Division of Science, NAOJ

Winding Motion of Planet-Forming Spirals Captured on Video for the First Time

How were our Solar System and exoplanets formed? Research to date has shown that planets are born within so-called protoplanetary disks – structures of gas and dust surrounding young stars. However, the detailed processes that lead to planet formation remain shrouded in mystery.

One feature that has long been thought to play an important role in planet formation is the spiral structure that can emerge within protoplanetary disks due to the self-gravity. Within these spirals, solid particles may efficiently collide and grow, eventually reaching planetary sizes. In some cases, the spirals themselves may directly fragment to planets. On the otherhand, it is also known that spiral structures can be created by massive planets that have already formed. This means that the presence of spirals alone does not allow researchers to determine whether planets are on the verge of forming or have already been born.

An international research team led by Tomohiro Yoshida, a graduate student at the National Astronomical Observatory of Japan (NAOJ) and the Graduate University for Advanced Studies (SOKENDAI), focused on a theoretical prediction that these two scenarios could be distinguished by the motion of the spirals. If the spirals are produced by the self-gravity of the disk prior to planet formation, they are expected to wind and eventually dissipate. In contrast, if the spirals are induced by an already-formed planet, they should retain their shape and co-rotate with the planet.

The team investigated the protoplanetary disk surrounding the young star IM Lupus, which exhibits prominent spiral arms. By combining images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) over four observation periods in 2017, 2019, and 2024, spanning seven years, the researchers created a “movie” of the disk. The results revealed dynamic winding motions of the spirals. This finding indicates that the spirals are caused by the self-gravity of the disk. Such spirals are believed to promote planet formation, suggesting that this disk is caught in the very moment before planets are born.

These results were published as Tomohiro C. Yoshida et al., “Winding Motion of Spirals in a Gravitationally Unstable Protoplanetary Disk” in Nature Astronomy on September 24, 2025.
https://www.nature.com/articles/s41550-025-02639-y

Press release from NAOJ/ALMA project:
https://alma-telescope.jp/en/news/press/vimage-202509.html