A hole in the dust disk surrounding our solar system would tell alien observers there are planets here, a new simulation shows. The new model, which tracks thousands of tiny particles beyond the orbit of Neptune, could help astronomers work out the properties of planets in other stars' dust disks.

"We're trying to create a new planet-search technique, and we're practicing on the solar system," said NASA exoplanet scientist Marc Kuchner, lead author of a paper describing the results in the Sept. 7 Astrophysical Journal.

The cloud of dust comes from the Kuiper belt, the region beyond Neptune that contains small, icy bodies, including Pluto. These giant snowballs sometimes smack into each other, sending up flurries of ice grains. These tiny clots of ice and minerals get tugged around by the gravitational influence of giant planets, as well as the solar wind and small nudges from sunlight.

Similar clouds of dust has been spotted around several other stars, including Fomalhaut, the first star to have its planets directly photographed. Most extrasolar planets are too dim to have their portraits taken directly, but their presence can warp the disk of dust and debris around their stars into distinctive shapes, telling outside observers that planets are there.

Kuchner and co-author Christopher Stark of the University of Maryland wondered how much information these dust clouds can offer.

"This field of studying shapes of debris disks has been around for a while, but it's been qualitative," Kuchner said. "We're trying to make it quantitative. We want to get to where you can give us a picture of a debris disk, and we can say bam – here are the planets, and here's how massive they are."

The researchers used a supercomputer at NASA's Goddard Spaceflight Center to simulate 75,000 particles bumping around the Kuiper belt. Their model is the first to include not just collisions between Pluto-sized bodies, but the tiny dust grains as well.

"You have something like a billion billion million particles, and they're all hitting each other," Kuchner said. "Nobody before had figured out how to keep track of all that stuff."

Rather than directly tracking all those particles, Kuchner's model looked at two separate pictures: how the particles moved without collisions, and the density and velocity of the particles. The model then integrated the two pictures to paint a fuller portrait of the dusty disk.

The results showed that a hole in the dust follows Neptune around in its orbit. Neptune's gravity traps some of the dust grains in a gravitational tango called a resonance, which pulls the dust into clumps that precede and follow the gas giant around the sun. Earlier studies have shown that the Earth does the same thing with dust released from the asteroid belt.

"When you have low dust levels, like in today's solar system, dust moves into resonances and makes a gap, which tells you where Neptune is," Kuchner said.

When the fragile dust grains collide, they can annihilate each other, he said. In today's wide, fuzzy Kuiper belt, the particles don't meet very often, so they stick around long enough to fall into resonances with Neptune. But earlier in the solar system's history – and in planetary systems around other stars like Fomalhaut – the dust grains are destroyed before they have a chance to wander away from where they were created.

Kuchner tweaked his model to simulate the solar system at 700 million, 100 million and 15 million years old. As he turned back the clock, the dust disk collapsed into a dense, bright ring.

"Our models of this ring let us sort of look back in time to when the solar system was young," said Marc Kuchner. "When we do that we find that this ring looks just like the rings we see around other stars, like Fomalhaut."

The model has some shortcomings. For one thing, it ignores grains smaller than a certain threshold, which could be important for creating dust. Also, astronomers don't have a very clear picture of what the Kuiper belt contains, so the model's input parameters could be off.

Still, the model is a welcome addition to other Kuiper-belt researchers. "I'm happy to see another well-studied Kuiper-belt–dust paper in the community. We need it," said astronomer Amara Graps of the Southwest Research Institute in Boulder, Colorado. "The dusty byproduct of those small bodies is still not well-understood, and I believe that Marc made an important contribution."

Images: 1) NASA/Goddard/Marc Kuchner and Christopher Stark 2) NASA/ESA/P. Kalas (Univ. of California, Berkeley) et al.

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