The following have been suggested as agents for capture:

Gas Drag Capture
Bloated atmospheres around the young gas giant planets could exert frictional drag forces on passing bodies, just like the Earth's atmosphere exerts drag on meteoroids that strike it. Small bodies would burn up, large ones would pass straight through while those of intermediate size could be captured. A necessary feature of this model is that the bloated atmosphere has to dissipate, so that the satellites are not dragged all the way into the planet. For historical reasons, this mechanism has received the most attention. Ice giants Uranus and Neptune do not hold much gas and their irregular satellites were probably not captured this way.

Pull-Down Capture
The region over which a planet exerts gravitational control (in competition with the Sun) is called the Hill sphere. As the planets grew, their Hill spheres expanded (but only like the cube root of the mass). Gas giants experience runaway growth, leading to a sudden expansion of the Hill sphere that might trap nearby objects from heliocentric orbit. Ice giants Uranus and Neptune did not experience runaway growth and their irregular satellites were probably not captured this way.

3-Body Capture
If two bodies collide (or gravitationally interact) in the vicinity of a third, one may be ejected while the other is trapped into a bound orbit about the major mass. This 3-body capture has always seemed unlikely (because it is hard to get 3 bodies in the right places at the right times) and there has not been much quantitative work in the literature to explore it. Still, I think it could be the right model, all the same!

It has been suggested that asymmetrically outgassing comets might propel themselves into permanently bound orbits after being temporarily trapped by entry to the Hill Sphere through a Lagrangian point. This seems an unlikely explanation, though, because many satellites are too large to be accelerated significantly by outgassing forces.

A new model of the Solar System hints that irregular satellites could be captured in a chaotic phase when the planets migrated through a swarm of scattered KBOs (Morbidelli et al. 2005 Nature 435, 462). This model has promise but awaits fuller explication.

David Jewitt.

Irregular Satellites Page