The Earth was formed about 4.5 billion years ago, and the early Earth was largely dry and rocky. Temperatures were too high to support an atmosphere, and any water would have boiled away. Water is an essential requirement for life as we know it, as well as helping to regulate the climate. Impacts with comets and asteroids are proposed for resupplying the Earth's water.
Asteroids and Comets in the Solar system
Outside the orbit of Neptune is the Kuiper belt of asteroids. These can become perturbed by the planets and they can then have highly eccentric orbits around the sun.
Volatile molecules, including water, are boiled off the asteroids as they near the sun. These volatiles give the astronomical bodies characteristic "tails" and they are then known as comets.
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