1,800? 5? 2007 190170 1,700? 9? 2007 230120 10,400? 140? 2006 140100 14,200 444 2003 100 80 3,770 137.3 2007 130100 64,000?1,700? 2003 250180 1,300? 4? 2007 150 80 2,300? 20? 2009 210120 10,480 97.05 2007 210120 87,000?7,400? 2005 240140 3,700? 22? 2008 140140 3,100? 28? 2008 140140 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.

General links:

Distant EKOs: The Kuiper Belt Electronic Newsletter.
Unusual Minor Planets--Minor Planet Center lists.
Kuiper Belt Page by David Jewitt.
Asteroid Comet Connection.