Solar System -
Trans-Neptunian Objects
The inner most TNOs are called Kuiper Belt Objects (KBO), which have orbits close to the plane of the solar system beyond Neptune. It extends from about 2.8 to 5 billion miles from the sun, with most of the objects concentrated in a central area from 3.7 to 4.5 billion miles from the sun. In fact, rather than a belt, it is shaped like a doughnut or torus. KBOs are heavily influenced by Neptune, which can destabilize their orbits forcing objects either into the inner solar system, or outwards into the scattered disk. There are two distinct populations of KBOs. The "dynamically cold" objects have orbits that are close to circular, and close to the plane of the solar system. They are believed to have formed in place in the outer solar system. The "dynamically hot" objects have orbits that are more inclined to the plane, and they are believed to have formed further in, perhaps around Jupiter, and been forced outwards by migration of the gas giant planets. The second largest KBO is Makemake, which is about ¾ the size of Pluto, and is covered in a layer of methane, ethane, and possibly nitrogen ice, at a temperature of 50K (-223ºC). KBOs are split into several categories:
Comparative orbits of some of the larger Cubewanos.
Credit
Credit
| Cubewano | Diameter (km) |
Diameter (miles) |
Orbit Period (years) |
Orbit Size (billion km) |
Orbit Size (billion miles) |
| Makemake | 1,360 - 1,480 | 850 - 920 | c. 310 | 5.76 - 7.94 | 3.58 - 4.93 |
| Quaoar | 820 - 960 | 510 600 | c. 288 | 6.27 6.72 | 3.90 4.18 |
| Varuna | 500 - 1,000 | 310 - 625 | c. 283 | 6.12 - 6.78 | 3.80 - 4.21 |
| Chaos | c. 460 | c. 286 | c. 309 | 6.12 - 7.52 | 3.80 - 4.67 |
Kuiper Belt Objects (KBOs)
Twotinos
The 1:2 resonance is at the outer edge of the Kuiper Belt and objects here are referred to as twotinos. The semi-major axis of their orbits is around 7.15 billion km (4.5 billion miles) and they orbit the sun approximately once every 330 years. Very few objects have been found in this resonance as the 1:2 resonance is less stable than 2:3 resonance. It is possible they were more numerous and that many have been lost over billions of years. They are small objects, from about 30 km (20 miles) across to the largest found at about 400 km (250 miles) across.
Cubewanos
Cubewanos, or Classical Kuiper Belt Objects, are not in an orbital resonance with Neptune. They orbit in the 40-50 AU range (5.9 to 7.4 billion km or 3.7 to 4.6 billion miles) and do not cross Neptune’s orbit, unlike some Plutinos like Pluto itself. Makemake, Quaoar, Varuna and Chaos are considered Cubewanos. Currently, the formal definition is under assessment. A particularly interesting Cubewano is Logos. It is about 77 km across, and orbits the Sun every 305.8 years at a distance of between 6 and 7.6 billion km. It is a binary with its companion Zoe which has a diameter of about 66 km. They orbit each other every 312 days or so.
| Plutino | Diameter (km) |
Diameter (miles) |
Orbit Period (years) |
Orbit Size (billion km) |
Orbit Size (billion miles) |
| Orcus | 850 - 950 | 528 - 590 | c. 245 | 4.54 - 7.19 | 2.82 - 4.47 |
| Ixion | 430 - 910 | 267 - 565 | c. 250 | 4.50 - 7.37 | 2.80 - 4.58 |
| Huya | 430 - 550 | 267 - 342 | c. 248 | 4.27 - 7.53 | 2.65 - 4.68 |
| (84922) 2003 VS2 | 525 - 925 | 326 - 575 | c. 246 | 5.45 - 6.30 | 3.39 - 4.29 |
| Twotino | Diameter (km) |
Diameter (miles) |
Orbit Period (years) |
Orbit Size (billion km) |
Orbit Size (billion miles) |
| (20161) 1996 TR66 | 139 | 87 | c. 330 | 4.3 - 10.0 | 2.7 - 6.25 |
| (26308) 1998 SM165 | 287 | 180 | c. 328 | 4.47 - 9.75 | 2.8 - 6.1 |
| (119979) 2002 WC19 | C. 400 | C. 250 | c. 329 | 5.3 - 9.0 | 3.3 - 5.625 |
Plutinos
Are in a 2:3 resonance with Neptune; that is, for every 2 orbits a plutino makes, Neptune orbits the Sun 3 times, and their orbits are around the 250 year mark. Named after Pluto, the name refers only to the resonance and not to physical characteristics. Plutinos are in the inner part of the Kuiper Belt, and include about a quarter of the known KBOs. Plutinos include Pluto itself, which has a highly eccentric orbit varying between approximately 7.376 and 4.437 billion km (4.583 and 2.757 billion miles) from the Sun. There is more information and pictures in the section Pluto System within Trans-Neptunian Objects.