Astronomical observations show that early in their evolution, stars experience stages of high activity associated with enhanced energetic particle fluxes1. The Sun’s early activity is often inferred from the spallogenic isotope record (for example, 10Be) in the Solar System’s oldest materials2, calcium–aluminium-rich inclusions (CAIs) in meteorites3,4. However, the 10Be record could be affected by processes other than in situ irradiation by solar particles5. Noble gases can give less ambiguous insights because they are inert volatiles and hence not incorporated into CAIs during their formation6. Here we show that hibonite-rich CAIs, considered to have formed before 26Al-rich CAIs7,8, contain helium and neon excesses that can be unambiguously attributed to in situ irradiation by energetic particles. Given their volatile nature, we infer that the noble gases were produced by irradiation in a relatively cold region at a considerable distance from the Sun (not at the inner disk edge), requiring high particle fluxes and thus high early solar activity. Because more evolved CAIs lack comparable noble gas irradiation records9, we conclude that the oldest Solar System materials experienced a phase of intense irradiation not recorded by materials that formed later. Consequently, disk properties or energetic particle fluxes changed significantly during the very early phases of Solar System evolution.

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We thank F. Ciesla, R. Trappitsch, T. Stephan and P. Boehnke for discussions. This work was supported by National Aeronautics and Space Administration through grants NNX15AF78G and 80NSSC17K0251 (to A.M.D.), a National Science Foundation Graduate Research Fellowship (grant numbers DGE-1144082 and DGE-1746045) (to J.G.) and an Ambizione Grant (PZ00P2_154874) from the Swiss National Science Foundation (to M.M.M.M.). H.B. acknowledges support from the ‘PlanetS’ National Center of Competence in Research (NCCR) of the Swiss National Science Foundation. P.R.H. and J.G. acknowledge funding from the Tawani Foundation.

Author information


  1. Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA

    • L. Kööp
    • , P. R. Heck
    • , A. M. Davis
    •  & J. Greer
  2. Chicago Center for Cosmochemistry, Chicago, IL, USA

    • L. Kööp
    • , P. R. Heck
    • , A. M. Davis
    •  & J. Greer
  3. Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA

    • L. Kööp
    • , P. R. Heck
    • , A. M. Davis
    •  & J. Greer
  4. Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland

    • H. Busemann
    • , C. Maden
    • , M. M. M. Meier
    •  & R. Wieler
  5. Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA

    • A. M. Davis


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L.K. planned the study and classified and prepared samples. L.K., J.G. and P.R.H. performed noble gas measurements, with assistance from H.B., C.M. and M.M.M.M. All authors contributed to the interpretation and writing of the paper.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to L. Kööp.

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