From Halopedia, the Halo wiki
Though overlooked in favor of Jupiter, Neptune is in a unique position to slingshot ships coming in from Pluto deep into the system with its gravity well. This only works when the two planets are relatively aligned (and when slipspace travel would be deemed unreasonable), so it is still a rather rare occurrence. Compared to the barren wastes that are its closest two neighbors, Neptune is relatively bustling.
Neptune's orbit has anomalies, though, which were believed to be caused by Pluto. However, Pluto was ill-suited for causing them.
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