To determine the age of our solar system’s water, researchers focused on its ratio of hydrogen to deuterium, called “heavy hydrogen” because it has an extra neutron. Interstellar ice has a very high ratio of deuterium to hydrogen because it formed in very cold temperatures. Scientists already knew this from looking at the composition of comets and asteroids.

But, confounding the matter, deuterium levels in the solar system’s water have also been rising ever since the sun formed. So to determine if the sun alone could produce today’s levels of the isotope, researchers built a computer model that essentially wound back the clock to the beginning of the solar system and assumed no inherited deuterium.

However, the model system was incapable of producing deuterium to hydrogen ratios that were as high as those found in our solar system. Therefore, researchers estimate, 30 to 50 percent of our solar system’s water was already a part of the ancient molecular cloud that spawned the Sun and planets.

Up to half of it, but still. Something about this discovery is strangely poetic. On the scientific side, I’ve read that this means water is not all that rare; there might actually be a lot of water-based organism out there.