The conditions that make life possible are exquisitely rare. Yet researchers are finding that the universe today is far more welcoming to life than it was when microbes first emerged on Earth—a fact that makes our existence all the more remarkable. Plus, it will only grow even more habitable in the future.
"The universe of the future will be a much better place for planets," says Pratika Dayal, a researcher at the University of Groningen’s Kapteyn Astronomical Institute in the Netherlands who studies the evolution of early galaxies.
As star formation winds down, the dangerous radiation levels produced by dying stars drops, creating an environment up to 20 times as habitable as the Earth when life first evolved. At the same time, the sheer number of tiny dim stars—each of which could potentially support life-promoting planets—increases the likelihood that life might evolve in the future. These facts render Earth's current inhabitants “premature” in the life of the solar system, according to a study published online today in the Journal of Cosmology and Astroparticle Physics.
Avi Loeb, lead author of the new study and a researcher at the Harvard-Smithsonian Center for Astrophysics, focused on small, dim stars known as red dwarfs (our sun is a yellow dwarf). The long lifetimes and simple ubiquity of these stars, which make up about three-fourths of the stars in the Milky Way, make them the most likely candidates for hosting life. Assuming that life is possible around red dwarfs, Loeb and his colleagues found it is a thousand times more likely to arise in the distant future than it is today.
"That's surprising," says Loeb, whose research focused on life that resembled ours. "It means that life around the sun is probably a bit early."
However, it is still a matter of debate whether red dwarfs can in fact support life. Early in their lifetimes these stars are incredibly active, and the parts of nearby planets where liquid water can remain on the surface lies very close to the star. This puts planets under constant fire from flares and radiation. Scientists continue to debate whether life can handle these extremes, though Loeb says that the answer may come in the next few decades with help from instruments such as the upcoming Transiting Exoplanet Survey Satellite and James Webb Space Telescope.
"If it turns out that low-mass stars are able to support life, then we are special because we are one of the early forms of life," Loeb says. However, if no signs of life exist around the dim stars, the equation changes and Earth's inhabitants are right on schedule. "If you consider the minimum mass of the star that allows life to emerge to be the sun, then we are most likely to exist today," Loeb adds.
The new study contributes to a growing body of research that finds that the universe’s habitability has increased over time. In separate research, Dayal and her colleagues compared all of the major producers of radiation that can damage emerging lifeforms. They confirmed that supernovae dominate radiation production, while active young galaxies and powerful gamma ray bursts play a negligible part. Among the various types of supernova, Type II play the starring role as single stars explode in violent deaths. Type Ia supernovae, which involve a dying white dwarf star reignited by its companion, also make a significant contribution to damaging radiation.
"It's basically a numbers game," says Dayal, who led the radiation research, and whose article is under review by the Astrophysical Journal. "In terms of the numbers of stars that form, it's supernovae that win."
Dayal and her colleagues simulated the universe through its 13.8-billion year lifetime to track how various astronomical objects contributed to damaging radiation, and found that radiation danger corresponded with star formation. Early on, the universe bustled with stellar births. But production rates slowed as most of the gas and dust became trapped in already living stars. Once the universe reached about 3.5 or 4 billion years, it had blown through most of its unused material.
That doesn't mean it isn't making any more stars, of course—only that they aren't producing them quite as rapidly. But the slowdown in star formation and the resulting stellar deaths do mean good news for worlds hoping to evolve life: Thanks to the decreased radiation, the universe today is as much as 20 times more habitable than it was when the Earth formed.
But potential life-cradling worlds aren't necessarily safe from radiation just yet. New Mexico State University astronomy Paul Mason, who studies how habitability changes within galaxies, says that events like galaxy mergers can jumpstart star formation throughout the lifetime of the universe. Mergers could create pockets new stellar births throughout the universe, potentially increasing the amount of radiation for nearby planets. However, Dayal says that mergers were more common in the early age of the universe than in its later stages.
Dayal’s simulations focuses on an “average” universe, in which matter and celestial bodies were evenly distributed. A more complex, realistic simulation would require significantly more computing time and resources. But existing simulations that focus on how galaxies slam into one another can't resolve individual stars, making it difficult to estimate how collisions affect the total radiation of the universe. Her research provided the first step of confirming what many scientists took as conventional knowledge: that supernovae provide the bulk of harmful radiation.
Loeb isn't quite as certain that the high levels of radiation from supernovae are quite as damaging as most scientists consider them to be. "My personal take on this is that it's very difficult to eradicate life on a planet," Loeb says, pointing to the variety of extreme environments on Earth capable of sustaining living organisms.
Together, Loeb and Dayal’s research suggest that the hunt for life will only improve in the future. However, that future may be significantly farther away than most astronomers would hope. After all, it took Earth somewhere from half a million to a billion years for life to evolve, and other 3 billion for technology to arise. "In some sense, that's good for astrobiologists, but that's 5 billion years hence," Mason says.