The growth of dust grains in protoplanetary disks is a necessary first step towards planet formation1. This growth has been inferred from observations of thermal dust emission2 towards mature protoplanetary systems (age >2 million years) with masses that are, on average, similar to Neptune3. In contrast, the majority of confirmed exoplanets are heavier than Neptune4. Given that young protoplanetary disks are more massive than their mature counterparts, this suggests that planet formation starts early, but evidence for grain growth that is spatially and temporally coincident with a massive reservoir in young disks remains scarce. Here, we report observations on a lack of emission of carbon monoxide isotopologues within the inner ~15 au of a very young (age ~100,000 years) disk around the solar-type protostar TMC1A. By using the absence of spatially resolved molecular line emission to infer the gas and dust content of the disk, we conclude that shielding by millimetre-size grains is responsible for the lack of emission. This suggests that grain growth and millimetre-size dust grains can be spatially and temporally coincident with a mass reservoir sufficient for giant planet formation. Hence, planet formation starts during the earliest, embedded phases in the life of young stars.

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This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01415.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We thank Allegro, the European ALMA Regional Centre node in the Netherlands, for providing the facilities and assistance in recalibrating and imaging of the data. Furthermore, D.H. thanks M. Hogerheijde, A. Bosman and E. van Dishoeck for discussions. D.H. is supported by EU ERC Advanced Grant 291141 “CHEMPLAN” and by a KNAW professor prize awarded to E. van Dishoeck. D.H. and L.T.M. are part of Allegro, which is funded by the Netherlands Organisation for Scientific Research (NWO). P.B. acknowledges support by the Swedish Research Council (VR) through contracts 2013-00472 and 2017-04924. The group of J.K.J. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 646908) through ERC Consolidator Grant “S4F”. Research at the Centre for Star and Planet Formation is funded by the Danish National Research Foundation (DNRF97).

Author information


  1. Leiden Observatory, Leiden University, Leiden, The Netherlands

    • Daniel Harsono
    •  & Luke T. Maud
  2. Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden

    • Per Bjerkeli
  3. ASTRON, The Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands

    • Matthijs H. D. van der Wiel
  4. Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen, Copenhagen , Denmark

    • Jon P. Ramsey
    • , Lars E. Kristensen
    •  & Jes K. Jørgensen


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D.H. and L.T.M. were responsible for the data re-calibration and reduction. DH was responsible for the analysis and wrote the manuscript together with P.B., M.H.D.v.d.W. and J.P.R. M.H.D.v.d.W. and P.B. composed the observing proposal, with contributions from D.H., J.P.R. and J.K.J. All authors contributed at various stages to the data analysis, discussed the results and contributed to the manuscript.

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The authors declare no competing interests.

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Correspondence to Daniel Harsono.

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