Alan Stern, principal investigator for NASA’s New Horizons mission to Pluto, and a planetary scientist at the Southwest Research Institute in Boulder, Colorado, has a new answer for one of the oldest conundrums in astrobiology. If alien civilizations exist, why can’t we find them?
It’s a puzzle known as the Fermi Paradox, and Stern’s answer is an unusual one. Perhaps, he said this week at a meeting of the American Astronomical Society’s Division for Planetary Sciences in Provo, Utah, aliens exist, but are trapped in subsurface oceans on cold worlds such as Saturn’s moon Enceladus or Jupiter’s moon Europa.
The Fermi Paradox was first propounded by Nobel laureate physicist Enrico Fermi, who famously wondered why, if alien civilizations are out there, we haven’t had unambiguous contact with them. It’s a conundrum that’s done nothing but intensify since Fermi first propounded it in the 1950s, as radio astronomers have vainly scoured the skies seeking to intercept alien transmissions similar to those our own planet has been producing for the better part of a century.
Perhaps we really are alone, unique in the universe. Or perhaps something, ranging from differences in technology to Borg-like marauders that home in on advanced civilizations and destroy them the moment they reveal their presence, is keeping us from spotting them. Perhaps habitable planets capable of developing life are simply so rare that we have yet to point our radio telescopes in the right direction.
Stern’s idea is simpler. Perhaps, he says, most worlds capable of sustaining life aren’t like Earth, with its continents, surface water, and atmosphere. Maybe they are icy bodies with thick “lids” enclosing interior oceans.
Until recently we didn’t even know such worlds existed. But now, we know they are quite common, at least in our own solar system. Ours has only one Earth, but it may eventually prove to have dozens of Europas.
Alien civilizations living on these worlds, Stern says, might never come into contact with anyone else. Partly that’s because the thick crusts covering them would block us from ever hearing their internal transmissions.
But such civilizations might not even know there was anything interesting above their own “roof.” And if they were motivated to drill through it to reach the unknown, they might not even know what those lights in the sky mean – assuming they have eyes able to see them.
And if they were nonetheless disposed toward space travel, they would have to carry large volumes of water, rather than air, for life support. That’s a huge impediment, Stern says, because “everything we know about space travel involves making the spacecraft lighter.”
Yet another factor, he says, is that “exterior” ocean worlds like Earth are “shooting galleries” for bad things descending from space. “We all know about impacts like those that very likely terminated the Cretaceous Period and the dinosaurs,” he says. “But there are many other dangers. This doesn’t apply to interior oceans.”
Such planetsl, he adds, don’t even have to be in the traditional habitable zone close to their suns, or even anywhere close to it. “An ocean inside of Pluto is just as wet and just as warm as an ocean in the interior of a Galilean moon,” he says.
The response from others has been mixed. Award-winning science fiction writer and space scientist David Brin agrees that Stern is correct in pointing out that the vast majority of ocean worlds are likely to be ice-covered. “And Alan is right that few or none would ever get free of their roof,” he says.
But that has no bearing on the Fermi Paradox, he adds, because all that Stern has done is to vastly multiply the number of worlds that might develop intelligent life, without explaining why none of the other “roofless” ones like Earth have so far revealed themselves.
NASA astrobiologist Chris McKay agrees. “We already know enough about exoplanets to say that even if earthlike worlds are not the dominant habitable world in the galaxy, there should still be gazillions of earthlike worlds out there. The Fermi Paradox still holds for that set,” he says.
Nor is it clear that lidded ocean worlds will be suitable for the origin of any life, let alone intelligent life. To begin with, McKay says, there is serious debate over whether life can develop in an ocean, where currents can disperse and dilute its chemical precursors. Many biologists believe it needs surface ponds that periodically dry out, thereby concentrating the available chemicals to critical levels.
But even if life can develop in an interior ocean world, there’s another problem. “It is generally thought multicellular life requires high oxygen levels,” he says. “There is no evidence for this in the plume of Enceladus, [and] the presence of hydrogen and methane would suggest its strong absence.”
Paul Davies, director of the BEYOND Center for Fundamental Concepts in Science at Arizona State University, Tempe, however, is more sanguine. In his 2010 book The Eerie Silence, he too examined the prospect of life on “lidded” ocean worlds such as Europa, noting that there is a difference between what it takes for life to reach intelligence and what is needed for an alien race to develop science and technology.
“So many cultural factors contributed to what we call ‘the scientific method’ that even if intelligence is common, scientific inquiry may not be,” he tells Cosmos.
One of these might well be the ability to see the stars. “Astronomy was the midwife of human science,” he says. “If you can’t see the sky or develop a belief in gods or an intelligible cosmic order, why on Europa would you ever think of science?”
Stern, on the other hand, notes that his paper is simply the starting point for continued discussion. “We are at the beginning of understanding,” he says. “Fasten your seat belt.”