The water that surrounds us is older than the solar system itself.

The water that surrounds us—the water we drink, the water in the oceans, and even the water that makes up our bodies—is older than our planet. In fact, some of that water existed before the Sun began to shine. We often say we are stardust, but we are also cosmic water.

In the universe, water can originate from various astrophysical and cosmochemical processes. It can form within the immense molecular clouds where stars are formed; vast masses of molecular hydrogen and dust that travel through the interstellar medium of the galaxy. One of the most famous is the Eagle Nebula, with its spectacular "Pillars of Creation." Others, like the Orion Nebula, can even be seen with the naked eye, as a faint smudge beneath the "three stars" of the Orion constellation on winter nights.

When ultraviolet radiation passes through these clouds, it breaks apart certain molecules, releasing oxygen atoms, which combine with hydrogen to form water on the surface of dust grains. This water is slightly "heavy" due to its isotopic composition. Therefore, when a star like our Sun is born in one of these clouds, it already has water available for its planetary system. But the process doesn't stop there: during the Sun's birth, new water molecules form at the edges of the solar system, where the ultraviolet rays from the young Sun itself trigger similar chemical reactions. Finally, during the formation of a new star and its system, a third type of water is formed simply by gas condensation when it reaches the appropriate temperature; this time, "light" water, the most abundant kind.

Recently, the journal Nature published a study in which a team of scientists detected, for the first time, "heavy" water in the disk of gas and dust surrounding a very young star, V883 Ori, using the Atacama Large Millimeter Array (ALMA) radio telescope. This discovery confirms what the study of meteorites in the laboratory had long revealed: that a solar system contains water from various origins. Earth, however, formed too close to the Sun to preserve water in its initial parent material. Water-rich asteroids were located much farther out, in cold, distant regions of the solar system where water could not only survive but also be preserved in asteroids that were not swallowed by the nascent Sun. During the formation of our planet, the gravitational interaction between Jupiter and Saturn destabilized the orbits of distant asteroids, causing many to be flung into the inner regions of the solar system. For about 200 million years, a shower of primitive meteorites laden with water and organic matter bombarded the early Earth, providing the essential ingredients for life. In fact, the craters on the moon record this violent period of large impacts, known as the Late Heavy Bombardment.

Ultimately, this time astrophysics and meteoritics have intertwined to reconstruct the history of water: from interstellar space to our blue planet.