So far, the only examples of sentient life we've found are right here on our own planet. It's not for lack of trying, though -- we've sent out spacecraft deep into the reaches of our galaxy and, so far, still remain alone. What if the problem isn't where where looking, though, but when? Artist's conception of an Earth-like planet orbiting a low mass star (Image: Christine Pulliam (CfA))
A forthcoming study in Journal of Cosmology and Astroparticle looks at the possibility that life as we know it may not require a star similar to our sun but could emerge on planets orbiting much smaller, weaker stars. If we do allow for the possibility of life around non-sunlike stars, then it turns out that the universe is likely to be much more habitable in the distant future than it is today.
"It's natural for us to think that we are the most common form of life, simply because that's the only one that we know of," lead author of the paper, Harvard University's Avi Loeb, told Gizmodo. "Therefore, people assumed that being next to star like the sun was the most likely place for life to emerge."
If you throw out the assumption that we need a sun-like star, though, then there's a whole new class of stars -- smaller and less powerful than the sun, but far more common -- that suddenly start to look like good candidates. They're called low mass stars.
Although these stars throw out less light and heat than our powerful sun, they still emit enough to create potentially habitable zones that could support liquid water on close-orbiting rocky planets. Not only are these types of stars more common in the universe than sun-like stars, but they also have much considerably longer lifespans of more than 1000 times that of the sun.
Using this information, Loeb calculated that it was much more likely for life to have emerged in the distant future around one of those low mass stars than to have emerged in our time on a sun-orbiting planet like Earth.
"If you allow low mass stars to have life, just like we find here on Earth, then the probability of life emerging in the future 10 trillion years from now is one thousand times bigger to find life," noted Loeb.
Proxima Centauri, a low mass star just four light years away from us (Image: ESA/Hubble/NASA)
And yet, we are not orbiting a low mass star, trillions of years in the future. We are here and now, orbiting our Sun -- and this is the only place we've ever found sentient life. That suggests an intriguing explanation. Perhaps we are simply searching way too soon.
In other words, we may be alone in the universe right now. But that's only because we showed up long before life really started to get going. If this hypothesis is the correct one, then the real explosion of life in the universe hasn't yet happened -- and likely won't happen for trillions of years after us.
There's also a second, alternate explanation that would account for all the facts. Perhaps there's something about low mass stars which, even in zones that are technically habitable, suppresses life from ever forming.
"We still keep the notion that, perhaps, we are at the center of the biological universe, that we are really the only ones or special in that regard, or in terms of intelligence," Loeb said. "If it turns out that we are rare and early on in the game then that would be really surprising to me because, so far, whenever we look we have found that we are not special and we are not the center of the universe."
Figuring out which of these two possibilities are correct hinges on the question of whether low mass stars can indeed support life. We won't necessarily have to wait several trillions of years to find out, though. Instead, Loeb suggests that the answers could be found in the next decade or so.
By sampling the atmospheres of planets around nearby low mass stars, researchers can search for biomarkers that would suggest whether these planets are capable of supporting life. If they keep finding atmospheres devoid of signs that they are capable of supporting life, then it's likely that something about these low mass stars -- perhaps their frequent solar flares or some other attribute -- renders the planets orbiting them sterile.
If, however, they find that these planets do appear able to support life, then it may be that the lack of other life in the universe is simply because we showed up too soon to see any of it.