Apr15

April 15 lecture

The Kuiper Belt

The Kuiper Belt is a relatively recent development in planetary science. Although long theorized, the first KBO was not discovered until 1992. Now there are thousands of known objects. There are several important classes:

Centaurs are objects that orbit between Jupiter and Neptune.
Classical KBOs are the "main belt" of the Kuiper Belt, well outside the orbit of Neptune (which is at 30 AU). They are in relatively circular, co-planar orbits between 40 AU and 47 AU from the Sun.
Plutinos, or "little Plutos," occupy the 3:2 resonance with Neptune, at 39.4 AU from the Sun. That is, Plutinos orbit the Sun exactly twice for every 3 Neptune orbits. Pluto, of course, is in this same resonance. This relationship gives Plutinos a certain amount of stability, even though their orbits can be very eccentric and inclined. A few KBOs can also be found in the 2:1 resonance with Naptune, at 47.7 AU from the Sun. These objects have been dubbed "Twotinos".
Scattered KBOs are in much more eccentric orbits than the Classical KBOs, but are not in any resonance like the Plutinos. They are probably former Classical KBOs that have been ejected from their nice-and-tidy orbits by an encounter with Neptune.

From Wikipedia:
KBO positions

Known objects in the Kuiper belt. Objects in the main belt are green, while scattered objects are orange. The four outer planets (J, S, U, N) are blue. Neptune's few known Trojan asteroids are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.

Outer solar system animation (5.0 MB -- Better to download rather than play on your browser)

Symbols in the animation
Red circles	Classical KBOs
White circles	Plutinos
Magenta circles	Scattered KBOs
Orange triangles	Centaurs
Cyan triangles	"High-eccentricity objects"
Blue squares (filled)	Short-Period Comets
Blue squares (unfilled)	Long-Period Comets

The Oort Cloud

The Kuiper Belt is one of the two main comet reservoirs; the other is the Oort Cloud. In 1950, Jan Oort was the first to notice that long-period comets all have closed orbits (that is, they don't come from interstellar space), that their aphelion distance tends to cluster around 50,000 AU, and that they don't have any preferred direction. Using these observations, Oort hypothesized a comet reservoir that forms a spherical shell around the Sun, with the greatest concentration at 50,000 AU. Although Oort Cloud objects are much too far from the Sun to be observed, subsequent observations of comets have refined the Oort Cloud model, and computerized models have helped to explain its origin and structure.

Oort cloud

Large Kuiper Belt Objects

Some KBOs have proven to be quite large, and at least one is larger than Pluto.

eight TNOs

The largest KBO so far discovered is Eris, discovered in 2003, which is slightly larger than Pluto and has at least one moon, Dysnomia. A year earlier, Quaoar was discovered. At 1200 km diameter, it is nearly as large as Charon (Pluto's moon) More info on Quaoar. Other large KBOs have also been discovered, driving home the point that not much is particularly special about Pluto, except for the fact that it is one of the larger KBOs.

some large KBOs	Diameter (km)	Class	Discovered
Pluto	2320	Plutino	1930
Charon	1270	Plutino (moon)	1978
Quaoar	1200 +/- 200	Classical	2002
Ixion	1065 +/- 165	Plutino	2001
Varuna	900 +/- 140	Classical	2000
Eris	2400 +/- 100	Scattered	2003

Multiple Kuiper Belt Objects

One of the most fascinating discoveries about the Kuiper Belt is that many of them have one or more large moons. The first binary KBO to be discovered is called 1998 WW31, and more are now known. Each of these binaries is made up of two objects orbiting one another, and it is difficult to say which one is the "moon" and which is the "primary." WW31 is less than twice as large as its companion! By comparison, Pluto (the next closest thing to a "double planet") is 8 times more massive than its moon Charon, while Earth is 80 times more massive than the Moon. On the other hand, these binaries are nowhere near planet-size: the combined mass of WW31 is 5,000 times smaller than the combined Pluto and Charon.

Binary KBOs can also have unusually high eccentricity. WW31 has one of the more eccentric satellites in the known Solar System. The separation between the two bodies varies by a factor of 10: from 4,000 to 40,000 km!

binary KBO

Theories of Satellite Formation

Binary Kuiper Belt Objects are like nothing else in the Solar System. Not only is the "moon" practically as large as the "primary," but they can have large eccentricity, moving quite far from each other while remaining in orbit. In contrast, although some asteroids have also been found to have moons, asteroid moons are always very close to the asteroid they orbit, and much much smaller. Let's review the three major ways to form a satellite:

Co-Accretion -- Regular systems of satellites, like those of Jupiter and Saturn, probably formed out of a "mini-nebula," very similar to the larger nebula out of which the whole Solar System formed.
Re-Accretion of Impact Debris -- Asteroid moons are probably the result of impact debris that was accelerated into orbit. Once in orbit, a ring of debris can re-accrete into a single moon. On a much larger scale, Earth's Moon (and possibly Pluto's Charon) formed in essentially the same way.
Capture -- An object from elsewhere in the Solar System can be captured into orbit if something slows it down while it is approaching. Neptune's Triton is probably a captured KBO, having collided with (and obliterated) a pre-existing Neptunian moon.

The method by which binary KBOs are formed is still unknown, and all three of these mechanisms are possibilities. They could have formed as "twins" out of the solar nebula, they could be the result of a large impact that split a KBO in two, or they could be the result of capture. Like many things about the Kuiper Belt, this is an ongoing frontier of research.