Trans-Neptunian objects

Trans-Neptunian Objects

by Wm. Robert Johnston
last updated 3 September 2012

Dynamical classes:

Classes of outer solar system objects, with numbers of known objects (1,731, as of 3 September 2012) in parenthesis (see note below):

Centaurs--asteroid-like objects which (generally) cross the orbit of Neptune. First one discovered in 1977. (124)
Neptune Trojans--objects orbiting in Neptune's L4 and L5 positions (similar semimajor axis to Neptune, but orbiting 60° ahead of or behind Neptune). Of those known, 6 objects are in the L4 position and 2 in the L5 position. First one discovered in 2001. (8)
Trans-Neptunian Objects (TNOs) or Kuiper Belt Objects (KBOs)--non-cometary objects with semimajor axes beyond Neptune's orbit. First one discovered in 1992. (1,476) Subgroups include:
- 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. (243)
- Resonance objects--TNOs in resonances with Neptune (other than Plutinos and Neptune Trojans). Resonance locations with known objects include: 4:5 (5), 3:4 (10), 3:5 (15), 4:7 (26), 5:9 (3), 8:15 (1), 1:2 (19), 3:7 (5), 5:12 (1), 2:5 (17), 1:3 (4), 1:4 (1), and 1:5 (1). (108 total)
- Cubewanos--TNOs with eccentricities below 0.15 and with semimajor axes between 41.8 AU and 48 AU. First one discovered in 1992. (839)
- Haumea family members--A subgroup of Cubewanos, TNOs which are likely fragments of dwarf planet Haumea. These are identified based on similarities in both orbit and apparent composition. (11)
- 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 resonance objects.) (253)
- Other TNOs not fitting in the above groups. (21)
- Inner Oort cloud objects?--objects orbiting beyond the classical Kuiper belt, with perihelia greater than 55 AU. (1)
Other asteroids with aphelion distances greater than 7.5 AU. (123) 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. (66)
- Apollos--Asteroids with perihelion distances less than 1 AU (those included here are those with aphelion distances greater than 7.5 AU. (2)
- Amors--Asteroids with perihelion distances between 1 AU and 1.2 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. (54)

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)

Five trans-Neptunian objects--(87269) 2000 OO67, (90377) Sedna, (308933) 2006 SQ372, 2007 TG422, and 2012 DR30--have aphelion distances beyond 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 2012 OP; these may be inactive comets.) A total of 80 TNOs (plus 18 more unusual objects) 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 51.6 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 1,731 outer solar system objects counted above, 44 have inclinations from 40° to 90°, and 40 "unusual" asteroids 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 34 largest known TNOs (plus one satellite) are, with estimated diameters:

(134340) Pluto (plutino), 2350 km
(136199) Eris (scattered disk object), 2330 km
(136472) Makemake (cubewano), 1470 km
(225088) 2007 OR10 (scattered disk object), 1280 km
(50000) Quaoar (cubewano), 1170 km
Charon (satellite of Pluto), 1212 km
(136108) Haumea (Haumea family), 1150 km
(84522) 2002 TC302 (2:5 resonance object), 1150 km
(90377) Sedna (Inner Oort cloud object?), 1000 km
(202421) 2005 UQ513 (cubewano), 920 km?
(19308) 1996 TO66 (Haumea family), 900 km
(120347) Salacia (cubewano), 900 km
(307261) 2002 MS4 (cubewano), 840 km
(120178) 2003 OP32 (Haumea family), 840 km?
(174567) 2003 MW12 (cubewano), 820 km?
(90482) Orcus (plutino), 810 km
(20000) Varuna (cubewano), 780 km
2006 QH181 (scattered disk object), 760 km?
(55565) 2002 AW197 (cubewano), 750 km
(278361) 2007 JJ43 (cubewano), 730 km?
(28978) Ixion (plutino), 730 km
(208996) 2003 AZ84 (plutino), 720 km
(24835) 1995 SM55 (Haumea family), 700 km
2010 KZ39 (cubewano), 700 km?
(145453) 2005 RR43 (Haumea family), 700 km?
(230965) 2004 XA192 (scattered disk object), 700 km?
(145452) 2005 RN43 (cubewano), 680 km
(90568) 2004 GV9 (cubewano), 680 km
(55637) 2002 UX25 (cubewano), 650 km
(42301) 2001 UR163 (scattered disk object), 640 km?
2003 UZ413 (plutino), 640 km?
2010 RF43 (scattered disk object), 640 km?
2010 RE64 (scattered disk object), 610 km?
2008 ST291 (scattered disk object), 610 km?
2010 FX86 (cubewano), 610 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 52 TNOs appear to be at least 500 km in diameter and 139 at least 300 km. The total volume of known TNOs (including Pluto) is estimated at 152% of that of the Moon (35% in Pluto/Charon and 30% in Eris). (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 (with additional discoveries later bringing Pluto's total to five satellites). Data for the 79 known TNO binary or multiple systems are given below:

TNO
(134340)
	(26308) 1998 SM165
	(38628) Huya
	(42355) Typhon
	(47171)
(48939) 1995 TL8	(47171)
(50000) Quaoar
(55637) 2002 UX25
(58534) Logos
(60458) 2000 CM114
(60621) 2000 FE8
(65489) Ceto
(66652) Borasisi
(79360) Sila-Nunam
(80806) 2000 CM105
(82075) 2000 YW134
(82157) 2001 FM185
(88611)
(90482) Orcus
(119067) 2001 KP76
(119979) 2002 WC19
(120347) Salacia
(123509) 2000 WK183
(134860) 2000 OJ67
(136108)
(136108)	(136199) Eris
(139775) 2001 QG298
(148780) Altjira
(160091) 2000 OL67
(160256) 2002 PD149
(174567) 2003 MW12
(182933) 2002 GZ31
(208996) 2003 AZ84
(229762) 2007 UK126
(275809) 2001 QY297
(303712) 2005 PR21
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 RZ143
2001 XP254
2001 XR254
2002 VF130
2002 VT130
2002 XH91
2003 FE128
2003 HG57
2003 QA91
2003 QR91
2003 QW111
2003 QY90
2003 TJ58
2003 UN284
2003 WU188
2003 YS179
2003 YU179
2004 KH19
2004 PB108
2005 EF298
2005 EO304
2005 GD187
2005 VZ122
2006 BR284
2006 CH69
2006 JZ81
2006 SF369
2007 TY430
[hst5]

companion(s)component diameters (km)separation (km)orbital period (d)year
reported align="left"> PlutoCharon 23501212 19,571 6.3871978 S/2012 (134340) 1 15 42,000 20.2 2012 Nix 70 49,240 25.49 2005 S/2011 (134340) 1 25 59,000 32.1 2011 Hydra 60 65,080 38.73 2005 S/2001 (26308) 1 280 90 11,370 130.2 2002 390200 1,800? 3? 2012 Echidna 150 80 1,630 18.9712006 1999 TC36 290260 870 1.9072007 S/2001 (1999 TC36) 1 140 7,410 50.38 2002 330150 420? 0.5? 2005 Weywot 890 60 14,500 12.44 2007 650210 5,000? 8? 2007 Zoe 80 70 8,220 309.9 2002 170130 2,200? 15? 2006 150110 1,200? 8? 2007 Phorcys 180140 1,840 9.5542006 Pabu 160130 4,530 46.29 2003 Nunam 240250 2,780 12.5 2005 160120 2,700? 22? 2005 440220 1,900? 4? 2005 140220 3,100? 32? 2011 TeharonhiawakoSawiskera 150110 27,700 828.8 2001 Vanth 900370 9,010 9.5392007 160140 8,900? 130? 2008 400130 2,800? 7? 2007 Actaea 930320 5,600 5.4942006 110100 2,370 30.92 2007 140110 2,360 22.04 2005 HaumeaNamaka 1300160 25,660 18.2782005 Hi'iaka 320 49,900 49.46 2005 Dysnomia 2330300 37,400 15.7722005 140120 180? 0.5742004 160150 9,900 139.6 2007 150120 7,800? 120? 2008 190150 24,000? 470? 2007 820420 4,200? 4? 2011 200110 2,100? 12? 2007 680 70 7,200? 13? 2007 910160 3,600? 3.1? 2011 160150 9,960 138.1 2007 230140 3,700? 23? 2008 100 90 1,400? 15? 2008 S/2000 (1998 WW31) 1 150120 22,300 574 2001 80 70 3,270 84.12 2005 100 70 3,300? 64? 2006 120 80 1,500? 15? 2009 210180 2,700? 14? 2009 60 50 33,000 4,000 2002 140120 5,900? 90? 2004 150140 5,000 56.45 2006 200170 2,800? 16? 2009 140100 1,900? 17? 2007 240200 3,810 19.23 2002 170160 4,000? 33? 2008 130130 10,200 6,200 2001 170160 1,400? 7? 2007 110 80 1,200? 12? 2011 170140 9,300 125.6 2007 120110 22,000? 740? 2009 240190 2,500? 10? 2009 300180 20.000? 190? 2009 180130 2,100? 24? 2012 140140 13,000? 260? 2012 250240 1,800? 5? 2007 160150 1,700? 10? 2007 140 80 6,700 101.1 2006 80 90 8,500 309.7 2003 60 50 3,800 137.3 2007 120 80 56,000?3,190 2003 260190 1,300? 4? 2007 130110 7,800? 150? 2011 150 80 2,000? 18? 2009 180130 13,000? 210? 2011 240130 10,400 97.02 2007 210160 3,300? 20? 2011 150 80 70,000 3,580 2005 120100 7,600? 102? 2011 120 50 2,300? 50? 2011 90 70 25,300 1,500 2011 100 80 27,600 1,420 2010 120 80 32,300 1,500 2011 140140 3,100? 28? 2008 80 80 21,000 961.2 2008 140 ? 16,000? 135? 2011

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 79 binary/multiple TNOs (or SDOs/centaurs) (through September 2012) in a population of 1,608 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.