According to the widely accepted nebular hypothesis, the planets and moons in the solar system, including Earth, formed from a huge cloud of mostly hydrogen and helium. Contraction, rotation, and dropping temperatures resulted in the formation of small particles, the first being nickel and iron. These began to stick together, and after tens of millions of years of condensation and accretion, the earth was formed about 5 billion years ago. Although the earth has been cooling ever since and has formed a hard outer crust, part of the interior is still hot and molten.