Contact Binaries

Contact binaries in the trans-Neptunian belt


A contact binary is made up of two objects that are almost touching or in contact with each other. These systems have been found in the Near-Earth Object population, the main belt of asteroids, the Jupiter Trojans, the comet population and even in the trans-Neptunian belt. Several studies suggest that up to 30% of the Trans-Neptunian Objects (TNOs) could be contact binaries (Sheppard & Jewitt 2004, Lacerda 2011). Contact binaries are not resolvable with the Hubble Space Telescope because of the small separation between the system’s components (Noll et al. 2008). Only lightcurves with a characteristic V-/U-shape at the minimum/maximum of brightness and a large amplitude can identify these contact binaries (Fig 1.). Despite an expected high fraction of contact binaries, 2001 QG298 is the only confirmed contact binary in the Trans-Neptunian belt, and 2003 SQ317 is a candidate to this class of systems (Sheppard & Jewitt 2004, Lacerda et al. 2014).


Figure 1: Lightcurve of a potential contact binary.


Until the summer 2017, the basic characteristics of 2014 MU69 (nicknamed Ultima Thule) were a mystery, but thanks to a successful stellar occultation, scientists have been able to obtain some information about its shape, albedo, size, and binarity. One of the most striking results is the shape of 2014 MU69 (Fig. 2.). So far, three potential shapes can interpret the stellar occultation data: Ultima Thule can be a close binary, a contact binary, or a single object with a “potato” shape.


Figure 2: Artistic representation of the potential shapes of 2014 MU69 based on stellar occultation data. Credits: James Tuttle Keane.


To provide context for this second flyby of the New Horizons mission, it is important to find more contact binaries to evaluate their fraction in the trans-Neptunian belt, understand their formation and evolution, and derive their rotational and physical characteristics. Recently, using the 6.5m Magellan Telescope and the Lowell’s 4.3m Discovery Channel Telescope,  Scott S. Sheppard and I started a search for contact binaries in the trans-Neptunian belt.  We already discovered six new potential contact binaries. Early results have been published in Thirouin et al. 2017, Thirouin & Sheppard 2017 and Thirouin & Sheppard 2018.

So far, contact binaries have been found in several resonances with Neptune, in the Haumea family and in the dynamically cold classical population. We estimated that the fraction of contact binaries in the 3:2 mean motion resonance with Neptune is ~40-50%, but it is only ~10% in the dynamically cold classical population (Thirouin & Sheppard 2018, paper in preparation).

This work is funded by the National Science Foundation (NSF). Our work was highlighted in the AAS Nova and in the Planetary Mechanics Blog.