How do planets form?

Chuck Bednar for – @BednarChuck

During the early stages of their formation, stars are surrounded by rotating disks of dust and gas, but how do these particles manage to avoid getting sucked into the star’s gravitational field for long enough to accumulate into celestial bodies?

Dr. Alan P. Boss from the Carnegie Institution for Science’s Department of Terrestrial Magnetism and his colleagues wanted to know, and they tackle the question in new research published earlier this week in The Astrophysical Journal. The current prevailing theory of rocky planet formation states that grains of dust collide and aggregate, growing increasingly larger until they form new worlds.

However, one of the problems with this theory is that the pressure gradient of the gas in the disk would create a headwind, pushing the still-forming pebble- and boulder-sized planetoids inwards to the forming protostar, thus destroying these young planets. Objects between one- and ten-meters in radius would be most susceptible to the gas drag, and if too many such objects wound up being lost, there would not be enough material left to form a planet.

Spiral arms play a key role

According to Dr. Boss and his team, observations of protostars that are still surrounded by their dust disks have revealed that those about the same size of the Sun often experience periodic bursts of explosive activity that last about 100 years. During these events, the star becomes more luminous and the disk experiences a period of gravitational instability.The study reveals that this phenomenon can cause smaller bodies to be scattered away from the developing star instead of towards it.

Furthermore, recent studies have shown that young stars have spiral arms that are believed to play a key role in the short-term disruptions of the disk, Boss and his co-authors said.

The gravitational forces of these spiral arms could scatter boulder-sized objects, making it possible for them to accumulate and form objects large enough to overcome gas drag. Modeling techniques used in the Carnegie team’s study could further demonstrate how these spiral arms help contain smaller planetoids on their way to becoming planets.

“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,” Boss said in a statement. “Once these bodies are in the disk’s outer regions, they are safe and able to grow into planetesimals.”


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