The last decade witnessed the discovery of numerous binaries in nearly every small-body population in the solar system and the realisation that such objects are much more abundant than had been anticipated. This could mean that binaries are abundant in exoplanet systems as well.
Most known KBO binaries are resolved. That means the two binary components can be seen in images taken by telescopes, mostly by Hubble which is in space and does not suffer from atmospheric distortion of the images. You can read more about resolved binaries here.
A particularly interesting subset of the binary population are the contact binaries which are identified from their extremely variable lightcurves (∆m>0.9 mag; see Figures 1 & 2). So far, 1 contact binary has been identified in the Kuiper belt and 3 contact binaries have been found in the Trojans (Mann, Jewitt & Lacerda 2007). These discoveries suggest high intrinsic contact binaries abundances (>10%; Lacerda 2011) among both Trojans and KBOs.
Asteroid (624) Hektor is the most famous contact binary in the solar system. Hektor is the largest known Jovian Trojan, measuring more than 350 km along its largest dimension. By far the most striking feature of Hektor is its extreme shape, first inferred from its rotational lightcurve. Depending on position along the orbit, the lightcurve range of Hektor varies between ∆m = 0.1 magnitudes and an extreme ∆m = 1.2 magnitudes (see Fig. 1). The 1.2 mag variability is indicative of a highly elongated shape with a long- to short-axis ratio a/b ≈ 3, making Hektor the most elongated object of its size.
Importance of Contact Binaries
Contact binaries are particularly useful for a number of reasons. Firstly, their abundance relative to more distant, resolved binaries will depend strongly on (and hence constrain) the collisional history of the population. Secondly, and more importantly, their bulk densities (proxy for composition) can be estimated from careful modelling of their lightcurves (Lacerda & Jewitt 2007). Densities are hard to measure remotely but are extremely important as an indicator of the bulk composition of an object. Modelling of 1 of the KBOs and 2 of the Trojan CBs mentioned above has revealed densities near 0.6 g/cm³. This surprisingly low density requires an almost pure icy composition and significant porosity. Even more surprising is that the remaining 1 KBO and 1 Trojan (Hektor) have densities in excess of 2 g/cm³ uncovering a significant compositional diversity in these populations. As more contact binaries are discovered in the future we hope to understand how the diversity relates with other properties.