for National Geographic News
The model could explain some characteristics of Mars and Mercury that have long puzzled scientists, said Brad Hansen, an astronomer at the University of California, Los Angeles.
"In this picture, Mars and Mercury are essentially byproducts" of Earth and Venus, said Hansen, who presented his research at a recent meeting of the American Astronomical Society in Long Beach, California.
Scientists generally agree that Earth and the other rocky planets in the solar system formed from a wispy disk of gas and dust that ringed the infant sun some 4.5 billion years ago.
(Related: "Planet-Forming Disk Spotted Around Dead Star" [April 5, 2006].)
Over time, the microscopic dust particles coalesced into pebble-size clumps. The pebbles became boulders that became mountain-size "planetesimals," which merged into full-fledged planets.
In computer simulations of this process, scientists typically assume that the initial dust particles were distributed evenly in a disk around the sun.
"While this is a logical first guess, there are some problems," said Andrew Youdin, a planetary modeler at the Canadian Institute for Theoretical Astrophysics (CITA) who was not involved in the research.
If the rocky planets formed from a homogenous debris disk, they should all be roughly the same size and orbit the sun in similar circular orbits, Youdin explained.
In reality, however, Venus and Earth are much more massive than Mercury and Mars, and the orbits of the latter two planets are more elliptical, or eccentric, than expected.
"The traditional explanation is that this is luck—or more scientifically, 'chaos'—with Jupiter's proximity to Mars perhaps playing a role," Youdin said, referring to the large gravitational pull of a planet as big as Jupiter.
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