Contents:

• Dynamical classes
• Distribution by distance
• Largest objects
• Binary objects
• General links

Link to List of known trans-Neptunian objects and Centaurs

Dynamical classes:

Classes of outer solar system objects, with numbers of known objects with MPC designations (2,946, as of 22 April 2018) in parenthesis (see note below):

• Centaurs--asteroid-like objects which (generally) cross the orbit of Neptune. First one discovered in 1977. (230+129)
• Uranus Trojans--object orbiting in Uranus's L4 position (similar semimajor axis to Uranus, but orbiting 60° ahead of Uranus). First one discovered in 2011, identified as such in 2013. (1)
• 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, 13 objects are in the L4 position and 4 in the L5 position. First one discovered in 2001. (17)
• Trans-Neptunian Objects (TNOs) or Kuiper Belt Objects (KBOs)--non-cometary objects with semimajor axes beyond Neptune's orbit. First one discovered in 1992. (2,330) 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. (246+100 possible members)
  • Resonance objects--TNOs in resonances with Neptune (other than Plutinos and Neptune Trojans). Resonance locations with known objects include: 4:5 (7), 3:4 (21), 5:8 (2), 3:5 (38), 4:7 (27), 5:9 (5), 6:11 (4), 1:2 (48), 4:9 (3), 3:7 (8), 5:12 (4), 2:5 (40), 3:8 (3), 4:11 (1), 1:3 (8), 4:13 (1), 3:10 (1), 2:7 (4), 3:11 (2), 1:4 (2), 2:9 (2), 2:11 (2), 1:6 (3), and 1:9 (1). These resonance locations range from 1.16 to 4.33 times the semimajor axis of Neptune's orbit, or from 34.9 to 130.1 AU. (237 total)
  • Cubewanos--TNOs generally with eccentricities below 0.15 and with semimajor axes between 41.8 AU and 48 AU, or otherwise classfied as "classical" TNOs. First one discovered in 1992. (1,151)
  • 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. (12)
  • 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.) (316)
  • Other TNOs not fitting in the above groups. (266)
  • Inner Oort cloud objects?--objects orbiting beyond the classical Kuiper belt, with perihelia greater than 55 AU. The first one, (90377) Sedna, was discovered in 2003. (2)
• Other asteroids with aphelion distances greater than 7.5 AU. (239) This includes a few members of the following groups:
  • Damocloids--A loosely defined class of asteroids with cometary orbits, those counted here include "unusual" minor planets with semimajor axes greater than 7.5 AU. First one discovered 1991. (118)
  • Apollos--Asteroids with perihelion distances less than 1 AU (those included here are those with aphelion distances greater than 7.5 AU. (6)
  • 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. (3)
  • Other unusual objects--Minor planets identified by the MPC as unusual. (110)
  • Other asteroids--other asteroids with aphelion distances greater than 7.5 AU. (2)

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.

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)

Including Sedna, 12 trans-Neptunian objects have aphelion distances beyond 800 AU. (Another 5 unusual asteroids have aphelia over 800 AU; these may be inactive comets.) A total of 66 TNOs (plus 13 more unusual objects) have aphelion distances exceeding 200 AU. Relatively few (10) have been discovered with perihelion distances greater than 47 AU, and only 6 with perihelion distances greater than 50 AU. Data thus far suggests than there is indeed a cutoff to the classical TNO population around 47 AU, although the discoveries of Sedna and 2012 VP113 suggest an unrecognized population of much more distant objects (see below).

Of the 2,946 outer solar system objects counted above, 92 (3.1%) have inclinations from 40° to 90°, and 91 (3.1%) have retrograde orbits. Most if not all of the 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 30 largest known TNOs (plus one satellite) are, with estimated diameters:

• (134340) Pluto (plutino), 2377 km
• (136199) Eris (scattered disk object), 2326 km
• (136108) Haumea (Haumea family), 1600 km
• (225088) 2007 OR10 (3:10 resonance object), 1540 km
• (136472) Makemake (other TNO), 1430 km
• Charon (satellite of Pluto), 1212 km
• 2013 FY27 (scattered disk object), 1100 km?
• (50000) Quaoar (cubewano), 1070 km
• (90377) Sedna (Inner Oort cloud object), 1000 km
• (307261) 2002 MS4 (cubewano?), 930 km
• (90482) Orcus (plutino), 910 km
• (120347) Salacia (cubewano?), 850 km
• 2015 RR245 (2:9 resonance object), 810 km?
• 2014 EZ51 (scattered disk object), 810 km?
• 2010 RF43 (scattered disk object), 810 km?
• (208996) 2003 AZ84 (plutino), 770 km
• (55565) 2002 AW197 (other TNO), 770 km
• (120178) 2003 OP32 (Haumea family), 740 km?
• 2015 KH162 (scattered disk object), 740 km?
• (174567) Varda (cubewano?), 720 km
• 2014 WK509 (scattered disk object), 700 km?
• 2012 VP113 (Inner Oort cloud object), 700 km?
• 2010 KZ39 (cubewano?), 700 km?
• (145453) 2005 RR43 (Haumea family), 700 km?
• 2010 JO179 (scattered disk object), 700 km?
• (24835) 1995 SM55 (Haumea family), 700 km
• V774104 (Inner Oort cloud object?), 700 km?
• (90568) 2004 GV9 (cubewano?), 680 km
• (145452) 2005 RN43 (other TNO), 680 km
• (20000) Varuna (cubewano?), 680 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 75 TNOs appear to be at least 500 km in diameter and 318 at least 300 km. The total volume of known TNOs (including Pluto) is estimated at 205% of that of the Moon (36% in the Pluto system, 31% in the Eris system, and 10% in the Haumea system). (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.) For more discussion of large TNOs see:

• The 1000 km Scale KBOs,
• Beyond Pluto, and
• Frequently Asked Questions about Quaoar.

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:

Below is a plot of TNO and asteroid size ratios vs. separation:

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 six more TNOs has been suggested (for (20000) Varuna, (32929) 1995 QY9, (33128) 1998 BU48, (47932) 2000 GN171, 2003 SQ317, and 2010 WG9). 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). In 2013 the Centaur (10199) Chariklo was found to have a ring, followed by the TNO Haumea in 2017. Data for the 86 TNOs known to have satellites or rings are given below:

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:

• Binary TNOs and Centaurs
• Binary Kuiper Belt Objects.

With 86 binary/multiple TNOs (or SDOs/centaurs) (through April 2018) in a population of 2,707 objects, it appears 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.

© 2001-2017, 2018 by Wm. Robert Johnston.
Last modified 22 April 2018.
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