Spiral arms could solve planet formation problem

The presence of spiral arms surrounding a young star is being offered as a solution to a key problem in the theory of planetary formation.

Artist’s concept of planets forming around a young star
Credit: David A. Hardy/www.astroart.org

Artist’s concept of planets forming around a young star. Credit: David A. Hardy/www.astroart.org


New work in the study of planet formation has offered a solution for a longstanding problem; how dust grains orbiting a young protostar avoid getting dragged inwards and destroyed before they can grow into planets.

Developing stars are surrounded by rotating disks of gas and dust.

Dust grains collide and build up slowly to form pebbles, then larger rocks and eventually rocky planets.

But this theory raises the issue of how the pebbles and boulders are prevented from being dragged into the young protostar by the headwind that would be created by gas in the star’s disk.

This problem is particularly an issue in bodies between 1 and 10 metres in radius, as they would be most susceptible to the gas drag.

Alan Boss at the Carnegie Institute for Science in Washington DC has come up with a theory involving explosive bursts that occur in the life of young stars and last about 100 years.

These bursts cause stars similar in size to the Sun to increase in luminosity and are linked to gravitational instability.

Boss’s theory shows that the bursts could scatter the smaller bodies outward, away from the developing star, rather than toward it.

The theory involves spiral arms, which are thought to be involved in this gravitational disruption.

They can be found around young stars and are formed from gas and dust in the circumstellar disk.

These spiral arms are well known in galaxies, but were only clearly observed around young stars in 2011, by researchers using the Subaru telescope in Hawaii.

Boss thinks these spiral arms could push the boulder-sized bodies away from the young star, which would allow them to accrete matter before they are dragged inwards and destroyed.

This idea could answer the question of how developing planetary systems avoid losing too many larger bodies before they can grow into something bigger.

“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 says.

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


While not every developing protostar may experience this kind of short-term gravitational disruption phase, it is looking increasingly likely that they may be much more important for the early phases of terrestrial planet formation than we thought.”