A Statistical Test for the Population of Contact Binaries in the Kuiper Belt

New Horizons images of Kuiper Belt object (486958) 2014 MU69 dramatically revealed that it is a contact binary, comprising two flattened lobes attached by a very narrow neck. In spite of its highly elongated shape, MU69 has a rotational light curve with an amplitude that was too small to be measured from Earth or during the approach of the spacecraft in December 2018. This surprising result is, in part, a consequence of MU69's bilobate shape; in general, contact binaries have light curve amplitudes that are systematically lower than those of equivalent-area ellipsoids.

The prevalence of contact binaries among the cold classical KBOs (CCKBOs) has been a topic of investigation on several fronts. We find that an important constraint on the answer to this question can be inferred from the statistical distribution of light curve amplitudes among CCKBOs. We have modeled these distributions based on a variety of assumptions, including rough or irregular shapes, albedo variations, degree of flattening, the size of the neck, and the size ratio of the two lobes. We have also explored the dependence on the statistical distribution of KBO orientations, assuming that the rotation poles are either completely random or are preferentially oriented normal to the orbit plane. In all cases, the distribution of light curve amplitudes is predicted to be lower if contact binaries are prevalent among the CCKBOs.

At this time, the limited available data on CCKBO light curve amplitudes does not permit a definitive answer to the question of how many CCKBOs are contact binaries. However, the statistical test holds promise as more CCKBOs are observed. It would complement the shape determinations based on the direct modeling of KBOs with high-amplitude light curves, combined with further occultation studies of KBO shapes. Needed is a large and an unbiased sample of light curve amplitudes, one which does not focus on the high-amplitude light curves that are often of greatest interest.