Our Solar System has eight planets, including Earth. Looking further abroad, around 6,000 exoplanets have been discovered outside our Solar System. How did these planets form? Previous studies suggest that planets develop in protoplanetary disks, which are rotating disks that surround young stars; however, the specific process of planetary formation remains largely unclear.

A spiral structure, formed by the gravitational influence of the protoplanetary disk itself, is considered a potentially crucial factor in the planet formation process that takes place in the disk. Planets form within the spiral either through the efficient aggregation of solid particles present in the disk, which eventually grow to planetary size, or through the disaggregation of the spiral into individual planets.

What complicates matters is that similar spirals can also be created by massive planets during their initial stages of formation. In other words, the presence of spirals alone cannot be used to differentiate between a planet is on the verge of forming and a planet in its early developmental phase. If it is known that an already formed planet is not present, the protoplanetary disk may be a perfect spot for studying planet formation.

The research team sought to solve this problem based on a theoretical prediction that enables the identification of planetary formation phases from the movement of the spirals. A spiral will wind around itself and eventually vanish if it is created by the gravitational influence of the disk prior to planet formation, whereas a spiral created by an already formed planet will maintain its shape and continue to rotate along with the planet.

ALMA observations of the spiral patterns in the disk around the young star IM Lup.（Credit: ALMA(ESO/NAOJ/NRAO), T. Yoshida et al.）

For verification, the research team focused on the protoplanetary disk surrounding the star IM Lupi. The disk has a spiral structure. Two opposing theories regarding the origin of the spiral have been suggested by different research groups. To settle the debate, the current researchers used images of the protoplanetary disk obtained by four ALMA observations over a span of seven years in 2017, 2019, and 2024, creating a flip-book style video. The results showed that the spiral exhibited dynamic, winding motion. Further detailed analysis of the results indicated that the winding speed matched the theoretical projection. This indicates that the spiral is created by the gravity of the protoplanetary disk itself. Since this type of spiral is known to facilitate planetary formation, the protoplanetary disk under study is assumed to be on the eve of planet formation—just prior to the emergence of a planet.

This marks the first successful detection of the winding motion of spirals. A further in-depth analysis of the properties of the protoplanetary disk under study may uncover how the planet formation progresses in more detail.

Tomohiro Yoshida, who led the research team, said, “When I saw the outcome of the analysis —the dynamic visualization of the spiral in motion— I screamed with excitement. This achievement was made possible by the long-term, stable operations of the ALMA telescope, which demonstrates the world’s highest performance. In the future, we plan to conduct similar observations on other protoplanetary disks to create a documentary of the entire planetary system formation process.”

https://www.youtube.com/watch?v=7xxSJws7Jj0
Video of artist’s impression of planet formation around a young star, showing spiral patterns which help the young planets to form. (Credit: ALMA(ESO/NAOJ/NRAO), T. Yoshida et al.)

Paper:
This research has been published in Nature Astronomy on September 24, 2025, under the title “Winding Motion of Spirals in a Gravitationally Unstable Protoplanetary Disk” by Tomohiro C. Yoshida et al. (DOI: 10.1038/s41550-025-02639-y)

Research team:
Tomohiro Yoshida, Hideko Nomura (NAOJ/SOKENDAI), Kiyoaki Doi (Max Planck Institute for Astronomy), Marcelo Barraza-Alfaro, Richard Teague (Massachusetts Institute of Technology), Kenji Furuya, Yoshihide Yamato (RIKEN), and Takashi Tsukagoshi (Ashikaga University)

Acknowledgements:
This research was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship (JP23KJ1008) and Grants-in-Aid for Scientific Research (JP19K03910, 20H00182).

About ALMA:
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.