The earth is a unique planet in the universe, and its formation is a story of violent cosmic events. Scientists have pieced together how a massive cloud of gas and dust collapsed under the force of gravity to form our sun and the planets. The earth's core, which is made of molten iron, creates a magnetic field that protects the planet from the sun's radiation. The moon, which is the earth's constant companion, was formed by a massive collision between the earth and another planet. The earth's water, which is essential for life, may have been brought to the planet by a shower of meteorites.