Trans-Neptunian objects

Trans-Neptunian Objects

by Wm. Robert Johnston
last updated 1 October 2007

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):

Centaurs--asteroid-like objects with semimajor axes between those of Jupiter and Neptune. First one discovered in 1977. (64)
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. (1,211) 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. (92+114 possible)
- Resonance objects--TNOs in resonances with Neptune (other than Plutinos and Neptune Trojans). Resonance locations with known objects include: 4:5 (4), 3:4 (4), 3:5 (4), 4:7 (8), 5:9 (2), 1:2 (14), 3:7 (1), 2:5 (14), and 1:3 (1). (52 total)
- Cubewanos--TNOs with eccentricities below 0.15 and with semimajor axes between 41.8 AU and 48 AU. First one discovered in 1992. (294+376 possible)
- 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 8 objects in 2:5 and 1:3 resonances.) (127)
- Other TNOs not fitting in the above groups. (155)
- 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. (61) 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. (15)
- Apollos--Asteroids with perihelion distances less than 1 AU (those included here are those with aphelion distances greater than 7.5 AU. (2)
- Other unusual objects--Minor planets identified by the MPC as unusual. (44)

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:

(136199) Eris (scattered disk object), 2660 km
(134340) Pluto (plutino), 2340 km
(90377) Sedna (Inner Oort cloud object?), 1500 km
(136472) 2005 FY9 (cubewano), 1500 km
(136108) 2003 EL61 (other TNO), 1380 km
(50000) Quaoar (cubewano), 1260 km
Charon (satellite of Pluto), 1212 km
(84522) 2002 TC302 (2:5 resonance object), 1200 km
(90482) Orcus (plutino), 910 km
(19308) 1996 TO66 (cubewano), 900 km
(20000) Varuna (cubewano), 870 km
2003 MW12 (cubewano), 840 km?
2005 QU182 (SDO), 800 km?
(55565) 2002 AW197 (other TNO), 790 km
2006 QH181 (SDO), 760 km?
(19521) Chaos (cubewano), 740 km
(145452) 2005 RN43 (cubewano?), 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?
2003 AZ84 (plutino), 690 km
(90568) 2004 GV9 (cubewano), 680 km
(120178) 2003 OP32 (cubewano), 670 km?
2005 UQ513 (other TNO), 670 km?
(55637) 2002 UX25 (cubewano), 650 km
(42301) 2001 UR163 (scattered disk object), 640 km?
(15874) 1996 TL66 (scattered disk object), 630 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 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:

object	component diameters (km)		separation (km)	orbital period (d)
(134340) Pluto Charon Nix Hydra	2,340	1,212	19,571	6.38723
		90	48,680	24.8562
		110	64,780	38.2065
(26308) 1998 SM165	290	100	11,300	130.1
(42355) Typhon Echidna	130	80	1,300?	11?
(47171) 1999 TC36	380	140	7,720	50.38
(48939) 1995 TL8	350	160	420?	0.5?
(50000) Quaoar	1260	95	11,000?	10?
(55637) 2002 UX25	650	205	5,000?	8?
(58534) Logos Zoe	80	70	8,010	312
(60458) 2000 CM114	150	120	2,200?	18?
(60621) 2000 FE8	150	115	1,200?	6?
(65489) Ceto Phorcys	170	130	1,840	9.55
(66652) Borasisi	170	140	4,660	46.26
(79360) 1997 CS29	300	290	2,300?	6?
(80806) 2000 CM105	220	130	2,700?	15?
(82075) 2000 YW134	430	240	1,900?	3?
(88611) Teharonhiawako Sawiskera	180	120	27,300	825
(90482) Orcus	910	260	8,700?	11?
(119979) 2002 WC19	400	130	2,700?	7?
(120347) 2004 SB60	550	190	3,500?	5?
(123509) 2000 WK183	170	140	2,400?	17?
(134860) 2000 OJ67	250	180	2,300?	9?
(136108) 2003 EL61	1,380	310	49,500	49.12
(136108) 2003 EL61	1,380	170	39,300	34.7
(136199) Eris Dysnomia	2,660	350	36,000?	14?
(139775) 2001 QG298	170	170	240?	0.5739?
(148780) 2001 UQ18	340	250	5,800?	23?
(160256) 2002 PD149	280	?	?	?
1998 WW31	130	110	22,300	574
1999 OJ4	170	90	2,200?	17?
1999 RT214	100	70	3,300?	55?
2000 CF105	170	120	23,000?	500?
2000 CQ114	160	130	5,900?	70?
2000 QL251	180	180	7,000?	70?
2001 FL185	140	100	1,900?	16?
2001 QC298	240	220	3,690	19.23
2001 QW322	90	90	130,000?	1,500?
2001 QY297	280	230	2,800?	10?
2001 RZ143	200	190	1,400?	5?
2001 XR254	240	230	3,400?	15?
2002 GZ31	190	120	2,100?	13?
2003 AZ84	690	70	7,200?	13?
2003 QA91	280	260	1,800?	5?
2003 QR91	190	170	1,700?	8?
2003 QW111	230	120	10,000?	90?
2003 QY90	140	100	14,000	460
2003 TJ58	95	75	3,500?	70?
2003 UN284	130	100	60,000?	3,300?
2003 WU188	250	180	1,300?	4?
2004 PB108	210	120	5,400?	45?
2005 EO304	210	120	85,000?	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:

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