This Week

5 April 2006

Missing: a fifth terrestrial planet

A FIFTH terrestrial planet may once have orbited between Mars and Jupiter. Although gravitational disturbances would have sent the planet hurtling into the sun or out into space long ago, traces of this long-gone world may still be visible in part of the asteroid belt today.

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 ...