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The natural history of ‘Oumuamua


The discovery of the first interstellar object passing through the Solar System, 1I/2017 U1 (‘Oumuamua), provoked intense and continuing interest from the scientific community and the general public. The faintness of ‘Oumuamua, together with the limited time window within which observations were possible, constrained the information available on its dynamics and physical state. Here we review our knowledge and find that in all cases, the observations are consistent with a purely natural origin for ‘Oumuamua. We discuss how the observed characteristics of ‘Oumuamua are explained by our extensive knowledge of natural minor bodies in our Solar System and our current knowledge of the evolution of planetary systems. We highlight several areas requiring further investigation.

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The authors declare that the main data supporting the findings of this study are available within the article. Extra data are available from the corresponding author upon request.


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We thank the International Space Science Institute (ISSI Bern), which made this collaboration possible. A.F., M.T.B. and C.S. acknowledge support from UK Science and Technology Facilities Council grants ST/P0003094/1 and ST/L004569/1. K.J.M. acknowledges support through NSF awards AST1617015, in addition to support for HST programmes GO/DD-15405 and -15447 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. Q.Y. is supported by the GROWTH project funded by the National Science Foundation under Grant No. 1545949. This research was partially supported by the project 2015/17/B/ST9/01790 funded by the National Science Centre in Poland. M.M.K. acknowledges support from NASA Near Earth Object Observations grant no. NNX17AK15G. A.G.-L. acknowledges funding from the European Research Council under grant agreement no. 802699. A.M. and D.E.T. are supported in part by Spitzer/NASA through an award issued by JPL/Caltech. S.N.R. acknowledges helpful discussions with P. Armitage related to the interstellar object number/mass density, and the Virtual Planetary Laboratory research team, funded by the NASA Astrobiology Program under NASA Grant Number 80NSSC18K0829. This work benefited from participation in the NASA Nexus for Exoplanet Systems Science research coordination network.

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M.M.K. and A.F. organized the ISSI team. K.J.M. created Figs. 1 and 2. S.N.R. conducted the modelling of inferred interstellar object number density and created Fig. 3. M.M.K., A.F. and R.J. created Fig. 4 from source data provided by T. Engelhardt. All authors discussed the topics in the paper, contributed to the writing and commented on the manuscript at all stages.

Correspondence to Matthew M. Knight.

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Fig. 1: Montage of images of ‘Oumuamua showing its point-like unresolved appearance with no hint of detectable activity.
Fig. 2: Montage of potential formation scenarios of ‘Oumuamua as a natural planetesimal.
Fig. 3: Inferred interstellar object number density — for a fixed estimate of the mass density of 0.004–3 Earth masses per cubic parsec — assuming different underlying SFDs.
Fig. 4: Predicted distribution of orbital elements of natural interstellar objects detected by the primary contemporary asteroid surveys.