Spiral Arms Cradled Baby Rocky Planets in Our Solar System

How did rocky planets form in our solar system? Scientists may now have an answer. Researchers may know how dust grains in the matter orbiting a young protostar avoid getting dragged into the star before they accumulate into bodies large enough that their own gravity allows them to rapidly attract enough material to grow into planets.

In the early stages of their formation, stars are surrounded by rotating disks of gas and dust. The dust grains in the disk collide and aggregate to form pebbles, which grow into boulders, and so on increasing in size through planetesimals, planetary embryos, and finally rocky terrestrial planets. But there are some questions raised by this theory. One of them is that the pressure gradient of the gas in the disk would create a headwind that would spiral the pebbles and boulders inward toward the young protostar, where they would be destroyed.

This issue is most acute in bodies that are between 1 and 10 meters in radius, because they would be most susceptible to the gas drag. If too many particles in this size range were lost, there wouldn't be enough remaining to collide with each other and accumulate into planetesimals and, eventually, planets.

Observations of young stars that are still surrounded by their gas disks demonstrate that those similar in size to our own sun often undergo periodic explosive bursts, about 100 years in duration, during which the star's luminosity increases. More importantly, these events can be linked to a period of gravitational instability in the disk.

Recent work has shown the presence of spiral arms around young stars, similar to those thought to be involved in the short-term disruptions in the disk. The gravitational forces of these spiral arms could scatter outward the problematic boulder-sized bodies, allowing them to accumulate rapidly to form planetesimals large enough that gas drag is no longer a problem.

In this case, the researchers created models of developing planetary systems. This revealed exactly how these systems may develop.

"This work shows that boulder-sized particles could, indeed, be scattered around the disk by the formation of spiral arms and then avoid getting dragged into the protostar at the center of the developing system," said Alan Boss, one of the researchers, in a news release. "Once these bodies are in the disk's outer regions, they are safe and able to grow into planetesimals."

The findings are published in the journal The Astrophysical Journal.

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