A majority of the observed Kuiper Belt Objects maintain large separations from Neptune even when at perihelion. The archetypal "Classical KBO" is 1992 QB1. Such objects are able to survive for the age of the solar system without the special protection offered by resonances to the Plutinos, simply because they are already Neptune-avoiding. The CKBOs are found mostly with semi-major axes between about 42 and 48 AU. The deficiency of more distant CKBOs is real: the Classical Belt has an outer edge at about 50 AU (Jewitt et al. 1998) (PS - about 3 Mbyte). The CKBOs are "classical" in the sense that their orbits tend to have small eccentricities as is expected of bodies formed by quiet agglomeration in a dynamically cool disk. There is still evidence for excitation, though, as discussed below.


The inclinations (see also PS version, PDF version) of the Classical KBOs range up to very high values (1996 RQ20 and 1997 RX9 have i > 30 degrees). This suggests that the inclinations have been excited by some agency yet to be identified. Two ideas have been suggested for the excitation mechanism:

i) A few massive planetesimals might have been scattered into the Kuiper Belt in the early days by Neptune. These objects could excite the inclinations of the CKBOs. One problem with this hypothesis is that massive planetesimals (they would have to approach Earth mass in order to be effective) would also disturb and depopulate the resonances. That we see many Plutinos is evidence against the action of massive planetesimals.

ii) A passing star might have stirred up the CKBOs. Proponents of this idea claim, based on numerical simulations, that the Classical objects can be excited while the Plutinos remain relatively undisturbed. One obvious problem with the external perturbation hypothesis is that passing stars rarely pass close enough to the sun (a miss distance of a few 100 AU is required). However, it is possible (likely?) that the sun formed with other stars in a cluster that might have been initially very dense. In this case, the early rate of close stellar passages might have been much higher than at present.

The outer edge of the Classical Kuiper Belt, near 50 AU, could also be a result of distrurbance by a close encounter with a passing star. This scenario has been explored by Ida et al. (2000) (PostScript - about 0.8 Mbyte)

It is worth noting that stellar close approaches and resulting tidal truncation have been suggested as the cause of the sharp edged and small disk-like structures known as Proplyds. Some proplyds are only 50 AU to 100 AU across, similar to the diameter of the known portion of the Classical Kuiper Belt.

Kuiper Belt

Last Update February 2000