Starship Asterisk*

New supercomputer simulations tracking the interactions of thousands of dust grains show what the solar system might look like to alien astronomers searching for planets. The models also provide a glimpse of how this view might have changed as our planetary system matured.

"The planets may be too dim to detect directly, but aliens studying the solar system could easily determine the presence of Neptune -- its gravity carves a little gap in the dust," said Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. who led the study. "We're hoping our models will help us spot Neptune-sized worlds around other stars."

The dust originates in the Kuiper Belt, a cold-storage zone beyond Neptune where millions of icy bodies -- including Pluto -- orbit the sun. Scientists believe the region is an older, leaner version of the debris disks they've seen around stars like Vega and Fomalhaut.

"Our new simulations also allow us to see how dust from the Kuiper Belt might have looked when the solar system was much younger," said Christopher Stark, who worked with Kuchner at NASA Goddard and is now at the Carnegie Institution for Science in Washington, D.C. "In effect, we can go back in time and see how the distant view of the solar system may have changed."

Kuiper Belt objects occasionally crash into each other, and this relentless bump-and-grind produces a flurry of icy grains. But tracking how this dust travels through the solar system isn't easy because small particles are subject to a variety of forces in addition to the gravitational pull of the sun and planets.

The grains are affected by the solar wind, which works to bring dust closer to the sun, and sunlight, which can either pull dust inward or push it outward. Exactly what happens depends on the size of the grain.

The particles also run into each other, and these collisions can destroy the fragile grains. A paper on the new models, which are the first to include collisions among grains, appeared in the Sept. 7 edition of The Astronomical Journal.

• Astronomical Journal 140(4) 1007 (Oct 2010) DOI: 10.1088/0004-6256/140/4/1007

[i]Credit: Boston University/NASA/Stereo[/i]

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Two satellites peering at the sun have snapped photos of Mercury's long, comet-like tail, but it took an amateur astronomer to bring the pictures to light.

The twin satellites are part of NASA's Stereo (Solar Terrestrial Relations Observatory) mission. They zip around the sun in Earth's orbit, one behind our planet and one ahead of it. Their main job is to monitor the sun and its atmosphere.

Since Mercury is so close to the sun, the planet occasionally wanders into the satellites' photos. Some of these images show a long, comet-like tail streaming off the planet, away from the sun.

Astronomers didn't notice the tail in the photos right away. But it didn't escape the eyes of Australian medical researcher Ian Musgrave.

Musgrave was poring over an Internet database of images when he discovered Mercury's wispy tail. He then asked scientists at Boston University to take a look, university researchers said.

The scientists presented their findings at the European Planetary Science Congress meeting in Rome.