The cold classical Kuiper belt objects have low inclinations and eccentricities1,2 and are the only Kuiper belt population suspected to have formed in situ3. Compared with the dynamically excited populations, which exhibit a broad range of colours and a low binary fraction of ∼10%4 cold classical Kuiper belt objects typically have red optical colours5 with ∼30% of the population found in binary pairs6; the origin of these differences remains unclear7,8. We report the detection of a population of blue-coloured, tenuously bound binaries residing among the cold classical Kuiper belt objects. Here we show that widely separated binaries could have survived push-out into the cold classical region during the early phases of Neptune’s migration9. The blue binaries may be contaminants, originating at ∼38 au, and could provide a unique probe of the formative conditions in a region now nearly devoid of objects. The idea that the blue objects, which are predominantly binary, are the products of push-out requires that the planetesimals formed entirely as multiples. Plausible formation routes include planetesimal formation via pebble accretion10 and subsequent binary production through dynamic friction11 and binary formation during the collapse of a cloud of solids12.
This is work is based in part on observations from the Large and Long Program GN-2014B-LP-1, obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation on behalf of the Gemini partnership: the National Science Foundation (USA), the National Research Council (Canada), CONICYT (Chile), Ministério de Ciência, Tecnología e Innovación Productiva (Argentina) and Ministério da Ciência, Tecnología e Inovação (Brazil). This work is also based on observations obtained with MegaPrime/MegaCam, a joint project of the Canada–France–Hawaii Telescope (CFHT) and CEA/DAPNIA at CFHT, which is operated by the National Research Council of Canada, the Institute National des Sciences de l’Universe of the Centre National de la Recherche Scientifique of France and the University of Hawaii. A portion of the access to the CFHT was made possible by the Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. M.E.S. was supported by Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Brazil, Canada, Chile and the United States. M.E.S. was also supported in part by an Academia Sinica Postdoctoral Fellowship. This research made use of the Giorgini, JD and Jet Propulsion Laboratory (JPL) Solar System Dynamics Group, NASA/JPL Horizons On-Line Ephemeris System (http://ssd.jpl.nasa.gov/?horizons). We thank J. Coffey for assistance in acquiring images of 2006 BR284.
Supplementary Tables 1–3 and Supplementary Figures 1–4.