Why Do They Matter?
The elongated, inclined orbits of the irregular satellites are inconsistent with formation in an equatorial pre-planetary accretion disk. The only plausible source of such orbits, especially of the retrograde orbits which numerically dominate the irregular satellite population, is capture.

Capture of a body initially in heliocentric orbit requires that some of the initial orbital energy be dissipated. In the modern-day Solar System, there is no obvious source of energy dissipation (e.g. there is not enough interplanetary gas to slow down passing asteroids and capture them around planets). For this reason, capture is presumed to have occurred when the gross properties of the Solar System were different from those prevailing now. Most likely this was at the epoch of planet formation, or soon afterwards.

The irregular satellites are scientifically important because they can tell us something about this early epoch of formation.

Here are some of the capture mechanisms.

David Jewitt.

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