Planet formation relied on sweeping up of small glassy beads, new model suggests

April 17, 2015, American Museum of Natural History
Planet formation relied on sweeping up of small glassy beads, new model suggests
Credit: NASA/JPL-Caltech

New research proposes that chondrules, small glassy beads that make up the bulk of the most primitive meteorites, played a crucial role in the formation of planets. Simulations developed by scientists at the American Museum of Natural History, Lund University in Sweden, and collaborating institutions show how asteroid-sized planetesimals—the building blocks of planets—can grow to observed sizes by sweeping up chondrules, each only about the size of a grain of sand. The work is published today in the journal Science Advances.

"The big question is, 'How did the planets come to be?'" said Mordecai-Mark Mac Low, a curator in the American Museum of Natural History's Department of Astrophysics and an author on the paper. "When the first started forming, the largest solids were sub-micron dust. The challenge is to figure out how all of that dust was gathered up into planet-building objects that then formed the diversity of planets and other smaller bodies that we see today."

In the early stages of planet formation, dust particles in the disk of gas and dust surrounding a young star collide and stick together to form dust bunnies, then pebbles, then boulders. However, models of the disk show that the gas imparts a drag on boulders when these reach a size larger than a person, causing them to slow and eventually fall into the star after only about 100 orbits. In addition, fast-moving boulders break apart, rather than sticking together, when they collide.

In 2007 Mac Low and collaborators, including the lead author on the current work, Anders Johansen of Lund University, proposed a mechanism called the streaming instability, which predicts a different outcome. The streaming instability occurs when one orbit contains more boulders than its neighbors. Boulders moving in the same orbit and close to one another start sweeping the gas with them, which reduces the drag and thus the rate at which the boulders move inward, acting like the peloton that forms in a bicycle race. As boulders from orbits farther out enter this orbit, the density of boulders becomes so high that gravitational attraction causes them to collapse together into a planetesimal large enough to overcome gas drag.

In their latest work, Mac Low, Johansen, and their collaborators—Martin Bizzarro from the University of Copenhagen and Pedro Lacerda from the Max Planck Institute for Solar System Research—ran high-resolution simulations of the streaming instability using the European supercomputer network PRACE, with the goal of predicting the size distribution of the planetesimals produced by this mechanism.

"The access to some of the world's fastest supercomputers allowed us to perform simulations of planetesimal formation in exquisite detail," Johansen said. "We were able to measure the size distribution of the newly born planetesimals and compare this to the sizes of the asteroids in the solar system."

Current evidence suggests that asteroids reflect the original size distribution of planetesimals, because, as Mac Low says, "asteroids are essentially fossil planetesimals—ones that never got swept up into planet embryos."

So the researchers were surprised to find that the size distribution predicted by their simulations does not agree with the distribution observed in asteroids. In particular, the simulations did not produce enough large planetesimals.

The research team's focus next turned to chondrules, which make up about 50 percent of the mass of the most primitive meteorites (aptly called chondrites).

"The interesting thing about chondrules is that they're just the right size to get slowed down by the gas around planetesimals, which causes them to fall down and accumulate like sand piling up in a sandstorm," Mac Low said.

When the researchers accounted for the accretion of these glassy beads, they were able to reproduce the size distribution observed in asteroids. Their model also shows that larger planetesimals capture chondrules more easily, making them grow, become unstable in their orbits, and in some cases, collide with other planetesimals to build big planets like Mars and Earth or bigger objects like the cores of gas giants such as Jupiter.

The scientists are now eager to see more asteroid surface sampling and characterization studies, looking for the chondrule-rich crusts that could provide hard evidence for their theory.

Explore further: Rocky planets may orbit many double stars

More information: Science Advances 17 Apr 2015: Vol. 1 no. 3 e1500109 DOI: 10.1126/sciadv.1500109

Related Stories

Rocky planets may orbit many double stars

March 30, 2015

Luke Skywalker's home in "Star Wars" is the desert planet Tatooine, with twin sunsets because it orbits two stars. So far, only uninhabitable gas-giant planets have been identified circling such binary stars, and many researchers ...

Dirty stars make good solar system hosts (w/ Video)

October 6, 2009

Some stars are lonely behemoths, with no surrounding planets or asteroids, while others sport a skirt of attendant planetary bodies. New research published this week in The Astrophysical Journal Letters explains why the ...

Wandering Jupiter accounts for our unusual solar system

March 23, 2015

Long before Mercury, Venus, Earth, and Mars formed, it seems that the inner solar system may have harbored a number of super-Earths—planets larger than Earth but smaller than Neptune. If so, those planets are long gone—broken ...

A rare snapshot of a planetary construction site

October 24, 2013

(Phys.org) —Planets are formed in disks of gas and dust around nascent stars. Now, combined observations with the compound telescope ALMA and the Herschel Space Observatory have produced a rare view of a planetary construction ...

Recommended for you

10 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

ian_miller
1 / 5 (1) Apr 17, 2015
Small lumps of sand do not stick together. Any turbulence on the gas will cause the "dust bunnies" to dissipate. As an aside, dust does not a boulder make, at least without some reason to stick together. Chondrules are only found in some meteorites. Dust closer to the sun would have melted and formed reasonably big stones, e.g, iron meteorites.
allergg
not rated yet Apr 18, 2015
I think that when tiny particles get super cold, they become somewhat super conductive and then resultant tiny magnetic fields in these tiny particles start attracting themselves to each other. And depending where these particles arise in a nebula will determine whether they become big enough to become stars or planets.
SnowballSolarSystem _SSS_
1 / 5 (3) Apr 18, 2015
I do admire the ingenuity of the pebble accretion bandwagon in attempting to salvage it from its multitudinous problems, but why is that the only game in town?

I agree that pebble accretion works great all the way up to the size of chondrules, but to my knowledge, no one on the bandwagon has ever attempted to parse chondrites for the least shred of evidence that they formed by core accretion?

I suggest that similar-color-and-sized-binary, cold classical Kuiper belt objects (in 'cold' low-eccentricity and low-inclination orbits) are best explained by in situ gravitational instability (GI)accompanied by 'fragmentation' (bifurcation due to excess angular momentum during GI) AFTER the formation of Neptune's outer resonances.
retrosurf
not rated yet Apr 18, 2015
"In the beginning, there were dust bunnies..."
SnowballSolarSystem _SSS_
1 / 5 (2) Apr 18, 2015
"In the beginning, there were dust bunnies..."

melted by the 3 million year flare star phase of our Sun following its binary spiral-in merger at 4,568 Ma, forming chondrules from a secondary debris disk laced with stellar-merger-nucleosynthesis f-process radionuclides (namely 26Al, 60Fe) and helium-burning stable isotopes (principally 12C and 16O).

Just for one minute, imagine that all our solar system theories were wrong...........

How would the scientific community ever go about untangling an interrelated mess when probably 99% of our intellectual effort goes into little mincing baby steps that resemble a random walk around bandwagon theories?
jsdarkdestruction
5 / 5 (3) Apr 19, 2015
"In the beginning, there were dust bunnies..."

melted by the 3 million year flare star phase of our Sun following its binary spiral-in merger at 4,568 Ma, forming chondrules from a secondary debris disk laced with stellar-merger-nucleosynthesis f-process radionuclides (namely 26Al, 60Fe) and helium-burning stable isotopes (principally 12C and 16O).

Just for one minute, imagine that all our solar system theories were wrong...........

How would the scientific community ever go about untangling an interrelated mess when probably 99% of our intellectual effort goes into little mincing baby steps that resemble a random walk around bandwagon theories?

What evidence do you have for this idea? Have you tried to talk to any of the people in the field? What issues did they find and point out to you? How did you respond?
Torbjorn_Larsson_OM
5 / 5 (1) Apr 19, 2015
Very good!

The hierarchical accretion process, to badly paraphrase Lewis Fry Richardson 'small bodies made bigger bodies that feed on their massivity, and big bodies made bigger bodies and so on to planetivity', must break down in the middle. The observational evidence is the meter size barrier [ http://astrobites...barrier/ ] and Rosetta's evidence that the larger rocks in Churyumov-Gerasimenko is made up of concretions. While we know the initial (dust production) and end stages (initial and late bombardment) are "all size accretion".

Earlier proposed instability mechanisms operate on a massive scale, and haven't been seen in planetary disk observations. Here the model turn the meter size barrier into the controlling factor that temporarily stopped all-size accretion. And they can, and do, test the outcome in many ways!

I think they broke the barrier. And I note that a swedish astronomer was the lead. (Yay! =D)
Torbjorn_Larsson_OM
5 / 5 (2) Apr 19, 2015
I forgot to note that their model works out to ~25 AU for the solar system, consistent with the seen planets and Rosetta's observations.

@Ian Miller, allergg, SSS: The same questions can be put to all of you.

- What evidence do you have?

(Some of your claims is directly inconsistent with the paper, which I haven't had time to read yet but browsed vital parts, such as that they predict the, correct, size distribution and chondrule size distribution of chondrites, early non-differentiated asteroids. Not the modern population of asteroids.)

- Why do you think the planetary scientists, and hence casual readers, think your ideas are chicken shit?

(As can be seen by that the scientists work with other, fruitful ideas which are based on evidence.)
Torbjorn_Larsson_OM
not rated yet Apr 19, 2015
Consistent with the Nice model I should have said, since Neptune now resides 30 AU out, I forgot the later migrations. My lapse!
SuperThunderRocketJockey
not rated yet Apr 21, 2015
That's how Woodstock formed too!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.