Sweeping Up A Planet

Research posts

The key to forming a planet could be found in some of the tiniest pieces of space debris—glassy beads the size of grains of sand that are known as chondrules. According to simulations developed in part by researchers at the Museum, asteroid-like objects known as planetesimals sweep up these glassy grains, growing into planets as they accumulate more and more dusty particles. The results of the simulations, carried out with collaborators at Lund University in Sweden and elsewhere, were published today in the journal Science Advances

NASA Planetesimaks

Disks of dust like these, imaged by the Hubble Space Telescope in 2014, can coalesce into “planetesimals.”

© NASA/Hubble Space Telescope


“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 solar system 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.”

Planets start out small, as 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 show that when those boulders get larger than a person, they begin to orbit faster than the surrounding gas. The resulting headwind brakes them in their orbit, so that they drift into their parent star within about 100 orbits. In addition, fast-moving boulders break apart, rather than sticking together, when they collide. So how do some of these objects stick around long enough to grow into planets?

In 2007 Mac Low and collaborators, led by the lead author on the current work, Anders Johansen of Lund University, proposed one answer: a mechanism called the streaming instability. The streaming instability occurs when one orbit contains more boulders than its neighbors. Boulders in the same orbit sweep the gas with them reducing the headwind, and slowing inward drift to their doom. This allows boulders further out to drift into that orbit, continually increasing their density there. In time, the density of boulders becomes so high that they can collapse together by their own gravity into a stable planetesimal.

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 on a network of supercomputers and found that the distribution of sizes resulting contained too few large objects when compared to modern asteroids.  They then realized that chondrules could play a key role in this model of planet formation.

Chondrules Picturing Science

This image was taken with an electron microprobe, which excites the atoms on the surface, revealing their mineral compositions: red for magnesium, green for calcium, and blue for aluminum. Small spheres of minerals called chondrules are key to the formation of some celestial objects like this meteorite. 

©AMNH/D. Ebel 


“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. 

Their model showed that the larger planetesimals are, the more easily they capture chondrules, and the bigger they grow. When these larger planetesimals collide, they can begin to build big planets like Mars and Earth or even larger 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.

Learn more in the Museum's press release. 

Tags: Astronomy