Space is "vastly, hugely, mind-bogglingly big," as the author Douglas Adams put it, and those gargantuan dimensions present a colossal roadblock in the search for alien life beyond Earth. Even if our own Milky Way galaxy is currently teeming with extraterrestrial beings, their worlds could be scattered thousands of light years distant from each other, passing blindly like cosmic ships in the night.
But what if the key obstacle to detecting alien pen pals is not spatial, but rather temporal? That's one of the questions posed by a forthcoming paper in the Journal of Cosmology and Astroparticle Physics.
Led by astrophysicist and professor Avi Loeb, who chairs the department of astronomy at Harvard University, the paper charts out the probability of life's emergence from the birth of the first stars 30 million years after the Big Bang to the death of the last stars trillions of years into the future. Loeb's team focused on "life as we know it," meaning terrestrial organisms on a rocky planet with liquid water, within the habitable zone of its star.
The results suggested that low-mass red dwarf stars are the most likely candidates for hosting habitable planets, thanks to their extreme longevity. These slow burners are only about ten percent as massive as yellow dwarfs like the Sun, but they outlive Sunlike stars thousands of times over.
Red dwarfs may also have some major setbacks, including a propensity for wildness in their youth. Flares emitted by adolescent red dwarfs may singe and sterilize the atmospheres of their surrounding planets, rendering life impossible. Though a recent study demonstrated that some planets within the habitable zones of red dwarfs do have compact atmospheres, similar to Earth, Mars, or Venus, the jury is still out on whether life can exist on these worlds.
But supposing red dwarfs could host life, it stands to reason that the long, stable, adult lifespans of red dwarf systems would amplify opportunity for fledgling ecosystems to bloom. "Our conclusion is that if low-mass stars can support life, then life is much more likely in the future," Loeb told me. "Since [low-mass stars] live so much longer, they are providing heat to keep a planet warm for longer."
Indeed, Loeb's team found that life would be about one thousand times more likely to arise in the distant future by calculating the probability of habitable, Earthlike planets forming over trillions of years.
The scenario casts Earthlings as early bloomers, prematurely born long before the universe's most fertile life-bearing years. Perhaps this is one possible explanation for the classic Fermi paradox: Have we struck out in our attempts to detect alien intelligence simply because we are the first example of it to show up to the cosmic party?
"It might be morning in the cosmos, to quote Reagan," Seth Shostak, senior astronomer and director of the Search for Extraterrestrial Intelligence (SETI) Institute, told me. "There's going to be a lot more life to come."
"But there's no reason for any of that to imply that there is a lack of habitation today," he added.
Indeed, we may be one of several precocious civilizations strewn across the cosmos. But Loeb's team is not alone in speculating that the real heydey of life in the universe lies billions or trillions of years ahead.
Another recent study led by Pratika Dayal, an astrophysicist based at the University of Groningen, came to a similar conclusion by delving into the role that radiation from sources like supernovae and gamma ray bursts have played in habitability over the course of the last 13 billion years.
Her team's models show that decreasing radiation may have resulted in a universe that is 20 times more liveable today than it was four billion years ago, when life first appeared on Earth. The study also projected that our cosmic surroundings will continue to evolve into a more nurturing environment for life in the future.
"People have been taking such different approaches," Dayal told me. "The paper that Avi [Loeb] wrote takes a different approach to what we're doing, but then all of us are, more or less, coming up with the same answers. It is encouraging because it shows we're on the right track."
Of course, all of this research will remain inherently speculative until we have built up more observational evidence of the universe's habitability over time, and more robust simulations to interpret that evidence.
"Even the best simulations aren't good enough to study habitability of the universe on an extremely large scale," Dayal said. "We're just trying to think of clever ways to get around the problem."
"We should be agnostic until we go and search," Loeb said.
Still, it's intriguing to ruminate on the implications of humans being the first, or among the first, intelligent life forms to emerge in the cosmos. Let's say we are, for kicks. Does that change how we view ourselves and our place in the universe? Are we elder brothers and sisters to societies that will emerge around stars that have yet to be born? In addition to trying to bridge communication gaps in deep space, should we also reach out across deep time, to the wealth of extraterrestrial beings that are projected to develop billions of years after our Sun dies?
We look for alien life in the stars because we want to learn from other intelligent civilizations, but we may be most valuable as teachers. Even if these speculative future lifeforms are advanced relative to humans, they might welcome information about our perspective on the universe. Imagine what a boon it would be for Earthlings to receive this kind of message-in-a-bottle from a bygone alien society, regardless of whether it was technologically superior to us.
Of course, it's possible that we already are receiving posthumous letters from aliens, but have no way to identify them. "This is one of the key outstanding questions: When you say you look for life in the universe, what do you mean?" Dayal said.
"From the astrophysical perspective, we are basically talking about Earthlike life. But what if it's different? Would you even be able to recognize it? How do we know there's not some form of life trying to communicate with us and we don't know?"
"It's quite possible that there are other forms of life, and that nature has more imagination than we do," Loeb told me. "We just have one example."
This is an important limitation to keep in mind for ourselves as well. The only thing more excruciating than the thought of alien attempts at contact falling on deaf ears on Earth is the opposite scenario, in which human messages reach civilizations that can't interpret them. Transcending this problem will be essential to securing even one link in a chain of cognitively sophisticated beings across time and space.
"It's quite possible that such forms of life spread throughout the galaxy, and are mostly in places we don't suspect."
"I bet you'd get a lot of people weighing in on how to communicate with critters that might spring up five, ten, or 100 billion years from now," Shostak said. "It's a tough problem."
There are some basic roadmaps for solving it, though each is contingent on major technological breakthroughs. We could, for instance, develop interstellar spaceflight in order to disperse ourselves more widely across the stars. It's a lot easier said than done, but theoretically, it would up the odds of humans sticking around long enough to interact directly with the more populous universe of the future.
"If you are an intelligent form of life like we are—a technological civilization—then everything changes because you are not restricted to live next to a star," Loeb said. "In principle, such a civilization, if it's very advanced, could move away from the star that hosted it in the beginning."
"It's quite possible that such forms of life spread throughout the galaxy, and are mostly in places we don't suspect," he continued. "There might be a lot of spacecraft moving through the galaxy that are not particularly visible to us because they are small. If you imagine a civilization that is hundreds of millions of years old in terms of its technology, the sky's the limit in terms of how widely it would be able to spread."
Humans may one day develop the capabilities necessary to become this type of star-hopping civilization. Projects like Breakthrough Starshot, which aims to send a fleet of tiny spaceships to the nearest star system, Alpha Centauri, are hoping to pave the way for this achievement.
"Our civilization will have to move somewhere [to survive long-term]," Loeb said. "The nearest example is Proxima Centauri, so we might consider traveling there."
It's difficult to predict if and when these efforts will come to fruition, and we may ultimately have to submit to our primitive planetary life and its star-exploding expiration date. If so, our plan B for contacting future life could be launching our own epitaphs in the form of robotic spacecraft or radio messages to civilizations that don't exist yet. It would be a shot in the dark, and we'd never know whether we'd succeeded. But it would be a small step towards the connection we so clearly crave from other living denizens of the cosmos.
For now, it's a fun thought experiment. However, if evidence continues to accumulate suggesting that we live in an era of biological sparseness relative to an abundance of future civilizations, it could reorient our attitude to our place in the universe, and our approach to the other creatures—past, present, and future—with whom we share it.