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A brief visit from a red and extremely elongated interstellar asteroid

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None of the approximately 750,000 known asteroids and comets is thought to have originated outside our Solar System, but formation models suggest that orbital migration of the giant planets ejected a large fraction of the original planetesimals into interstellar space1. The predicted interstellar number density2 of icy interstellar objects of 2.4 × 10−4 au−3 suggested that these should have been detected by surveys, yet hitherto none had been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of today’s Solar System, but until now there has been no way to tell if our Solar System is typical. Here we report observations and subsequent analysis of 1I/2017 U1 (‘Oumuamua) that demonstrate the extrasolar trajectory of ‘Oumuamua. Our observations reveal the object to be asteroidal, with no hint of cometary activity despite an approach within 0.25 au of the Sun. Spectroscopic measurements show that the object’s surface is consistent with comets or organic-rich asteroid surfaces found in our own Solar System. Light-curve observations indicate that the object has an extreme oblong shape, with a 10:1 axis ratio and a mean radius of 102±4 m, assuming an albedo of 0.04. Very few objects in our Solar System have such an extreme light curve. The presence of ‘Oumuamua suggests that previous estimates of the density of interstellar objects were pessimistically low. Imminent upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to produce more interstellar objects in the upcoming years.

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Author information

Author notes

    • Robert Weryk
    • , Marco Micheli
    • , Jan T. Kleyna
    •  & Olivier R. Hainaut

    These authors contributed equally to this work.


  1. Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

    • Karen J. Meech
    • , Robert Weryk
    • , Jan T. Kleyna
    • , Robert Jedicke
    • , Richard J. Wainscoat
    • , Kenneth C. Chambers
    • , Jacqueline V. Keane
    • , Andreea Petric
    • , Larry Denneau
    • , Eugene Magnier
    • , Travis Berger
    • , Mark E. Huber
    • , Heather Flewelling
    • , Chris Waters
    • , Eva Schunova-Lilly
    •  & Serge Chastel
  2. ESA SSA-NEO Coordination Centre, Largo Galileo Galilei, 1, 00044 Frascati (RM), Italy INAF

    • Marco Micheli
  3. Osservatorio Astronomico di Roma, Via Frascati, 33, 00040 Monte Porzio Catone (RM), Italy

    • Marco Micheli
  4. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany

    • Olivier R. Hainaut


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Corresponding author

Correspondence to Karen J. Meech.


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