Recent simulations have suggested that the gas giants of our solar system formed with circular orbits but moved into their more elongated paths about 4 billion years ago – 700 million years after the solar system formed. While the gas giants were in circular orbits, rocky planets should have formed in stable orbits out to a distance of 2.2 astronomical units (1 AU = 1 Earth- Sun distance).
However, there are no planets between Mars, which lies at 1.5 AU from the sun, and Jupiter at 5.2 AU. That puzzled Sean Raymond of the University of Colorado in Boulder and John Chambers of the Carnegie Institution in Washington DC. "There's room for another planet between Mars and Jupiter," says Chambers. "Given that planets formed everywhere else, why couldn't another planet have formed there?"
The researchers modelled what would have happened in that region, and found that a planet about the size of Mars could have formed 2 AU from the sun and remained stable there until the orbits of the gas giant changed.
Their simulations show that the migration of Jupiter and Saturn greatly disturbed the orbits of other planets, and this could have kicked the fifth rocky planet out of its orbit – either into the sun or out of the solar system altogether. If the planet was swallowed by the sun, it was probably too small to leave any measurable trace in the sun's composition. "But it's possible the orbits of the asteroids today show some memory of having had a planet in the asteroid belt," says Chambers.
Raymond believes this evidence lies in a family of asteroids called Hungaria, which are clustered at 1.9 AU. These bodies orbit in a plane tilted by about 25 degrees to the main disc of the solar system, which suggests they may have been swept off course in wake of the lost planet as it ploughed through the asteroid belt, says Raymond. "They're in this tiny little area that's just barely stable – I don't know how else they would have gotten there."
If it can be confirmed that the planet once existed, it would imply that planetary systems are dynamic environments, says Chambers.
The researchers presented their results at the Astrobiology Science Conference in Washington DC last week.
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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 8 APRIL 2006
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