Outer planets responsible for Mercury's oddities.

The weird properties of Mercury, such as its small size and large core, have been difficult to explain with simulations of the solar system's formation. However, researchers have found that the giant outer planets played a major role in shaping the inner solar system, depleting the innermost regions of the protoplanetary disk and triggering instabilities that sent material into the sun or out of the solar system. The researchers also found that young Mercury suffered dozens of collisions with similarly sized objects, which built up its core mass but not its mantle and crust materials.