Radial drift and the hazards of planet-building

When a huge cloud of dust and gas collapses under the weight of its own gravity, heats up and spins for millions upon millions of years, a star is born. But during that process, the angular momentum causes a disk to form, and tiny dust particles around the disk collide, bounce off each other, stick to one another and eventually form rocky planets like Earth or gas giants like Jupiter.

Except that it’s not quite that simple.

As the proto-planetary disk spins, it’s depleted of its precious planetary building blocks. Particles larger than a centimeter interact with gas and lose energy, shrinking their orbit and causing them to drift quickly towards the inner region of the disk. Eventually, these larger bodies get lost in the forming central star. The larger the particle, the faster it will drift. A meter-sized boulder of material one astronomical unit away from the infant star can disappear within one thousand years—a blink of an eye in celestial time.

This so-called radial drift depletes the disk of dust, but that’s not the only hazard for growing planets. As particles and larger rocks collide, they don’t always stick together. Sometimes they blast each other apart, forcing the particles back to planetary square one.

It’s easy to imagine the microscopic-sized particles slowly sticking together to form millimeter or even centimeter sized particles, says post-doctoral fellow Laura Perez of the National Radio Astronomy Observatory, but there’s a mystery—how do these centimeter-sized particles (which have already taken millions of years to form) get any bigger?

We’re all standing on a rocky planet, and since 2009, NASA’s Kepler probe has found hundreds more, so what gives? Somehow those tiny dust particles are growing into planet-sized bodies.

At this year’s American Association for the Advancement of Science meeting in Chicago, IL, Perez described how her team, using highly sensitive radio telescopes at the Very Large Array in New Mexico, analyzed young star systems to get more insight into planet growth. With the VLA, the researchers found that the distribution of dust was consistent with radial drift–smaller particles were more highly distributed in the outer parts of the disk, while larger particles populated the inner region.

Recent observations using the Atacama Large Millimeter/submillimeter Array, the newest and most sensitive radio telescope yet, showed that in most well-characterized proto-planetary disks, there is an asymmetry in the distribution of dust—one side of the disk has a higher concentration of dust than the other side. Perez speculates that there are areas where pockets of gas create pressure gradients, trapping dust particles and allowing them to grow larger without falling into the developing star.

Over the next year, Perez and her team will observe the distribution of gas in proto-planetary disks to find these high pressure pockets. Hydrogen, the most abundant gas in the universe, makes up most of the gas surrounding a developing star. But hydrogen is a “very picky molecule, in a way,” says Perez. Due to its symmetrical structure and low weight, hydrogen molecules are difficult to detect, even with the most sensitive telescopes. Instead, the team will be tracing carbon monoxide, a larger, more asymmetrical molecule that the telescopes can detect.

“This is why ALMA is going to be a revolution,” Perez says, “We’re going to see very clearly what the gas is doing, [and] see very clearly what the dust is doing.”

So far, the idea of a pressure gradient keeping dust particles together is a speculation, but the data gathered from ALMA will hopefully provide researchers with clues to the missing link between particles and planets.