Scientists today are quite sure about how long our universe has existed: it’s been 13.8 billion years, give or take 59 million years, since the cosmos burst into being via the big bang. But they’re much less certain about a related question: When could life have first arisen, somewhere out there? Our solar system formed a mere 4.6 billion years ago, after two thirds of cosmic time had already elapsed, and life seems to have happened here almost as soon as Earth cooled down from its fiery birth to harbor oceans of liquid water.
Could we be early arrivals in the universe—or even the first? Or are we instead late to the party, with life springing up far sooner in the universe’s history? Determining the timing of crucial prerequisites for life as we know it would be helpful here: Namely, when did water itself first form, and when could it find a nice planet to settle down with somewhere?
This line of thinking is what inspired a new paper, published in the journal Nature Astronomy, that looked at how much water might’ve been brewed up by some of the first stars— and found that they could’ve enriched the universe with the life-sustaining molecule surprisingly early. A follow-up preprint study by a group that includes the same authors, submitted for publication in the journal Science, suggests that rocky, potentially ocean-bearing planets could’ve coalesced from this water-rich material not long after.
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“What our simulations showed was that you could get sites for planet formation already enriched with water levels similar to [those in] the solar system today only 200 million years after the big bang,” says Daniel Whalen, an astrophysicist at the University of Portsmouth in England and lead author of both studies.
To understand the implications, imagine, for a moment, that the universe’s 13.8 billion years of history were compressed into a 70-year human lifespan. Whalen and his colleagues’ results suggest that habitable conditions could’ve existed when this now-elderly cosmic being was just one year old. The window of opportunity in which life could form and flourish may have been far wider and older than what researchers had previously considered. Living worlds with ancient oceans that formed within a few hundred million years of the dawn of time may be scattered throughout the cosmos.
Life’s Cosmic Recipe
To date, the earliest known water in the universe was detected by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which spied spectral signs of familiar H2O in a galaxy located some 12.88 billion light-years from Earth—and thus from a time when the universe was just under a billion years old.
But we know that, in the beginning, the universe was a cosmic desert, with not a drop to drink. That changed about 100 million years after the big bang as the first stars in the universe flickered into existence. Dense clumps of primordial hydrogen and helium left over from the big bang collapsed under their own gravitational weight, igniting thermonuclear chain reactions at their cores that would light up the universe. Inside these gigantic, bright nuclear furnaces, the first significant quantities of elements heavier than hydrogen and helium were forged.
Living fast and dying young, the first stars seeded their surroundings with elements like oxygen, carbon and silicon via their explosively violent deaths as supernovae. Subsequent generations of stars and planets formed from these fertile stellar ashes, and supernova-sourced oxygen could presumably combine with plentiful primordial hydrogen to make water.
“For 100 million years, the universe did not have the building blocks of life, like oxygen or carbon. Once nuclear fusion started in stellar interiors, the universe became far more interesting,” says Avi Loeb, an astrophysicist at Harvard University.
So in some respects, the early arrival of life’s main ingredients—water and heavier elements capable of forming complex molecules—isn’t so astonishing. But the specifics of how these feedstocks could have actually come together to set the stage for life have remained murky.
Breaking through Bottlenecks
Despite such early abundance, making water way back then wasn’t necessarily easy. The problem is that even though the first stars made lots of oxygen, it would have been dispersed over large areas when it spewed out into space via supernovae.
Consequently, relative to other elements concentrations of oxygen would have still been low, potentially bottlenecking water’s ready formation. And any water molecules that did form still would have been easily blasted back to atoms by the intense ultraviolet (UV) radiation emitted by stars in the early universe, which was smaller and more crowded than it is today.
But in 2015 Loeb, alongside Shmuel Bialy, now at Technion–Israel Institute of Technology, and Amiel Sternberg of Tel Aviv University, predicted that, despite these obstacles, plausibly temperate conditions could have jump-started water formation. All that was needed was for temperatures between 250 and 350 kelvins (–23 and 77 degrees Celsius) to prevail within some of the gas clouds that pervaded the early universe.
“At high gas temperatures, a set of very efficient chemical reactions that lead to water formation kick in,” Bialy says. “This increases the H2O formation rate so much that it can counter the low oxygen abundance and the destructive UV radiation.”
The new simulations by Whalen and his colleagues give additional weight to these earlier predictions.
For its Nature Astronomy study, the team built numerical models for the supernova explosions of two first-generation stars—one was 13 times heavier than the sun, and the other was 200 times heavier than our star. The smaller simulated star survived for roughly 12 million years before dying as a supernova, ejecting 17,000 Earth masses of oxygen into the surrounding interstellar medium. The larger star only survived for two and a half million years before experiencing its own explosive demise, generating 55 solar masses (more than 18 million Earth masses) of oxygen.
What happened next was surprising: As the shockwave of each virtual supernova emanated outward, ripples created density variations in the surrounding gas, causing some of the gas to condense into clumps. From there, these dense clumps were sprinkled with oxygen and other next-generation elements by the supernova’s expanding blast front of ejecta. Consistent with Loeb, Sternberg and Bialy’s prediction, the denser gas let the clumps hold more thermal heat which allowed faster water-generating chemical reactions.
“While the total water production in a given supernova explosion is modest, the water mass fraction in dense clumps created by the explosion can approach those that exist in the solar system today,” Whalen says. “That was the result that we weren’t expecting, and it’s important because those dense clumps are the only structures that can collapse to form stars and protoplanetary disks in the debris of the explosion.”
Whalen cautions that his group’s simulations only offer provisional answers at present. “We don’t have all the physics,” he says. “We are not sure what the masses of the first stars were, but it’s generally believed they were dozens to hundreds of solar masses.” The simulations also only modeled the formation of one star at a time, when consensus holds that the early universe was quite claustrophobic, with multiple stars forming in close proximity. Exactly how this might influence water production is unclear.
Just Add Water
But let’s assume, for now, that these theoretical speculations and computational models do reflect reality. If water was so abundant in regions of the early universe where second-generation stars could later form, could Earth-like planets emerge out of this cosmic mist?
That’s essentially the question Whalen and his co-authors asked in the study they submitted to Science. A second set of simulations tested whether the water-enriched gas clumps from the first could collapse into a low-mass star with a protoplanetary disk that could spawn rocky, wet worlds. And the answer, in short, is that they can.
In these follow-up simulations, a small star, about three quarters the mass of the sun, is birthed out of the dense gas with planetesimals—kilometer-scale precursors to terrestrial planets—in tow. Despite its potential formation so early in cosmic history, a star this size may not yet have burned through most of its thermonuclear fuel, meaning that, even now, so many billions of years later, it would still be shining. And that means such a star’s possible primordial ocean-hosting planets could still be out there, waiting for us to find and study them.
That isn’t to say that life would necessarily have an easy start on such worlds. Cataclysmic collisions with protoplanets, asteroids and comets are thought to be common during a planet’s formation and for the next tens to hundreds of millions of years thereafter. Life, if it ever emerged on one of these worlds, would still have had to endure that bombardment—or await its end.
Extrapolating from Earth’s history, in which life may have started just a few hundred million years into our planet’s existence, an approximate cosmic chronology emerges: 100 million years for the first stars to be born, 10 million for those stars to live, die and spread heavier elements, another 100 million for the second generation of lower-mass stars to form and another 100 million for rocky worlds to reach stable surface conditions suitable for life. This time line implies life could have begun scarcely 300 million years after the big bang, perhaps even before the first recognizable galaxies formed.
One enigma Whalen still wonders about is the provenance of the water in Earth’s oceans. “Somebody asked me if it’s possible some of this primordial water is here today—and we have to say: we can’t rule it out,” he says. “Some of the water on Earth is older than the solar system itself, but we don’t know exactly how old that water is; it’s possible that some of it is primordial.”
That is something to ponder the next time you raise a glass: some of those thirst-quenching molecules in your cup may have formed more than 13 billion years ago in the expanding shockwave of one of the universe’s first stars.
