Mercury and Mars - Leftover Planets.

Adding another wrinkle to the difficult task of categorizing planets, a new computer model suggests that Mercury and Mars were formed from particles ejected during the formation of Earth and Venus.

The computer model by UCLA astronomer Brad Hansen was a result of flaws in the traditional of assumption of an evenly distributed particle disk during planetary formation. One obvious flaw being that this should have resulted in the rocky planets all being essentially the same size. Hansen "proposes that the dust disk fragmented into bands of debris at various distances from the sun—much like the rings of Saturn." In this theory Earth and Venus formed in one large band, hence their similar sizes, while Mercury and Mars were formed mostly from particles ejected from the main band by the gravity of the larger planets.

One good aspect of theory is that it makes predictions that can be tested. This model predicts that "the composition of the solar system's four rocky planets should be strikingly similar" while the standard model predicts that they should vary in accordance with their distance from the sun.

<Read more at National Geographic>