Disk Vortices Keep Planets from Falling In express
A young planet in a gas disk is like a swimmer trying to swim against the current: friction with the gas slows it down, and it spirals toward the star. For small rocky planets, this fall can take less time than they need to grow. But the disk also has countercurrents — vortices that can push the planet outward.
Scientists built a computer model of a disk around a young star. They added heating from radiation and cooling, which naturally creates large vortices in the disk. These vortices move through the disk and sometimes collide with the planet. The planet is heavier than the vortex, so the vortex, like a light boat, flows around it and transfers some of its momentum. Such a "kick" slows the planet's fall.
This result is important for understanding where super-Earths come from — planets heavier than ours but lighter than Neptune. Previously, it was thought they couldn't grow because they fell onto the star too quickly. Now it's clear: vortices in the disk act as natural "life preservers," keeping planets at a safe distance.
The Solar System has no super-Earths, even though they are the most common type of planet in the Galaxy. Perhaps our gas disk lacked the vortices to keep such planets from falling into the Sun.