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Winding motion of spirals in a gravitationally unstable protoplanetary disk

Nature Astronomy volume 9, pages 1672–1679 (2025)Cite this article

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Abstract

The discovery of wide-orbit giant exoplanets has posed a challenge to our conventional understanding of planet formation through the coagulation of dust grains and planetesimals and the subsequent accretion of protoplanetary disk gas. As an alternative mechanism, the direct in situ formation of planets or planetary cores by gravitational instability (GI) in protoplanetary disks has been proposed. However, observational evidence for GI in regions where wide-orbit planets form is still lacking. Theoretical studies predict that GI induces spiral arms moving at the local Keplerian speed in a disk. Based on several high-angular-resolution observations over a 7-year time baseline using the Atacama Large Millimeter/submillimeter Array, here we report the evidence for spiral arms following the Keplerian rotation in the dust continuum disk around the young star IM Lup. This demonstrates that GI can operate in wide-orbit planet-forming regions, establishing it as a plausible formation mechanism for such planets.

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Fig. 1: Continuum images of the IM Lup disk in the three epochs used in the analysis.
Fig. 2: Deprojected continuum images used in the quantitative analysis.
Fig. 3: Deprojected continuum images after partially subtracting the axisymmetric component.
Fig. 4: Rotation curve of the dust disk.

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Data availability

The data used for this study are available via the ALMA science archive (https://almascience.nao.ac.jp/aq/). The project IDs are 2016.1.00484.L, 2013.1.00226.S, 2013.1.00694.S, 2013.1.00798.S, 2018.1.01055.L and 2023.1.00525.S.

Code availability

The codes used for the analysis are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank K. Tomida for the helpful discussion on the spiral speed. We also thank A. Sierra for providing the calibrated MAPS continuum data. This paper makes use of the following ALMA data: 2016.1.00484.L, 2013.1.00226.S, 2013.1.00694.S, 2013.1.00798.S, 2018.1.01055.L and 2023.1.00525.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSTC 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. This work was supported by a grant-in-aid for JSPS Fellows (Grant No. JP23KJ1008 to T.C.Y.) and JSPS (KAKENHI Grant Nos. 19K03910 and 20H00182 to H.N.).

Author information

Authors and Affiliations

  1. National Astronomical Observatory of Japan, Mitaka, Japan

    Tomohiro C. Yoshida & Hideko Nomura

  2. Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, Mitaka, Japan

    Tomohiro C. Yoshida & Hideko Nomura

  3. Max Planck Institute for Astronomy, Heidelberg, Germany

    Kiyoaki Doi

  4. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    Marcelo Barraza-Alfaro & Richard Teague

  5. Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo, Japan

    Kenji Furuya & Yoshihide Yamato

  6. RIKEN Pioneering Research Institute, Wako, Japan

    Kenji Furuya & Yoshihide Yamato

  7. Faculty of Engineering, Ashikaga University, Ashikaga, Japan

    Takashi Tsukagoshi

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  1. Tomohiro C. Yoshida
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Contributions

T.C.Y. led the observing proposal, data processing, analysis and preparation of the paper. R.T. and M.B.-A. were involved in the discussion regarding the analysis and results as well as in the preparation of the paper. H.N., K.D., K.F., Y.Y. and T.T. participated in writing the observing proposal and the paper.

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Correspondence to Tomohiro C. Yoshida.

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Supplementary Information

Supplementary discussion, Figs. 1–4 and Tables 1 and 2.

Supplementary Video 1

Animation of the three epoch images (2017, 2019 and 2024). Here 2019 is an average of the images 2019a and 2019b (Fig. 3). For visibility, the spiral structure is emphasized in the following way. First, we extracted the radial intensity profile by taking the azimuthal average of the normalized images. Then, axisymmetric images created using the radial profile were subtracted from the original images with 20% enhanced intensities so that we can easily see the non-axisymmetric structures together with the radial dependence of the intensity. We also plot the spiral ridges specified by ref. 17 with the temporal variations of the ridges expected for the local Keplerian rotation, as in Fig. 2.

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Yoshida, T.C., Nomura, H., Doi, K. et al. Winding motion of spirals in a gravitationally unstable protoplanetary disk. Nat Astron 9, 1672–1679 (2025). https://doi.org/10.1038/s41550-025-02639-y

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