What Aliens Would See When Spying On Our Solar System

Alien astronomers searching for planets in our solar system would at least be able to see Neptune inside the dusty disk that surrounds our planetary neighborhood, a new study suggests.

The method used in the study could be a new tool for astronomers on Earth seeking out strange new worlds, since the dust rings around other stars could be used to find their outer planets, too.

Our solar system has one of these dust clouds out beyond the orbit of Neptune, researchers said.

The new study models how our sun's dust cloud formed and evolved as our solar system matured. [Simulated view of our solar system's dust cloud.]

"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," study leader Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., said in a statement. "We're hoping our models will help us spot Neptune-sized worlds around other stars."

A haze of dusty debris

Much of our solar system's dust originates in the faraway Kuiper Belt, a cold-storage zone beyond Neptune where millions of icy bodies — including the dwarf planet Pluto — orbit the sun. Scientists think the region is an older, leaner version of the debris disks they've seen around distant stars like Vega and Fomalhaut.

Kuiper Belt objects sometimes crash into each other, producing a flurry of icy grains, the researchers said. This dust moves about, jostled by a variety of forces.  

There's the gravitational tug of the sun and planets, for starters. And the solar wind — the stream of charged particles flowing from the sun — can work to bring dust closer to the sun, researchers said. Sunlight can either pull dust inward or push it outward, depending on the size of the grain. The particles also run into each other, and these collisions can destroy the fragile grains.

So tracking the dust is a challenge.

"People felt that the collision calculation couldn't be done because there are just too many of these tiny grains to keep track of," Kuchner said. "We found a way to do it, and that has opened up a whole new landscape."

The scientists reported their findings in the Sept. 7 edition of The Astronomical Journal. NASA announced them Sept. 23.

Solar system in a supercomputer

Using NASA's Discover supercomputer, the researchers kept tabs on 75,000 simulated dust particles as they interacted with the outer planets, sunlight, the solar wind and each other.

The size of the model dust ranged from about the width of a needle's eye (0.05 inches or 1.2 millimeters) to less than a thousandth of that size, about as large as the particles in smoke. From the resulting data, the researchers created synthetic images representing how the solar system looks from afar, in infrared light.

They found that Neptune's gravity wrangles nearby dust particles into preferred orbits, creating a clear zone around the planet.

Astronomers on Earth might be able to spot alien planets by searching for such dust gaps in distant star systems, the researchers said.

Going back in time

The researchers also modeled the dust cloud's appearance at various stages in the past. The Kuiper Belt used to hold many more objects that crashed together more frequently, the researchers said, generating dust at a faster pace.

So the team sped up their models' dust-production rates to reflect conditions of the Kuiper Belt when it was 15 million years old, 100 million years old, and 700 million years old.

Today the Kuiper Belt, like the solar system itself, is about 4.6 billion years old.

"We were just astounded by what we saw," Kuchner said.

With more dust, collisions occur more frequently, and the likelihood that large grains would survive to drift out of the Kuiper Belt dropped sharply, the researchers said.

Stepping back through time, today's broad dusty disk collapses into a dense, bright ring, which looks a lot like the rings seen around some other stars, especially Fomalhaut.

Fomalhaut is a young star about 25 light-years from Earth; astronomers have confirmed that at least one planet circles the star.  

"The amazing thing is that we've already seen these narrow rings around other stars," said study co-author Christopher Stark of the Carnegie Institution for Science in Washington, D.C. "One of our next steps will be to simulate the debris disks around Fomalhaut and other stars to see what the dust distribution tells us about the presence of planets."