Kuiper Belt Objects

Deborah & William Hillyard

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Trans-Neptunian Objects

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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 50ºK (-223ºC). KBOs are split into several categories:

Comparative orbits of some larger Cubewanos.
Credit

	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

Artists impression of Varuna, a Cubewano. In truth, Varuna is likely to be ellipsoidal due to its rapid rotation. (no larger image).

Credit: NASA

Kuiper Belt Objects (KBOs)

Pluto & Charon taken by the Hubble space telescope in 1994.

A recent image of Pluto taken by the Hubble space telescope in Feb 2010, and computer enhanced in true color.

Images

Twotinos

Cubewanos

	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

	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

Pluto, its companion Charon and two tiny moons Nix and Hydra taken by the Hubble space telescope in 2005.

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, thus coming within Neptune's orbit. Pluto is 2,306 km (1,434 miles) across, compared to the Moon which is 3,474 km (2,159) in diameter, so it is relatively small. In fact, the SDO Eris is more than 25% heavier, though only a few percent larger. It has three moons: Charon (diameter 1,207 km/750 miles) and two that are much smaller Nix (diameter 46-136 km/29-85 miles) and Hydra (diameter 60-167 km/37-104 miles).

Plutinos

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.

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.

Credit

In mid-2011, the New Horizons science team used the Hubble Space Telescope, to identify Pluto's fourth moon. It

is smaller than the other three and, temporarily, is called by the less than romantic name P4. More information when New Horizons arrives at Pluto, I expect!

Credit: NASA, ESA & M. Showalter

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