Spirals in Planet-Forming Disk Caught Twisting for the First Time with ALMA

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have, for the first time, captured on video the winding motion of spiral structures in a planet-forming disk. These spirals, long suspected to play a crucial role in the birth of planets, have now been observed dynamically twisting around the young star IM Lupi, offering a direct glimpse into the earliest stages of planetary system formation.

Our Solar System contains eight planets, but outside it, nearly 6,000 exoplanets have been discovered. The exact process by which such planets form remains unclear. Protoplanetary disks—rotating disks of gas and dust around young stars—are thought to be the cradle of planets, and spiral structures within them are considered a key factor. Planets may emerge as solid particles accumulate within the spiral arms, or as the spirals themselves fragment into individual planets.

Until now, however, astronomers had been unable to distinguish whether a spiral was a precursor to planet formation or a product of a young planet already taking shape. Theoretical predictions suggest that spirals formed by the disk’s own gravity should wind up and eventually disappear. In contrast, spirals induced by a planet’s gravity remain stable and rotate along with the planet.

To test this idea, researchers examined the protoplanetary disk of IM Lupi, which displays striking spiral arms. Using ALMA observations taken in 2017, 2019, and 2024, the team created a flip-book style video spanning seven years. The spirals clearly showed a dynamic winding motion, with a speed matching theoretical models of disk-driven spirals. This confirms that the gravity of the disk itself creates the spirals in IM Lupi, and that the system is likely on the eve of forming new planets.

“This is the first successful detection of the winding motion of spirals,” said Tomohiro Yoshida, who led the research team. “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.”

This breakthrough opens a new window into the earliest phases of planet formation. Further studies of IM Lupi and similar systems may soon reveal, in unprecedented detail, how planetary systems like our own form.

Additional Information

This research appears 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).

This text is based on the original press release by the National Astronomical Observatory og Japan (NAOJ), an ALMA partner on behalf of East Asia.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (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 ALMA's construction, commissioning, and operation.

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