Much of Earth’s Water Is Older Than the Sun, and Came From Deep Space

“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars,” astronomer and science hero Carl Sagan famously said. “We are made of star-stuff.”

He might have also added that we are partially made up of what was once very, very old ice—from well beyond our solar system.

A new study published today (Sep. 25) in the journal Science suggests that between 10 to 30 percent of the Earth’s water is older than the sun, and likely hails from comets born outside our solar system. That means that the human body, which is 60 percent water, contains a significant percentage of extraterrestrial aqua; in that sense, we are all part alien.

The findings also suggest that if a significant portion of water could enter our solar system from interstellar space, then the same could happen in other planetary systems. This means water is probably even more prevalent in exoplanets outside our own than we already thought—and water is a precondition for life, says Conel Alexander, a “geochemist who studies space rocks” (in his words) at the Carnegie Institution of Washington.

This “probably means that most solar systems look similar to us [in terms of chemical makeup], and we know that life started in this solar system,” Alexander tells Newsweek.

Much of the water found on Earth was formed or reformed in the solar nebula, or cloud of hot material, that led to the formation of the sun and the planets in our good ol’ solar system. But it also stood to reason that a fair amount of the water came from beyond, from ice found in interstellar space. But until now there wasn’t a nice estimate of how much water was formed in this cloud, as opposed to what portion came in from beyond the solar system. The hard numbers in this study come from computer models developed primarily by study author L. Ilsedore Cleeves at the University of Michigan.

Despite the empty way it’s generally portrayed and thought of, interstellar space is not in fact entirely devoid of stuff. For example, comets containing water and organic material will occasionally pass through space, Alexander says.

Fred Ciesla, a scientist at the University of Chicago who wasn’t involved in the study, said the paper show that the “raw materials” of interstellar space include water and “organic materials, meaning such things would be commonly available to forming planetary systems.”

Besides shedding light on how our planet came to be, it also highlights the fact that there’s a pretty good chance that life could exist elsewhere besides our own planet: after all, many of the raw ingredients are just floating out there, waiting for the right chance.