According to a report in National Geographic News, the model, made by Brad Hansen, an astronomer at the University of California, Los Angeles, could explain some characteristics of Mars and Mercury that have long puzzled scientists.
“In this picture, Mars and Mercury are essentially byproducts” of Earth and Venus, said Hansen.
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.
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.
Hansen proposes that the dust disk fragmented into bands of debris at various distances from the sun—much like the rings of Saturn.
According to this scenario, Earth and Venus formed within one particularly thick band, or annulus, in the inner solar system.
As the young Earth and Venus circled the sun, they waded through a sea of pebble- and mountain-size debris.
The two planets captured and assimilated some of this debris, but hurled other chunks out of the annulus. Most of these ejected particles eventually circled back, returning to the annulus. But other bits collided with one another during their exile.
“If this happens, the particles are put on a new orbit,” Hansen said. “They become decoupled from the main annulus and don’t come back,” he added.
Computer simulations by Hansen suggest Mercury and Mars could have formed from such separated debris. According to Hansen’s estimates, about 90 percent of the debris in the annulus went into the formations of Earth and Venus, while the leftovers formed Mercury and Mars.