While for the first decade of the study of the Kuiper belt, a gap existed between the sizes of the relatively small and faint Kuiper belt objects (KBOs) that were being studied and the largest known KBO, Pluto, recent years have seen that gap filled and the maximum size even expanded. These large KBOs occupy all dynamical classes of the Kuiper belt with the exception of the cold classical population, and one large object, Sedna, is the first member of a new more distant population beyond the Kuiper belt. Like Pluto, most of the large KBOs are sufficiently bright for detailed physical study, and, like Pluto, most of the large KBOs have unique dynamical and physical histories that can be gleaned from these observations. The four largest known KBOs contain surfaces dominated in methane, but the details of the surface characteristics differ on each body. One large KBO is the parent body of a giant impact that has strewn multiple fragments throughout the Kuiper belt. The large KBOs have a significantly larger satellite fraction than the remainder of the Kuiper belt, including the only known multiple satellite systems and the relatively smallest satellites known. Based on the completeness of the current surveys, it appears that approximately three more KBOs of the same size range likely still await discovery, but that tens to hundreds more exist in the more distant region where Sedna currently resides.
The Solar System Beyond Neptune
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