Exoplanets form in protoplanetary disks, collections of cosmic dust and gas orbiting a star. The leading theory of planet formation, called core accretion, occurs when dust grains in the disk gather and grow, forming a planet’s core, like a snowball rolling down a slope. Once it gains a strong enough gravitational pull, other material collapses around it, forming an atmosphere.

A secondary theory of planet formation is gravitational collapse. In this scenario, the disk itself becomes gravitationally unstable and collapses, forming a planet, like snow being shoveled into a pile. This process requires the disk to be massive, and until recently, there were no known candidates to observe; previous studies had detected a pile of snow, but not what formed it.

However, in a new article published today in NatureMIT Kerr-McGee Career Development Professor Richard Teague and his colleagues present evidence that the motion of the gas surrounding the star AB Aurigae behaves as would be expected for a gravitationally unstable disk, consistent with numerical predictions.

Their discovery is similar to detecting a snowplow creating a pile. It suggests that gravitational collapse is a viable method for planet formation. Teague, who studies planetary system formation in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), answers some questions about the new work.

What made the AB Aurigae system a good candidate for observation?

There were a lot of observations that suggested some interesting dynamics going on in the system. Groups saw spiral arms inside the disk; people found hot spots that some groups interpreted as a planet; others explained it as some other instability. But it was really a disk, and we knew that there were a lot of interesting motions going on. The data we had before was good enough to see that it was interesting, but it wasn’t good enough to describe in detail what was happening.

What is gravitational instability in protoplanetary disks?

Gravitational instabilities occur when the gravity of the disk itself is strong enough to perturb the motions within the disk. We usually assume that the gravitational potential is dominated by the central star, which happens when the disk mass is less than 10% of the mass of the star (which is most often the case).

As the disk mass gets too large, the gravitational potential will act on it in different ways and drive these very large spiral arms in the disk. They can have a number of different effects: they can trap gas, they can heat it, they can allow very rapid transport of angular momentum in the disk.

If it’s unstable, the disk could break apart and collapse directly, forming a planet in an incredibly short time. Instead of the tens of thousands of years it would take for the core to accrete, it would happen in a fraction of that time.

How does this discovery challenge conventional wisdom about planet formation?

It shows that this alternative path of planet formation through direct collapse is how we can make planets. This is especially important as we find more and more evidence for very large planets—say, Jupiter-mass or larger—that are very far from their star.

These types of planets are incredibly difficult to form with core accretion because you usually need them close to the star where everything happens quickly. So forming something that massive that far from the star is a real challenge.

If we can show that there are sources massive enough to be gravitationally unstable, that would solve this problem. That’s a way that maybe newer systems could form, because it’s always been a bit of a challenge to understand how they formed with core accretion.

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