ALIEN PLANET DEBRIS DISCOVERED DEEP UNDER EARTH'S CRUST

A planetary collision so big that it formed the Moon also left behind remnants that changed Earth's mantle

- 

Around 4.5 billion years ago, a massive collision with another planet changed Earth forever. Now, scientists have discovered that debris from the impact is still lodged inside our world. The pieces of material, measuring tens of kilometres, could be relics of an ancient protoplanet (a large body of matter in the process of developing into a planet) known as Theia, according to the scientists. Theia would have been Mars-sized when it crashed into proto-Earth.

The new study, published in Nature, found that the debris of Theia buried and preserved under the crust forms up to two per cent of Earth's mass. The alien material is also denser than Earth's mantle and scientists think this is because it's enriched with iron, like Moon rock.