Planetary Disks Don’t Need Planets To Make Waves

In Exoplanets by Brian Koberlein

One of the biggest challenges in astronomy is observing the cold, dark dust surrounding a young star. These planetary disks, as they are known, are the birthplace of planets. Understanding them helps us understand how planetary systems form. But much of the gas and dust is so cold that they emit light mostly in the microwave range, which is difficult to detect. But with the construction of the Atacama Large Millimeter/submillimeter Array (ALMA) we can finally start to see details. 

A common feature of these planetary disks is their ringed pattern. The disks often have rings or arcs of thick dust separated by gaps. It has been thought that these gaps are caused by young planets, which tug on the gas and dust to make patterns in the disk, similar to the way Jupiter created gaps in the asteroid belt known as Kirkwood gaps. But new research finds that these ringed patterns might not be evidence of planets after all.

Computer simulations from a team at NASA show these gaps could be caused by ultraviolet light. When ultraviolet light strikes grains of dust, it can free electrons from the dust grains through the photoelectric effect. The free electrons then collide with surrounding gas, heating it up. As the gas heats and expands it tends to trap more dust grains. This reinforcing cycle is known as photoelectric instability. The computer simulations show that photoelectric instability can combine with other interactions to create the type of arcs and rings we see in young planetary disks.

This doesn’t mean there aren’t young planets orbiting these young stars, but rather that the presence of rings in a planetary disk doesn’t prove there are planets. Planetary formation is complex, and we will need to do further study to understand it.