If alien astronomers from a nearby star system pointed their version of the Hubble Space Telescope at Earth, astronomer Markus Landgraf believes they would not see our planet but they would find hints of our presence. With their infrared camera, the smart aliens would detect a vast donut-shaped ring of dust with a classic hole in the middle, all surrounding a yellow star. A little math, Landgraf says, and they could deduce the presence of a large planet, like Jupiter, that had cleared out the hole. They would also spot Neptune's signature scrawled in the dust.
If this culture's astronomical knowledge were as advanced as ours, they would then wonder if a planet like their own, a habitable world, also orbited the yellow star.
Landgraf, an earthbound astronomer working in Darmstadt, Germany, as a mission analyst for the European Space Agency, studies data collected by a pair of space probes that left Earth three decades ago. His research, along with that of some colleagues, is making a mirror out of our solar system, one that reflects on other stars and the planets they might harbor.
able --> Faster than a bullet
Our solar system's donut is made up of dust grains of many sizes. In order to see signs of planets in the dust, the alien astronomers would need to tune their instruments to spot the smallest dust. Landgraf studies this, too, bits that are just one-hundredth of a millimeter, or dozens of times tinier than a typical grain of sand. It is everywhere in our solar system, zipping around ten times faster than a rifle bullet.
Near Earth's orbit around the Sun, there is about one spec of this small dust in each cubic kilometer of space, Landgraf says. If there were no planets circling the Sun, the dust inside Jupiter's orbit would be at least twice as dense, he said.
The densest portion of the dust donut is beyond Saturn's orbit, at about fifteen times the Sun-Earth distance. Here, larger dust grains rule.
The Pioneer 10 and 11 spacecraft found this far-out, high concentration of dust in the 1970s and early '80s. Previously, astronomers had expected dust to thin out with distance from the Sun, because they knew that solar radiation created drag on the dust particles, causing them to spiral inward to the Sun.
Dust factories
Ever since the Pioneer findings, astronomers have wondered where all the dust comes from. Since theory held that such dust would eventually be shipped to the Sun, there must be some mighty dust factories out in the suburbs of the solar system, replenishing the supply. Landgraf and his colleagues have estimated that 50 tons of dust are produced every second.
As with many things in the universe, violent collisions are behind it all.
Using the old Pioneer data, Landgraf and his colleagues demonstrated through computer modeling that comet-like objects beyond the orbit of Neptune are the sources of the dust.
"They tend to collide with each other," Landgraf explains. "Much like a brick you let fall from your roof that hits the ground in your driveway, they explode into millions of pieces."
The solution might seem obvious. But the distant comets, often called Kuiper Belt Objects, were not known until the early 1990s. Only in the past five years or so has anyone tried to link them to the dust, Landgraf said.
The research revealed another dust-generation method at work, too. Alien dust -- typically even smaller particles -- wafts through the galaxy, and our solar system continually plows through it, Landgraf said. Like sandpaper, this interstellar dust rubs on comets and makes more dust.
Trapped
There's a hole in this whole explanation, of course: If the dust drifts toward the Sun, why is it not evenly distributed? What makes the hole?
An answer to this question comes from earlier work by Landgraf's colleague, Jer-Chyi Liou of Lockheed Martin Space Operations at the Johnson Space Center. Liou explains that the gravity of Neptune acts like a dam, and the dust particles pile up.
"Most of them are trapped there for million of years before they escape the traps and continue their journey toward the Sun," Liou says.
When the grains do get through, the larger gravitational fields of Jupiter and Saturn redirect much of the dust back out. Way out, as in clear out of the solar system. Only about 20 percent of the dust gets past Jupiter, Liou said.
The latest research by Landgraf and Liou, done with Eberhard Grn and the late Herb Zook, will be detailed in the Astronomical Journal this spring.
Liou said their new study is a first step in confirming the presence of the distant dust disk, called Edgeworth-Kuiper Belt dust disk, thought to extend well beyond Pluto. More observations will be needed to fully understand the structure of the disk, he said.
Next Page: How this helps find other solar systems
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