None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space1. The high predicted number density2 of icy interstellar objects (2.4 × 10−4 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of the Solar System today, but there has been no way of telling whether the Solar System is typical of planetary systems. Here we report observations and analysis of the object 1I/2017 U1 (‘Oumuamua) that demonstrate its extrasolar trajectory, and that thus enable comparisons to be made between material from another planetary system and from our own. Our observations during the brief visit by the object to the inner Solar System reveal it to be asteroidal, with no hint of cometary activity despite an approach within 0.25 astronomical units of the Sun. Spectroscopic measurements show that the surface of the object is spectrally red, consistent with comets or organic-rich asteroids that reside within the Solar System. Light-curve observations indicate that the object has an extremely oblong shape, with a length about ten times its width, and a mean radius of about 102 metres assuming an albedo of 0.04. No known objects in the Solar System have such extreme dimensions. The presence of ‘Oumuamua in the Solar System suggests that previous estimates of the number density of interstellar objects, based on the assumption that all such objects were cometary, were pessimistically low. Planned upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to result in the detection of more interstellar objects in the coming years.
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Pan-STARRS1 is supported by NASA under grant NNX14AM74G issued through the SSO Near Earth Object Observations Program. K.J.M., J.T.K. and J.V.K. acknowledge support through NSF awards AST1413736 and AST1617015. This work is based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 2100.C-5008(A), and in part on observations obtained under programme GS-2017B-DD-7 at the Gemini Observatory, which is operated by AURA under cooperative agreement with the NSF on behalf of the Gemini partnership: NSF (USA), NRC (Canada), CONICYT (Chile), MINCYT (Argentina) and MCT (Brazil). This work is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership between California Institute of Technology, the University of California and the National Aeronautics and Space Administration. This observatory was made possible by financial support from the W. M. Keck Foundation. UKIRT is owned by the University of Hawaii (UH) and operated by the UH Institute for Astronomy; operations are enabled through the cooperation of the East Asian Observatory. We thank all of the directors who evaluated the requests for telescope time and facilitated the execution of the programmes on the telescopes: L. Ferrarese (Gemini), R. Ivison (ESO), D. Simons (CFHT) and R. McLaren (UKIRT). We also thank the queue operators and support staff who made the data available. We thank K. Withington for processing the CFHT data, S. Isani for help with non-sidereal guiding, and W. Varricatt and M. Irwin for help with WFCAM non-sidereal scheduling. We thank K. Kimura (Director of the ‘Imiloa Astronomy Center) and L. Kimura (a Hawaiian linguistics expert) for their suggestion of a name. We recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.