If you grade earthlings on a cosmic curve, as recently hashed out by Harvard and Oxford University astrophysicists, we're at the head of the class.
So says a team of astronomers in a new study, to be published in the Journal of Cosmology and Astroparticle Physics. The researchers calculated the probability that life as we know it should exist at any given point in the universe. Based on their assumptions, Earthly life is quite likely premature.
"If you ask, 'When is life most likely to emerge?' you might naively say, 'Now,'" according to Avi Loeb, a scientist at Harvard-Smithsonian Center for Astrophysics and lead author of the new study, in a press release. "But we find that the chance of life grows much higher in the distant future."
Should you enjoy hearing that you are exceptional, this scientific narrative might give you a warm fuzzy feeling: By the standards of the universe, humans are some of the earliest intelligent life around. You may have heard of ancient aliens on basic cable television. But, according to cosmic probability, the ancient aliens are us.
For life as we know it to arise, organisms require three things: carbon-based chemistry, liquid water and an energy source, as Loeb and his colleagues wrote. The most crucial source of all three requirements is a star. Stars fuse photons and electrons into carbon and other elements; stars heat up water in the so-called habitable zone; and stars provide a steady sunny stream of radiation.
Stars play such an important role in our understanding of life that they dominate the researchers' equation. The scientists' timeline begins about 30 million years after the Big Bang banged, which as far as we can tell was 14.8 billion years ago. Their timeline ends far in the future due to the long-lasting red dwarf stars, which have lengthy wicks that burn for roughly 10 trillion years. (A yellow dwarf, by comparison, has a measly 10 billion years of fuel in the tank.)
Crucially, what red dwarf stars also have going for them is strength in numbers. The Milky Way is chock full of the little slow-cookers. Of the roughly 100 billion stars in the galaxy, some three-quarters are red dwarfs.
Our sun is not a red dwarf. It is a rarer thing, a yellow dwarf, a star 10 times more massive but one that will flare out much sooner. That we exist on around a yellow dwarf, per the scientists' equation, makes us the true space oddities.
"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," as Loeb told Smithsonian Magazine.
All things being equal, the sheer amount of time a planet could tick around a red dwarf gives those solar systems the astrobiological edge. "Our conclusion is therefore that the most conservative estimate put the probability of our existence before the current cosmic time at 0.1 percent at most," the authors write. To say that another way, in the far future they believe the chances of life -- on a dim red dwarf -- increases by a thousandfold.
"Many more stars that all last much longer than our Sun ensures there's simply more opportunity for life to arise in the future on a small dwarf star than in the last 13 billion years of cosmic time," as Alan Duffy, an Australian astronomer at Swinburne University of Technology who was not involved in the study, wrote in an email to The Washington Post.
But let us deflate that warm fuzzy feeling with a deep space chill. Namely, most experts do not believe all stars are created equally.
"One possibility is we're premature," as Loeb admitted in the press release. "Another possibility is that the environment around a low-mass star is hazardous to life."
To Charley Lineweaver, an astrobiologist at the Australian National University, the belief that life could equally appear on a yellow dwarf or red dwarf is a "crazy assumption." He told The Washington Post as much by phone early Wednesday.
Around red dwarfs, "it's much more believable that life is suppressed," he told The Post, "based on the assumption that we are an average life form." (He invoked Princeton University cosmologist J. Richard Gott, whose famous idea is that we are observers plucked at random and therefore we are nothing special.)
"There's a lot of papers that say that life may be suppressed around those stars," said James Kasting, a Penn State exoplanet researcher, to Popular Science.
Even where a planet might be in the sweet spot for liquid water around a red dwarf, the other environmental conditions may be grim. Such a world would have to be much closer to the dimmer red stars. The likelihood that the planet is baked in intense radiation therefore increases. So does that chance a planet is tidally locked -- that is, one half caught in perpetual day and the other in forever night.
That said, Lineweaver believes the scientific theory in this paper is solid. "The method I like," he said. "I disagree with the main conclusion."
Should you subtract the assumption that tiny stars could support life, Lineweaver believes the timeline makes more sense. Duffy -- who called the analysis "clever" -- is also concerned that the authors ignored issues concerning small stars.
"The authors calculated that if you limit life to stars just 90 percent of the mass of our Sun," Duffy wrote, "then the most likely time and star to find ourselves in and around is a Sun-like star."
The conclusions for why we are alone, then, become a bit grimmer. Earth may be in fact be a latecomer to the habitable party, based on Lineweaver's analysis of metal formation within stars. And as The Post reported in January on another one of his papers, we may be exceptional only because everyone else has already died.
"We don't know what does matter as we only have only our one Solar System to examine," Duffy said, "and can't know yet what makes it, seemingly, unique in harboring life."
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