"Where is everybody?" the Italian physicist Enrico Fermi once asked, when faced with the apparent contradiction between the high probability for the existence of alien life and the lack of any conclusive evidence for the existence of extraterrestrials.
Over the years, several solutions to the "Fermi's paradox" have been put forward, including one that points out that the universe is staggeringly huge and that Earth and its denizens are mere specks dust floating in one of its billions of galaxies.
The other, a version of which has now been described in a theoretical work published online in the Journal of Cosmology and Astroparticle Physics, is that perhaps we came too early to the party.
"If you ask, 'When is life most likely to emerge?' you might naively say, 'Now,'" lead author Avi Loeb from the Harvard-Smithsonian Center for Astrophysics, said in a statement released Monday. "But we find that the chance of life grows much higher in the distant future."
While trying to find out when — in the universe's estimated 10 trillion year long lifetime — life is most likely to arise, the researchers found that the dominant factor was the lifetime of stars. The higher a star's mass, the shorter its lifetime. This means that stars heavier than roughly three times then our sun's mass will expire before life on planets around it has a chance to evolve.
"So then you may ask, why aren't we living in the future next to a low-mass star?" Loeb said in the statement. "One possibility is we're premature. Another possibility is that the environment around a low-mass star is hazardous to life."
In order to understand if we really are a lucky, premature accident in a universe that is yet to witness a significant burst of life, the researchers recommended studying low-mass stars, particularly red dwarfs, which are small and relatively cool. The last generation of these stars are likely to remain aglow for 10 trillion years.
While their longevity makes them the most common stars in the universe and thus ideal candidates, probabilistically, for hosting planets with life, these stars come with their own baggage. When they are young, they emit strong flares and ultraviolet radiation that can strip the atmosphere from any rocky world in the habitable zone.
Although astronomers are still not sure whether life can exist in such conditions, they have previously estimated that at least six percent of red dwarfs have an Earth-sized planet in the "habitable zone" for life as we know it.
"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 told Smithsonian magazine. Conversely, "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."
In either case, the study suggests that the cosmos of the future — about 5 billion years hence — would be a much better hunting ground for alien life.