The first water molecules may have formed just 100 million to 200 million years after the big bang – before even the first galaxies – kicking off a process that led to life on Earth… and possibly elsewhere.

Shortly after the big bang, most of the matter in the universe was hydrogen and helium, with only trace amounts of other lighter elements, like lithium. Heavier elements like oxygen didn’t yet exist, making it impossible for water to form.

Those initial elements came together in the first stars, which then produced heavier elements through nuclear fusion including, crucially, oxygen. When these stars reached the end of their lives, they exploded as supernovae, releasing these heavier elements and allowing oxygen to mix and combine with the pre-existing hydrogen to create H₂O – water.

Previous research has shown that even the relatively low amounts of oxygen produced in the earliest stars could have made water molecules, but until now nobody had simulated exactly what would happen when a primordial star went supernova and and how the elements it released would mix with the cosmological environment in which the star formed, says Daniel Whalen at the University of Portsmouth in the UK. “To do anything less, you really just don’t know what’s happening,” he says.

To investigate this, Whalen and his team used computer models to simulate the birth and death of the first stars in a realistic context. These early stars are thought to have ranged from 13 times as massive as the sun to 200 times as massive, so the researchers modelled both extremes.

As you might expect, the larger stars spewed out more oxygen, and so produced more water, in the form of vapour clouds about the mass of Jupiter, while the smaller stars produced an Earth’s mass of water, says Whalen.

Depending on the mass of the star, the researchers found that water took between 3 million and 90 million years to form after the supernovae explosions, meaning that the first water molecules formed 100 million to 200 million years after the big bang.

Importantly, however, the team found that this water didn’t simply diffuse throughout the cosmos. Instead, gravity caused it and other heavy elements produced by the first stars to clump together. That in turn meant these clumps were the breeding grounds for the second generation of stars, and perhaps the first planets. “That was a huge result,” says Whalen.

“This idea that water formed even before galaxies did basically overturns decades of thought about when life could have first emerged in the universe,” says Whalen. Team member Muhammad Latif at the United Arab Emirates University says the researchers now plan to simulate if the water vapour could survive the destruction and harsh radiation of the formation of the first galaxies, meaning those early molecules may still exist – potentially even on Earth – today.

“The chemistry of life as we know it requires liquid water, and that you can get only on a planet or some object that has a surface with an atmosphere,” says Avi Loeb at Harvard University. A lot of time would have passed before this first vapour would have condensed into liquid water, but searching for second-generation stars – and their planets – using instruments like the James Webb Space Telescope will help us to understand this process more and whether these planets might have been habitable millions of years after the big bang, he says.

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