Formation of the moon

A new theory proposed that a Mars-sized planet collided with Earth, forming the Moon from Earth's outer layers. This theory is a favored explanation due to computer simulations supporting its feasibility.
 William K. Hartmann United States 1974 Formation of the moon

In 1974, American astronomer William K. Hartmann suggested that a planet the size of Mars had a glancing collision with Earth in the early days of the Solar System, forming the Moon from Earth's outer layers while the cores merged.