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An artist's rendering of a gas giant planet orbiting a distant star

A Jupiter-like gas giant planet orbits a young star in an artist's conception.

Picture courtesy T. Pyle (SSC), NASA/JPL-Caltech

Brian Handwerk

for National Geographic News

January 8, 2010

Of the billions of stars in our Milky Way galaxy, 15 percent may host "twins" of our solar system, a new study says.

While that might not sound like much, the find suggests that several hundred million star systems look a lot like the one we call home, the study authors say.

The research is based on surveys of stars with gas giant planets—similar to Jupiter and Saturn—that orbit far from their stars.

As in our solar system, vast distances stretch between these stars and their gas giants. This creates ample room for rocky planets to thrive in the stars' habitable zones, the regions where liquid water can exist.

And that boosts the likelihood that other Earths, and maybe even other forms of life, abound in the Milky Way.

"For the first ten years of planet hunting, we were feeling a bit worried—other systems looked so different from our own solar system," noted Debra Fischer, an astronomer at San Francisco State University who was not involved in the research.

"[These] results are reassuring us that there are solar systems akin to our own. This is real data that strengthens the hypothesis that there are many habitable worlds like our Earth."

Cosmic Magnifying Glass

Astronomers think that gas giants generally form farther from their stars, while rocky worlds like Earth form closer in.

But in some star systems it's thought that gas giants migrate inward, knocking any smaller planets out of their orbits or destroying the rocky worlds outright.

Meanwhile, star systems like ours have gas giants in stable outer orbits.

"In these systems there is room for terrestrial planets to prosper and not get knocked out of their orbits," said study co-author Andy Gould, an astronomer at Ohio State University. 

What's more, studies of Jupiter suggest that outer gas giants can act as gravitational shields, protecting inner rocky worlds—and any life-forms on them—from frequent asteroid impacts.

To find such star systems, nearly a hundred scientists joined forces as part of the Microlensing Follow-Up Network, or MicroFUN, to scour the galaxy using a technique called gravitational microlensing.

In this method, when one star passes in front of another, as seen from Earth, the nearer star's gravity acts like a lens, bending and magnifying the more distant star's light.

If the nearer star has orbiting planets, keen-eyed observers can spot the subtle clues of their presence in the magnified light.

If all the stars in the Milky Way hosted solar system twins, astronomers should have found at least six such systems them by now, according to a statistical analysis of four years' worth of microlensing data.

But so far, only one other system like ours has been spotted: In 2006 astronomers found a star with its own versions of Jupiter and Saturn.

That means just 15 percent of the galaxy's stars must have solar systems like ours, Gould and colleagues announced this week at the 215th meeting of the American Astronomical Society in Washington, D.C.

Rough Estimate

Gould cautioned that his team's "very rough" estimate is based on limited data and is likely to change as other star systems are spotted in the coming years.

For instance, some systems could house only Earthlike planets and no gas giants, Gould said. But for now we don't know, since most stars are too far away for current instruments to detect small, rocky worlds.

That might change as newer planet-hunting missions, such as the Kepler space telescope, begin to bear fruit.

Still, the new findings are in line with recent studies that say lower-mass planets such as "super Earths" might be relatively common in the galaxy, said Michael Meyer, of the Institute for Astronomy in Zurich, Switzerland.

"It may turn out that stars harboring even lower-mass terrestrial planets may be the rule rather than the exception," said Meyer, who was not involved in the study.

"If so, understanding the dynamical relationship between the smaller terrestrial planets and the more massive gas and ice giants may help us to understand how common Earthlike planets might be in our galaxy."

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