Planet Earth formed 4.5 billion years ago, when boulders, rocks, debree, and meteors collected and slowly started to form Earth. But, one day a planetesimal, (which was about the size of mars) crashed and collided with Earth, this caused some boulders and chunks of what was part of Earth to break off. Over time the planetesimal became part of Earth and slowly got its shape that it has today. But, the boulders and chunks of rock that were once part of Earth came together and slowly stated to form the moon. When Earth was first formed it was filled with mountains and volcanoes, also Earth was made up of helium and hydrogen gases.