http://www.newsweek.com/scientists-discover-secrets-earth-venus-violent-youth-planet-formation-hubble-1633023

Researchers have discovered that planets in the inner solar system could have been born as the result of multiple "hit and run" collisions.

Using computer modeling and simulations of tremendous impacts, the team at the Lunar and Planetary Laboratory (LPL), University of Arizona, found that Earth and Venus could have been formed when smaller planetary building blocks collided and stuck together.