In the past decade, we’ve developed amazing capacities to directly image planetary systems forming around young stars. With the Atacama Large Millimeter/submillimeter Array (ALMA) in particular, we have seen amazing systems with delicate ring structures that sometimes even have knots of material thought to be planets.

It’s the “sometimes” in that statement that is a bit troubling.

We like to think that planetary formation is a nice clean process. Giant molecular cloud starts to collapse and fragment. The fragments will spin and flatten as they collapse, with a star taking shape in the center, and planets cleaning out rings in the disk as they gather up their own mass. Over time, the star will brighten and push out leftover remains, the system will be left with some number of planets, maybe an asteroid belt, and icy stuff out in the outskirts.

It’s a nice picture. It is also overly idealistic. Solar system formation is a violent, planet-hit-planet kind of process, and thanks to a new paper in The Astrophysical Journal and led by Kazuhiro Kanagawa, we are learning that the gravitational interactions between newly formed planets and their surrounding rings of unused material migrate planets into the centers of gaps where we may not see them. 

This work was done using supercomputers to simulate all the motions of gas, dust, and forming planets in a variety of systems and evolve those simulations to match actual systems observed by ALMA. Three different planet and ring stages were uncovered in these simulations, and here I quote from the NAOJ release: In Phase I, the initial ring remains intact as the planet moves inwards. In Phase II, the initial ring begins to deform and a second ring starts forming at the new location of the planet. In Phase III, the initial ring disappears and only the latter ring remains.

These results help us understand why planets are only rarely spotted interacting next to rings.

Unfortunately, we are only able to really see the outer parts of disks clearly and resolve different structures at great distances from stars. It is hoped that the next generation of space and ground telescopes will allow us to understand the inner workings of systems more and verify that our simulations match the reality of planet formation at all distances from a star.

More Information

NAOJ press release

“Dust Rings as a Footprint of Planet Formation in a Protoplanetary Disk,” Kazuhiro D. Kanagawa, Takayuki Muto, and Hidekazu Tanaka, 2021 November 12, The Astrophysical Journal