At some point, our solar system gained access to water — the molecule that would one day be a vital component to life on Earth. But how did it get here? It seems like a simple question, but scientists have been puzzled until now. The biggest mystery has been the timing of water’s arrival: Did it come from the same cloud of space dust that would also create our sun, or was it formed later, by chemical reactions that took place after the sun had formed?
According to a study published Thursday in Science, something like 30 to 50 percent of the water on our planet does indeed predate the sun. Researchers determined this with a model that traced deuterium, a modified form of hydrogen that forms what we call “heavy water.” Based on the ratio of deuterium to hydrogen found in water on earth, scientists can estimate which chemical processes formed it — and how that water came to be.
When the sun was formed, it took most of the mass of the cold space cloud that birthed it, first author and University of Michigan PhD candidate Ilse Cleeves said. But what was left over formed something called a protoplanetary disk — the matter from which all of our planets would be formed. There was definitely water around when the sun was born — but until now, researchers thought the star’s birth might have destroyed it, forcing the protoplanetary disk to start its new solar system’s water production from zero.
"The question was whether that violent, hot process of star creation scrambled the chemicals and broke the molecules of this pre-solar gas’s heavy water," Cleeves said. "But if it did, then water would have been formed locally in this protoplanetary disk."
According to the model created by Cleeves and her colleagues, our solar system actually maintained the water it inherited pre-star.
It’s certainly a gee whiz discovery, but Cleeves and her team think it might have some universal applications. “This is speculative, but if the sun’s formation was typical — and we have no reason to think it wasn’t — then the fact that this water survived the formation of a star means that they can survive it everywhere,” she said.
If water can be present in any protoplanetary disk, it means that it’s available as a building block when young planets are born — and that means we can hope to find other planets that have the same molecule that made life possible here.
"This is very different from a model where everything is erased during star formation, and you start from the protoplanetary disk," said Ralph Pudritz, a professor of astronomy at McMaster University who wasn’t involved in the study. This study, he said, is a "compelling" and "solid" step toward erasing the doubt surrounding the origin of water in our solar system.
Finding the water’s origin is vital in understanding the origin of life — and if this new research is correct, it means that our life-giving gift of water might be run-of-the-mill. “If most of the water is being put together before the star is there,” Pudritz said, “it smacks of a greater universality.”