A study finds that Earth's water may have come from materials that were already present in the inner solar system at the time the planet was formed and not from comets or asteroids delivering such water. Earth is the only planet to have liquid water on its surface, a fundamental characteristic when it comes to explaining the dawn of life.

Most of the water present on Earth today has probably been there right from the very beginning.  Earth was formed in a region of the Solar System where temperatures were too high for water to condense and clump together with other solids as ice, long supporting the hypothesis of a late addition of water. However, the amount of water present in the rocks that were the building blocks of the Earth had never been accurately estimated. Hence, scientists focus on meteorites that contain sufficient hydrogen to deliver at least three times the amount of water contained in the Earth's oceans, called enstatite chondrites.

The hydrogen present in these meteorites has the same isotopic composition as that of the water stored in the Earth’s mantle,  while the composition of the oceans is consistent with a mixture containing 95% of water from the enstatite chondrites and a mere 5% of water delivered by comets or asteroids rich in water. Therefore Earth appears to have obtained the overwhelming majority of its water from its constituent materials.