Planet Formation? It’s a Drag

Planet Formation? It’s a Drag

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It’s not easy to make a planet.

You start with a disk of gas and dust swirling around a newborn star. Grains of rock and minerals in the disk somehow clump together and eventually grow into an entire world. But exactly how dust particles stick together had long perplexed scientists. Electrostatic forces would build pebble-size clumps, similar to how dust bunnies form under a couch. But that process peters out at larger scales, where bigger objects bounce or shatter rather than sticking together when they collide. Something else must be guiding the early growth of planets—but what?

A potential answer to the mystery came about a decade ago, when astrophysicists Andrew Youdin, now at the University of Arizona, and Jeremy Goodman of Princeton University discovered whirling grains of dust dragging against gas in a disk can form the seeds of planets. Youdin and Goodman called their proposed planet-building mechanism the “streaming instability.”

The behavior of the dust, Youdin says, is akin to how cyclists in a race pack together to minimize drag. “It’s one of the best ways we have right now to understand how the planet formation process really gets going,” he says. That drag—the same force that makes it harder to drive against a headwind—coaxes the grains into clusters that then rapidly collapse into solidity via their own gravity, forming large chunks that glom together to make full-size planets. “There’s been a flurry of work in the last decade because [the streaming instability] has potentially represented a breakthrough in our understanding of how planets might form,” says Philip Hopkins, an astrophysicist at California Institute of Technology.

But now there may be more to the story.

In a series of recent papers Hopkins and his Caltech colleague Jonathan Squire describe how dust dragging through gas can have profound effects that transcend planet-forming disks to extend into the universe at large. The gas in a disk causes orbiting grains of dust to slow down—allowing other particles to pile up behind them. These piled-up particles can create wakes in the gas, attracting other nearby particles like a conga line at a dance party. Depending on conditions such as the presence or absence of electromagnetic fields, the dust can then form clumps, sheets, filaments or other structures across a wide variety of scales.

Dubbed “resonant drag instability,” this process is a broader class of phenomena that includes the streaming instability. They all rely on the same kind of interactions with the drag force, but the streaming instability is a special case that happens in a planetary disk. In theory, however, drag instabilities should occur anywhere there is dust and gas—such as around black holes and stars, and even in the depths of interstellar space. “Anytime you try to move particles through gas, a version of this instability would appear,” Hopkins says. The instability could play an important role in everything from how certain stars age and die by blowing off gusts of stellar wind to how volcanic ash settles in a planet’s atmosphere.

In particular, researchers discovered a new type of drag instability that could also happen in a planet-forming disk. In this process, called the “settling instability,” dust could clump up into concentric rings as it settles within a disk—potentially augmenting the streaming instability and supercharging the rapid formation of planets. “The general feeling is there needs to be more than just the streaming instability,” Hopkins says. “The concern a lot of people had was that the specific circumstancesunder which this particular instability could operate were quite narrow.” Streaming instabilities work best in extremely dust-rich environments, in which only a certain subset of grain sizes settle into a thin disk within the gas. Only a fraction of planet-forming disks might meet those criteria, yet statistics suggest all stars have planets. In theory, other drag-driven planet-forming mechanisms much like Hopkins and Squire’s proposal—such as the settling instability—could make up the difference.

“Planet formation is one of the great intellectual curiosities of humankind,” says Konstantin Batygin, an astrophysicist at Caltech who did not take part in either group’s research. “This problem of how do you form the building blocks has plagued the field for decades. The fact that there’s this emergent understanding of how the smallest pieces form—I think is a really important breakthrough.”

Others are not so sure. The streaming instability is already fairly general and plays such a big role that it does not seem to need much if any help, says Jim Stone, an astrophysicist at Princeton who studies gas dynamics but was not involved with any of the work. “It’s hard to imagine anything else to be more important,” he says.

Researchers agree, though, that more work needs to be done to examine Hopkins and Squire’s mechanisms and their implications. In particular, better computer simulations are needed to model how the specific scenarios—such as the pair’s proposed ring-forming settling instability—arise and evolve in more realistic, turbulent and chaotic conditions. Such simulations are already underway, Hopkins says. Furthermore, new radio telescope observations that peer into the murky hearts of planet-forming disks could also provide more evidence for or against processes like the settling instability, Youdin says.

“What’s really interesting about this work is they unified the mathematics,” Stone says, showing drag instabilities, including the streaming instability, are fundamentally the same. What may be most exciting, he notes, is these phenomena might be occurring all over the universe.

They could even be happening here on Earth. Hopkins says he and Squire have heard reports from volcanologists who suspect they have witnessed evidence of drag instabilities occurring in volcanic ash raining through Earth’s atmosphere. The more ash clumps as it falls, the less sunlight it blocks, reducing its cooling effect on the planet; understanding how drag instabilities affect volcanic eruptions, then, could prove important for improving climate change models.

Still, at this point the blossoming study of drag instabilities has yet to definitively solve any mysteries. “I see it as something that’s not closing questions off,” Youdin says, “but really opening new avenues of things to look at.”

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