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Researchers say key to life may lie in planets' orbits

NU scientists say the regularity of planets' orbits in the Milky Way might

By Elizabeth Sabrio

April 22, 2005

The reason life might be unique to Earth may lie in its special orbit, according to Northwestern scientists.

While the major planets in our solar system travel around the sun in near-circular patterns, the planets in the nearby Upsilon Andromedae system take more irregular, egg-shaped orbits.

It is this elliptical orbit that might cause planets to be unable to sustain life as seen on Earth.

In a paper published April 14 in the science journal Nature, astrophysicists at Northwestern and a post-doctoral student at University of California-Berkeley reported evidence explaining the origins of this unusual orbit.

Frederic Rasio, research team member and associate professor of physics and astronomy at NU, explained the findings.

When planets travel around the same star in elliptical orbits, they continue on their paths as long as they don't get too close to one another.

But because of the planets' irregular orbits, at some point the pull of each planet becomes stronger than the gravitational force pulling the planets around the system's central star.

As the two planets pull each other closer, one planet will eventually repel the other. The repelled planet will be sent either into space or on an even more elliptical path.

"They slingshot each other," Rasio said.

The findings, named "planet-planet scattering," are based on more than 10 years of observations of the Upsilon Andromedae planetary system.

Before these observations began, scientists weren't sure if there were planets orbit a star as Earth orbits the sun.

Now the findings on the Upsilon Andromedae system can tell scientists more about our solar system.

"Sure there may be planetary systems that orbit a typical star like the sun," Rasio said. "But to remain stable in our orbit -- that may be why we are special."

By using computer simulation, scientists can predict that Earth's orbit won't change in another two billion or three billion years, he said.

But if they change the parts of the equation just a bit, things wouldn't be the same.

"If Uranus is, say, twice as massive, our orbit may be very different," Rasio said. "It doesn't take much of a change to alter the stability. Our stability may be a rather rare property."

Reach Elizabeth Sabrio at

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