UK Daily Mail 
Sept 21, 2011
Scientists believe our solar system may have once had a fifth gas planet that was ejected from the solar system and ‘orphaned’.
A new study by David Nesvorny, from the Southwest Research Institute in Colorado, U.S., used different computer simulations to explore what the solar system looked like four billion years ago.
He discovered that back then planets had not yet settled into their existing orbits as they migrated and moved around.
However, after a series of tests he worked out that the solar system we recognise today could never have emerged without the existence of a fifth planet.