April 26, 2006

Big Planets’ Gravitational Dance Made Them Tilt

By Deborah Zabarenko

WASHINGTON -- An early gravitational dance made the giant planets tilt the way they do -- which is different from the way Earth and the other smaller planets tilt, an astronomer reported on Wednesday.

The shift probably happened billions of years ago when the bigger planets in our solar system were closer together than they are now, and the gravity of each one exerted a pull on the others, said Adrian Brunini of the Facultad de Ciencias Astronomicas y Geofisicas in Buenos Aires.

This "neutral gravitational interaction" caused Jupiter, Saturn, Uranus and Neptune to have tilted axes that were determined as they moved through the solar system to take their current positions far from the sun, Brunini said in a telephone interview.

This is a departure from an earlier theory that holds that the massive planets' tilts -- or obliquities, as astronomers call them -- were caused by collisions with Earth-sized space rocks during the early period of the solar system.

"This model has some problems that were not clear how to solve," Brunini said. "For example, we believe that such a big object never existed in the outer solar system."

In research published in the current edition of the journal Nature, Brunini used numerical models to show that the outer planets' obliquities could have been created by gravitational interactions.

All the planets in our solar system have tilted axes but the bigger ones have axes that lean at a constant angle, while the smaller ones like Earth have obliquities that can change.

Despite the potential for change, Earth's axis has been leaning about 23 degrees for millions of years and is almost completely stabilized by the moon's gravitational pull, Brunini said, but Mars' axis might change over tens of millions of years.

For humans, the reliability of Earth's tilted axis is important since it is responsible for the change of seasons. At the point in its annual orbit where Earth's northern hemisphere leans away from the sun, it's winter; when the southern hemisphere tilts away, it's winter there.

While the more massive planets have stable obliquities, they range in size from a nearly perpendicular 3 degrees for Jupiter to about 97 degrees for Uranus. Brunini said.