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How a Dust Bunny Becomes a Planet

Published: 02-23-2018
Source: JILA Scientific Communications

Cosmic dust surrounds a new star. Within the cosmic dust are small pebbles that create turbulence. This turbulence allows the dust to accrete and form planetesimals. Photo Credit: Armitage Group, JILA and NASA/JPL-Caltech.

Jupiter is large enough to fit 1,300 Earths inside, and still have room. But like all planets, Jupiter was once nothing more than a cosmic dust bunny.

A team of physicists at JILA and the University of Arizona, led by JILA Senior Research Associate Jake Simon, are studying how cosmic pebbles­­—­starting only a millimeter in size—can lead to the formation of planetesimals, the football-field-to-Delaware-sized primordial asteroids whose development defined our solar system’s architecture.

It all starts in the cosmic dust, which is composed of tiny, micron-sized particles, surrounding freshly collapsed stars. Before planets can form, planetesimals must form; and before planetesimals can form, the cosmic dust must cluster. But like all dust bunnies, cosmic dust only clusters when stirred. The technical term for this stirring is turbulence.

There are many possible sources of initial turbulence. One source, known as streaming instability, is the presence of small pebbles within the cosmic dust. ­­

“So what’s going on here is the small solids, which you might think of as the pebbles within the disk, those feel aerodynamic forces, sort-of headwind forces, against the gas. And it turns out in some circumstances that doesn’t just cause them to spiral towards the star... it also causes them to cluster,” says JILA Fellow Philip Armitage.

“But what is not so clear is how you make them so clustered that they actually collapse gravitationally, and that’s really the point of these simulations.”

Understanding the formation of these early planetesimals may reveal the architectural rationale of our solar system. Is it a coincidence that our solar system’s gas giants, Jupiter and Saturn, are fenced away from our moderately sized terrestrial planets by an asteroid belt?

In terms of forming a giant planet, Armitage says the tinier the starting materials, the better. “If the planetesimals are small, then they themselves are damped aerodynamically by the gas disc, and so then they actually accrete faster onto the core of a forming giant planet.”

The accretion of planets is not unlike the accretion of a snowball. Rolling a small snowball among the tiny snowflakes on the ground quickly generates a giant snowball. Trying to combine several snowballs, however, results in a scattered mess.

“Large objects will tend to be scattered by the growing core more,” explains Armitage, and so they are not as readily accreted.

And much like snowflakes, which precipitate in different sizes, planetesimals have a wide variation of initial sizes. To quantify this variation, scientists use the mass function. The mass function describes many properties of the planetesimals, such as the average size, and the ratio of small to large particles.

Armitage and his fellow researchers wondered if the mass function of planetesimals depended on the cosmic pebbles that initiated their formation. “In a disc, you can imagine that in close to a star, where the gas is dense, the particles have quite different aerodynamic interactions. So we’ve been asking, do you get the same outcome whether you’re forming planetesimals in region of the terrestrial planets, in the asteroid belt, or in the Kuiper belt, where you might be forming comets.”

And the answer was, within error, a surprising yes. They found universality, or the idea that the slope of the mass function is the same regardless of the size of the initial pebbles. But this result is not too surprising.

“In other astronomical situations where we have gravitational collapse, which is for example in the formation of stars, we see observationally that the distribution of stellar masses appears to be the same in many different environments. So in that case, we have a similar result,” says Armitage.

This result also agrees with previous research1,2,3 of planetesimal formation that considers similar mechanisms, but not initial pebbles as small as one millimeter. A surprising agreement, given that smaller particles act differently, and are more likely to form bands or rings within a disc, than large particles.

Armitage is eager to explore more sources of turbulence. “It remains the case that we’ve only explored a small fraction of all the conditions that might be present in discs, and it may be that some other parameter, which we vary, will eventually be shown to make a difference to the mass function.”

This research was led by Jacob Simon and Philip Armitage, both of JILA, as well as Andrew Youdin and Rixin Li from the University of Arizona. The research was supported by NASA and the NSF, and was published in The Astrophysical Journal Letters.

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