No one really knows how the oceans filled with water. One theory is that volcanoes released enough water vapor to allow the oceans’ waters to condense. Another theory states that comets bombarded Earth just after the formation of the solar system, bringing enough water to eventually fill the oceans.

The origin of Earth’s atmosphere is also debated, but not as intensely. In this case, it is more likely that some of the atmosphere originated from gases that were part of the solar nebula, gases brought by comets, and those produced from volcanic activity. Earth probably would have had a thicker atmosphere, too, but the young, active Sun’s heat boiled away the lighter materials—elements that are still found today around the gas giant planets Jupiter, Saturn, Uranus, and Neptune.