The wide-binary origin of (2014) MU69-like Kuiper belt contact binaries

An Author Correction to this article was published on 22 May 2020

This article has been updated


Following its flyby and first imaging of the Pluto–Charon binary, the New Horizons spacecraft visited the Kuiper belt object (KBO) 2014 MU69 (also known as (486958) Arrokoth). The imaging showed MU69 to be a contact binary that rotates at a low spin period (15.92 hours), is made of two individual lobes connected by a narrow neck and has a high obliquity (about 98 degrees)1, properties that are similar to those of other KBO contact binaries inferred through photometric observations2. However, all scenarios suggested so far for the origins of such configurations3,4,5 have failed to reproduce these properties and their probable frequent occurrence in the Kuiper belt. Here we show that semi-secular perturbations6,7 operating on only ultrawide KBO binaries close to their stability limit can robustly lead to gentle, slow binary mergers at arbitrarily high obliquities but low rotational velocities, reproducing the characteristics of MU69 and other similar oblique contact binaries. Using N-body simulations, we find that approximately 15 per cent of all ultrawide binaries with a cosine-uniform inclination distribution5,9 are likely to merge through this process. Moreover, we find that such mergers are sufficiently gentle to deform the shape of the KBO only slightly. The semi-secular contact binary formation channel not only explains the observed properties of MU69, but may also apply to other Kuiper belt or asteroid belt binaries and in the Solar System and extra-solar moon systems.

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Fig. 1: Roadmap to collisions of MU69.
Fig. 2: Cumulative distributions of the impact characteristics.
Fig. 3: Shape and spin period of MU69.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Change history

  • 22 May 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.


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We acknowledge discussions with D. C. Fabrycky and E. Kite. H.B.P. acknowledges support from the MINERVA Center for Life Under Extreme Planetary Conditions and the Kingsley Fellowship at Caltech. C.M.S. and O.W. acknowledge support by the High Performance and Cloud Computing Group at the Zentrum für Datenverarbeitung of the University of Tübingen, the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant number INST 37/935-1 FUGG. C.M.S. acknowledges support from the DFG through grant number 398488521.

Author information




E.G. led the project, performed the analytic calculations and ran and analysed the N-body simulations. U.M. led the hydrodynamical modelling, its analysis and wrote the hydrodynamical sections. H.B.P. initiated the project and supervised it, suggested the main ideas and concepts and took part in all of the analysis. O.W. ran the hydrodynamical simulations and was the main developer of the porosity module in the hydrodynamical code. C.M.S. developed the hydrodynamical code and supervised the development of the porosity module. E.G. and H.B.P. wrote the main text.

Corresponding author

Correspondence to Evgeni Grishin.

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Extended data figures and tables

Extended Data Fig. 1 Additional results of the collision models.

a, 40° impact angle, medium-strength material. b, 40° impact angle, low-strength material. c, d, Low-density model (0.5 g cm−3) with an impact angle of 55° and medium-strength material. The edge (c) and face (d) views are given.

Extended Data Fig. 2 Additional results of the collision models.

5° impact angle, high-strength material and large escape velocity, v = 10vesc.

Extended Data Table 1 Merger rate of the binaries in the non-secular regime
Extended Data Table 2 Crush curve, plasticity and fragmentation parameters

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Grishin, E., Malamud, U., Perets, H.B. et al. The wide-binary origin of (2014) MU69-like Kuiper belt contact binaries. Nature 580, 463–466 (2020).

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