Astronomers Discover ‘Missing Link’ in Planet Formation

An image of a protoplanetary disk, made using results from the new model, after the formation of a spontaneous dust trap, visible as a bright dust ring. Gas is depicted in blue and dust in red. (Credit: Jean-Francois Gonzalez)

(CN) – Astronomers may have identified the cause of a cosmic phenomenon that has eluded researchers and left a crater in the current theory of planetary formation.

In a new study published Monday, a team of astronomers presented a new theory for the intermediate stage of planetary formation in which pebble-sized fragments collect and stick together in “dust traps” which are fairly common – refuting the previous theory that dust traps require very specific environmental conditions.

“Until now we have struggled to explain how pebbles can come together to form planets, and yet we’ve now discovered huge numbers of planets in orbit around other stars,” lead author Jean-Francois Gonzalez, of the Centre de Recherche Astrophysique de Lyon, said. “That set us thinking about how to solve this mystery.”

Planetary formation begins when pieces of dust form together into tiny grains, each a few millionths of a meter – a micron – across, which then combine into grains that are a few centimeters long.

While this phase and the mechanism for making mile-sized “planetesimals” into planet cores, are both well understood, the middle step of the process has been murky.

There are two major barriers that must be overcome for pebbles to become planetesimals. First, the pull of gas on dust grains in a disk makes them drift quickly toward the central star only to be destroyed, leaving no material to form planets. The second challenge is the vulnerability of the growing grains. High-speed collisions can break them into a large number of small pieces, reversing the aggregation process.

Dust traps are the only locations in planet-forming disks where dust grains can overcome these challenges, allowing them to accumulate. They also slow the drift motion of the grains, which helps them avoid fragmentation when they collide.

The team’s computer simulations show that dust traps are common, and their models pay close attention to the way the dust in a disk drags on the gas component. Gas causes dust to move – in most astronomical simulations. But in the dustiest settings, the dust acts more strongly on the gas.

This effect, known as aerodynamic drag back-reaction, is typically negligible and has been ignored in previous studies of growing and fragmenting grains. However, its effects become more important in dust-rich environments, including where planets form.

The back-reaction slows the inward drift of the grains, giving them more time to grow in size. Once large enough, the motion of the grains is no longer affected by the gas. Back-reaction also pushes gas outward, forming a high-pressure region: the dust trap. These traps then concentrate the grains coming from the outer disk regions, creating a dense ring of solids.

“We were thrilled to discover that, with the right ingredients in place, dust traps can form spontaneously, in a wide range of environments,” Gonzalez said. “This is a simple and robust solution to a long-standing problem in planet formation.”