Earth's Life-Nourishing Water Came From Interstellar Space, Scientists Find

Earth’s water originated in interstellar space more than five billion years ago, making it more ancient than the Sun, reports a new study that reveals fascinating insights about the evolution of water in budding star systems. The finding will inform efforts to find other water-hosting habitable planets. 

Astronomers captured an unprecedented glimpse of water vapor surrounding the baby star V883 Orionis, located some 1,300 light years from Earth, enabling them to measure the ratio of two key versions of water for the first time in a developing star system. The results revealed that the composition of water in the V883 Orionis system is very similar to water content in many bodies of our own solar system, a discovery that adds weight to the theory that much of Earth’s water is sourced from interstellar dust grains that existed before the birth of the Sun. 

Liquid water is essential for life on Earth, which is why scientists hoping to find life elsewhere in the universe focus on the so-called “habitable zones” of other stars, where water could conceivably exist. But in order to assess whether habitable water worlds elsewhere in the galaxy, we need to understand how Earth came to host seas, rivers, and lakes that teem with life.

V883 Orionis offers an excellent laboratory for this question, because it is surrounded by a relatively warm expanse of gas and dust—known as a protoplanetary disk—from which comets, planets, and other celestial bodies may eventually emerge. In this natal phase of most star systems, water is mostly locked up in the form of ice on interstellar dust grains, but hot bursts from the young star have hinted at the possibility of observing water vapor sublimating from these grains. 

To search for this vapor, a team led by John Tobin, an astronomer at the National Radio Astronomy Observatory, observed V883 Orionis with the Atacama Large Millimeter/ submillimeter Array (ALMA), one of the world’s most sensitive radio telescopes. 

The researchers not only report “the direct detection of gas phase water” in the system, they were also able to measure, for the first time, the ratio of regular water (made from two hydrogens and an oxygen) and water that contains a heavier version of hydrogen, called deuterium, according to a study published on Wednesday in Nature.  

“This is the first time that we were able to actually measure the deuterium-to-hydrogen ratio of the water in a disk, which is a powerful tool that lets us link it to comets in our own solar system, water on Earth, and water in earlier phases of star formation,” Tobin said in a call. 

“We think the water that eventually ends up in the disk and in comets formed before the star even formed,” he continued. “Those water molecules formed out in interstellar space on the surfaces of very tiny dust grains.”

The unusually warm disk surrounding V883 Orionis presented Tobin and his colleagues with a unique opportunity to examine water vapor across much larger distances than other systems. Whereas previous studies have detected water vapor in the warm region very close to baby stars, Tobin’s team spotted it at distances 120 times as far as the orbit of Earth around the Sun. Indeed, the researchers calculated that the disk of V883 Orionis currently contains at least 1,200 times the amount of water in all Earth’s oceans. 

This enormous abundance of vapor allowed the team to capture the ratio of enriched deuterium water against the regular water, which was a close match with the ratio seen in solar system comets. The results imply that Earth inherited at least some of its water from comets carrying this ancient agua, which impacted our planet billions of years ago. As a result, interstellar ice grains appear to be the primordial wellsprings of water in star systems across the galaxy, which has big implications for the debate over the origin of Earth’s water, and the search for other wet worlds elsewhere that might host life.

“It's further confirmation that there's going to be a lot of water available that could become incorporated into planets in newborn solar systems, which also suggests that exoplanet systems also potentially have a lot of water available to them,” Tobin said.

“That water is present in large quantities and could become part of planets that are forming, but we just don't quite know what fraction ends up on planets because in our solar system, a lot of the things have changed over billions of years,” he continued. “There's only a few places where we see that there's still tons of water on the surfaces—Earth has tons of water, Europa has tons of water, as do some of the moons of Saturn, but then there are also a lot of places that are still incredibly dry, like the Moon.”

In other words, the new study confirms that water is plentiful in other star star systems, but that does not necessarily mean aliens are slurping it up on countless lush exoplanets. A lot can happen over the lifetime of a planet—just look at Mars, which hosted water, and perhaps life, some four billion years ago—but now is a desiccated husk of a world. 

Ultimately, we know of only one watery planet that ended up producing life—spoiler alert, it’s Earth—and we’re still in the process of understanding how we ended up here.

“The water that ended up on Earth might have been a mixture—some of it could have come from comets, but some of it might have had to be processed at higher temperatures to get rid of some of that deuterium,” Tobin noted.”

“I hope this helps spur more studies, or contributes to studies, of how water was delivered to Earth ultimately, because that's still an important open question,” he concluded.