Fig. 39. Color-color diagram for Centaurs showing evidence for bimodality. From [32]

KBOs, as noted in a previous section (Figs. 31-33), are not. It is tempting to imagine that this effect (which is formally statistically significant) could be caused by past or present activity on the Centaurs. However, a search for correlations between Centaur color and such likely indicators as perihelion distance, semimajor axis, nucleus size, or current outgassing activity has revealed nothing of importance. The Centaur bimodality, if it is real, is unexplained.

4.7 Irregular Satellites

There are two, largely distinct types of planetary satellite, based on dynamical characteristics. The most familiar satellites have small eccentricities and inclinations, and orbit from a few to a few dozen planetary radii from their parent planets. These are the regular satellites, most thought to have formed by accretion within circumplanetary disks that were present around the planets during the formation epoch (the details of satellite formation in disks remain obscure and are the subject of interesting speculation and ongoing research). Other satellites, out-numbering the regulars by a considerable margin, follow eccentric and highly inclined orbits with large semimajor axes. These "irregular satellites" in fact sweep-out a considerable fraction of the Hill spheres of their planets. The Hill sphere is the region in which planetary gravity is dominant over Solar gravity and has radius (roughly the distance from the planet to the innermost Lagrange point) of

Table 7. Planetary hill spheres


Mass/M® a

a [AU]6

«H [AU]C

On [deg]d


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