Binaries

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.

Resolved Binaries

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.

Contact Binaries

Figure 1 – Model of a contact binary.
Figure 1 – Model of a contact binary.

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.

Hektor

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.

Figure 1 — Lightcurves of Trojan (624) Hektor taken at four different points along its orbit. The lightcurve range is seen to vary substantially, from ∆m ∼ 0.1 mag in 1965 to ∆m ∼ 1.1 mag in 1968. The red line is the theoretical lightcurve produced by a Roche binary model of Hektor’s shape. The insets show the Roche binary model at its minimum cross-section configuration (see Fig. 2). In the top-right panel the spin axis lies only 25 deg from the line-of-sight so the minimum cross-section is not very different from the maximum cross-section and the lightcurve variation is small. In the bottom-right panel the spin axis lies ∼90 deg from the line-of-sight so the lightcurve range is maximum. Adapted from Lacerda & Jewitt (2007).
Figure 2 — Lightcurves of Trojan (624) Hektor taken at four different points along its orbit. The lightcurve range is seen to vary substantially, from ∆m ∼ 0.1 mag in 1965 to ∆m ∼ 1.1 mag in 1968. The red line is the theoretical lightcurve produced by a Roche binary model of Hektor’s shape. The insets show the Roche binary model at its minimum cross-section configuration (see Fig. 3). In the top-right panel the spin axis lies only 25 deg from the line-of-sight so the minimum cross-section is not very different from the maximum cross-section and the lightcurve variation is small. In the bottom-right panel the spin axis lies ∼90 deg from the line-of-sight so the lightcurve range is maximum. Adapted from Lacerda & Jewitt (2007).
Roche binary model of (624) Hektor as seen from Earth at the times the lightcurves in Fig. 1 were taken. Pictures from left to right illustrate the rotation motion of Hektor and the values θ on the left indicate the aspect angle, i.e. the angle between the spin axis and the line of sight. Adapted from Lacerda & Jewitt (2007)
Figure 3 – Roche binary model of (624) Hektor as seen from Earth at the times the lightcurves in Fig. 1 were taken. Pictures from left to right illustrate the rotation motion of Hektor and the values θ on the left indicate the aspect angle, i.e. the angle between the spin axis and the line of sight. Adapted from Lacerda & Jewitt (2007)

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.

Relevant research and publications

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