Proxima Centauri lies in the constellation of Centaurus (The Centaur), just over four light-years from Earth. Although it looks bright through the eye of Hubble, Proxima Centauri is not visible to the naked eye.
ESA/Hubble & NASA
The study of the formation and logic of the universe — cosmology — and the study of exoplanets and their conduciveness to life do not seem to intersect much.
Scientists in one field focus on the deep physics of the cosmos, while the others search for the billions upon billions of planets out there — and seek to unlock their secrets.
But astrophysicist and cosmologist Avi Loeb, a prolific writer about the early universe from his position at the Harvard-Smithsonian Center for Astrophysics, sees the two fields of study as inherently connected and has set out to be a bridge between them. A result of his efforts is a theoretical paper that seeks to place the rise of life on Earth, and perhaps elsewhere, in cosmological terms.
His conclusion: Earth may well be a very early example of a living biosphere, having blossomed well before life might be expected on most planets. And in theoretical and cosmological terms, there are good reasons to predict that life will be increasingly common in the universe as the eons pass.
By eons, here, Loeb is thinking in terms that don’t generally get discussed in geological or even astronomical terms. The universe may be an ancient 13.7 or so billion years old, but Loeb sees a potentially brighter future for life not billions — but trillions — of years from now. Peak life in the universe, he says, may arrive several trillion years hence.
“We used the most conservative approaches to understanding the appearance of life in the universe, and our conclusion is that we are very early in the process and that it is likely to ramp up substantially in the future,” says Loeb, whose paper was published in the Journal of Cosmology and Astroparticle Physics. “Given the factors we took into account, you could say that life on Earth is on the premature side.”
This most intriguing conclusion flows from the age of the universe, the generally understood epochs when stars and then planets and galaxies formed, and then how long it would take for a planet to cool off enough to form the chemical building blocks of life and then life itself. Given these factors, Loeb says, we’re early.
In the long term, the authors determined, the dominant factor in terms of which planets might become habitable proved to be the lifetime of stars. The higher a star’s mass, the shorter its lifetime. Stars larger than about three times the sun’s mass will burn out well before any possible life has time to evolve.
Our sun is a relatively large and bright star, which is why its lifetime will be relatively short in cosmological terms (altogether, maybe 11 billion years, with 4.5 billion already gone). But smaller stars, the “red dwarf,” low-mass variety, are both far more common in the universe and also much longer lived — as in trillions of years.
These smallest stars generally have less than 10 percent the mass of our sun, but they burn their fuel (hydrogen and helium) much more slowly than a larger star. Indeed, some may glow for 10 trillion years, Loeb says, giving ample time for life to emerge on any potentially habitable planets that orbit them. What’s more, there’s every reason to believe that the population of stars in the galaxy and cosmos will increase significantly, giving life ever more opportunity to commence.
As a result, the relative probability of life grows over time. In fact, chances of life are 1,000 times higher in the distant future than now.
This calculation, however, comes with a major caveat: Scientists are sharply divided about whether a star much smaller than ours can actually support life.
The potential obstacles are many — an insufficient amount of heat and energy emanating from the star unless the planet is close in, the fact that red dwarf stars have powerful, luminous beginnings that could send a nearby planet into a runaway greenhouse condition that might result in permanent sterilization, and that many planets around red dwarfs would be close to the stars and consequently tidally locked. That means that one side of the planet would always face the star and be light, while the other would continue in eternal darkness. This was earlier considered to be a pretty sure deterrent to life.
Recent theoretical analyses of planets around these red dwarfs, however, suggests that life could indeed emerge. It could potentially survive at the margins — where day turns into night and the temperatures would likely be stable — and also in other day-side regions were temperatures could be moderated by clouds and winds. But no observations have been made to substantiate the theory.
Because of their relatively cool temperatures and resulting low brightness, individual red dwarfs are nearly impossible to see with the naked eye from Earth. But they’re out there.
The nearest star to our sun, Proxima Centauri, is a red dwarf, as are 20 of the next 30 nearest stars. Scientists announced Wednesday they had discovered that a potentially habitable planet about the size of Earth orbits Proxima Centauri. Data from the Kepler Space Telescope suggest that as many as 25 percent of red dwarfs have planets orbiting in their habitable zones — neither too hot nor too cold to keep liquid water from sometimes pooling on their surfaces.
“I think we can and we should test these theories in the years ahead with observations,” Loeb says. “We should be able to tell if nearby low-mass stars have life around them” in the decades ahead.
And if red dwarfs can support life, then the future for life in the universe is indeed grand.
The merging of cosmological theory and astronomical observation that Loeb has in mind would indeed be unusual, but it is nonetheless consistent with the interdisciplinary nature of much of the broader search for life beyond Earth. That effort has already brought together astrophysicists and geoscientists, astronomers and biologists. It’s just way too big for one discipline.
An interesting sidelight to Loeb’s argument that Earth may well be among the earliest planets where life appeared and continued is that it would provide a solution to the extraterrestrial life puzzle known as Fermi’s Paradox.
It was in 1950 that renowned physicist Enrico Fermi was talking with colleagues over lunch about the predicted existence of billions of still-to-be-discovered planets beyond our solar system, and the likelihood that many had planets around them. Fermi also was convinced that the logic of the vast numbers and of evolution made it certain that intelligent, technologically advanced life existed on some of those planets.
It was an era of fascination with aliens, flying saucers and the like, but there actually were no confirmed reports of visitations by extraterrestrial life. Ever, it seemed.
If intelligent life is common in the universe, Fermi famously wondered, “Then where is everybody?”
There are many potential answers to the question, including, of course, that we are alone in the universe. The possibility that Earth might be among the very early planets with life has not been put forward before, but Loeb says that now it has been.
“Our view is that we’re at the very beginning of life in the universe, we’re just ramping up,” he says. “So of course we haven’t been visited by anything extraterrestrial.”
As a congenital thinker in the very long term, Loeb also raises the issue of whether it makes sense for human life to remain on Earth and in our solar system. The sun, after all, will run out of fuel in those remaining 6 billion years, will expand enormously as that occurs, and then will re-emerge as a superdense white dwarf star. Any biology in our solar system would have been destroyed long before that.
But is Proxima Centauri one of those very long-lived stars?
“It will be there a very long time,” he says. “If the conditions are right, then maybe a time will come to migrate to any planets that might be around Proxima. It’s four light-years away, so it would take generations of humans to get there. Certainly very difficult, but some day in the far future people may be faced with an alternative that’s considerably worse.”