Water could have first formed when the first stars died in supernova explosions only 100 million to 200 million years after the Big Bang — billions of years earlier than scientists previously thought.
Astronomers have used computer simulations to study the supernova explosions of the first generations of stars in the Universe. The simulations show that when these stars died, they left behind oxygen and hydrogen, which reacted to form water over the course of millions of years to form the first water molecules.
This discovery means it may have been possible for the first habitable planets to form early on in the history of the Universe, even before the first galaxies did.
“Our simulations show that water was probably a key constituent of the first galaxies,” said Daniel Whalen from the University of Portsmouth. “This implies the conditions necessary for the formation of life were in place way earlier than we ever imagined — it’s a significant step forward in our understanding of the early Universe.”
Supernova simulations
Right after the Big Bang, the Universe was made entirely of light elements, mostly hydrogen and helium. Heavier elements, such as carbon and oxygen, appeared later — they were created through nuclear fusion reactions in the first stars.
Water, however, is a more complex and fragile molecule that requires cooler temperatures to form and remain stable. The aftermath of the first supernova explosions may have provided just the right conditions, and Whalen’s team has been the first to simulate how this may have happened.
“Before the first stars exploded, there was no water in the Universe because there was no oxygen,” said Whalen. “Oxygen, forged in the hearts of these supernovae, combined with hydrogen to form water, paving the way for the creation of the essential elements needed for life.”
The researchers simulated two types of supernova explosions that were likely to happen among the first generation of stars, which were much larger and lived shorter than the stars we observe in the Universe today — the simulated stars had 13 and 200 times the mass of the Sun.
The calculations showed that after the supernovae expanded and cooled down, oxygen reacted with hydrogen to form water. These water molecules were highly concentrated in cloud cores, dense clumps of gas that are thought to be where stars and planets first formed.
There was a key difference between both types of supernovae. The explosion of the larger star could produce water within “just” three million years — between 10 and 30 times faster than the smaller supernova. Most likely, the first water molecules appeared in the Universe when the largest stars died, as early as 100 million years after the Big Bang.
“Although the total water masses were modest, they were highly concentrated in the only structures capable of forming stars and planets,” said Whalen. “These early water-rich regions likely seeded the formation of planets at cosmic dawn, long before the first galaxies took shape.”
These results challenge previous theories of how early life could have first emerged in the Universe. However, more research will be needed to better understand what happened to the first water molecules after they formed in early supernovae. “How much water survived the harsh radiation environments of the first galaxies remains to be determined,” said Whalen.
The simulations only considered the simplest possible case, where a single star explodes. Future studies could look into how several overlapping supernova explosions — a more likely scenario — may have affected the formation of the first water molecules. The researchers stated in their study that, while further explosions could destroy some of the water initially formed, they could also contribute to the creation of denser cores where it would be easier for water to form and survive radiation.
Reference: D. J. Whalen et al., Abundant water from primordial supernovae at cosmic dawn, Nature Astronomy (2025). DOI: 10.1038/s41550-025-02479-w
