Things were not looking so good for alien life in 1976, after the Viking I spacecraft landed on Mars, stretched out its robotic arm, and gathered up a fist-size pile of red dirt for chemical testing. Results from the probe’s built-in lab were anything but encouraging. There were no clear signs of biological activity, and the pictures Viking beamed back showed a bleak, frozen desert world, backing up that grim assessment. It appeared that our best hope for finding life on another planet had blown away like dust in a Martian windstorm.
What a difference 33 years makes. Back then, Mars seemed the only remotely plausible place beyond Earth where biology could have taken root. Today our conception of life in the universe is being turned on its head as scientists are finding a whole lot of inviting real estate out there. As a result, they are beginning to think not in terms of single places to look for life but in terms of “habitable zones”—maps of the myriad places where living things could conceivably thrive beyond Earth. Such abodes of life may lie on other planets and moons throughout our galaxy, throughout the universe, and even beyond.
The pace of progress is staggering. Just last November new studies of Saturn’s moon Enceladus strengthened the case for a reservoir of warm water buried beneath its craggy surface. Nobody had ever thought of this roughly 300-mile-wide icy satellite as anything special—until the Cassini spacecraft witnessed geysers of water vapor blowing out from its surface. Now Enceladus joins Jupiter’s moon Europa on the growing list of unlikely solar system locales that seem to harbor liquid water and, in principle, the ingredients for life.
Astronomers are also closing in on a possibly huge number of Earth-like worlds around other stars. Since the mid-1990s they have already identified roughly 340 extrasolar planets. Most of these are massive gaseous bodies, but the latest searches are turning up ever-smaller worlds. Two months ago the European satellite Corot spotted an extrasolar planet less than twice the diameter of Earth (see “The Inspiring Boom in Super-Earths”), and NASA’s new Kepler probe is poised to start searching for genuine analogues of Earth later this year. Meanwhile, recent discoveries show that microorganisms are much hardier than we thought, meaning that even planets that are not terribly Earth-like might still be suited to biology.
Together, these findings indicate that Mars was only the first step of the search, not the last. The habitable zones of the cosmos are vast, it seems, and they may be teeming with life.
The Solar System Habitable ZoneOne of the guiding tenets in the search for life as we know it (the only kind we can meaningfully speculate about) is that it requires water. Until recently, that rule led scientists to think only in terms of places just like home: temperate, rocky planets with bodies of liquid water on their surfaces. From there it was a simple matter to calculate where such worlds could exist within our solar system.
“If you define a habitable zone in terms of favorable climate, you get a pretty narrow band of orbits around the sun,” says Greg Laughlin of the University of California at Santa Cruz. “You can move the Earth inward toward the sun a couple of percent or move it outward by at most about 30 percent before the climate runs into a serious problem.” From this perspective, there is no other promising location for life in our solar system. Even if many other stars have solar systems too, planets that happen to orbit in just the right place to support life could be pretty rare.
That would be a depressing end to the story of habitable zones, if not for a series of amazing findings that life on Earth is not what everyone thought it was. “No one really expected it,” says Chris McKay, one of the pioneers of astrobiology—the hybrid field that studies how life could arise and evolve elsewhere in the universe. “People found strains of bacteria that don’t use food from the surface, don’t use oxygen from the surface, and don’t use sunlight from the surface.”
These newly revealed life-forms, called extremophiles, thrive in conditions so harsh a biologist 50 years ago would not have dreamed it possible. Giant tube worms, crabs, and shrimp live in the dark, a mile below the ocean surface, huddled around superheated geothermal vents. These vents are known as black smokers for the plumes of dark hydrogen sulfide they belch into the ocean. The organisms around them survive off chemicals from the vents in an ecosystem that operates without photosynthesis.
To McKay, these creatures are not the most exciting types of extremophiles, however. “They still rely on oxygen that is indirectly created by sunlight,” he says. Far more compelling are the bacteria that have been found thriving deep underground. One type lives five miles deep in the bowels of South African gold mines. “These creatures get their energy from sources we never imagined,” McKay exclaims. “The South African extremophile bacteria are powered by the radioactive decay of unstable atoms in the rocks. Sunlight and surface water play no role. It’s amazing!”
Extremophiles feeding on nonsolar energy sources show how alien life might similarly arise and thrive deep underground, far from surface water and sunlight. “Habitable planets don’t need to be like Earth,” McKay says. “That realization has driven the biggest expansion in our understanding of habitable zones.”
By happy coincidence, the discovery of extremophiles coincided with new studies showing that the solar system might have many previously unexpected warm, wet locations. In the 1990s the Galileo space probe collected convincing evidence that Jupiter’s large moon Europa has a global ocean of liquid water beneath its frozen surface. (NASA just announced plans to return there in 2027 to get a better look.) The recent discovery of the geysers on Enceladus added a second twist, making planetary scientists wonder if there are even more such hot spots scattered around the solar system. These locations lack sunlight and access to the surface—but apparently some kinds of life do nicely without either.
“When you take the discovery of liquid water below the surface of Europa and Enceladus and put it together with our understanding of terrestrial extremophiles,” McKay says, “you can see why the definition of ‘habitable zone’ had to change.”
The Galactic Habitable ZoneAstrobiologists’ new, grander view of habitability gets even more expansive when they look out to the galaxy around us. The Milky Way contains perhaps 200 billion stars. Now that we know a significant fraction of stars have planets, that number translates into (as Carl Sagan might say) billions and billions of worlds. Red dwarf stars, which are by far the most common stars in our galaxy, were once considered unlikely places to find Earth-like planets, but new studies contradict that view. And the extremophiles tell us that life could potentially take hold even on planets not much like our own.
All of that is the good news. But things are not quite so simple, because galaxies—like solar systems—have habitability zones of their own. Not all parts of a galaxy are suited to life. In 2004 astrobiologist Charley Lineweaver of Australian National University published a paper that broadly mapped out our galaxy, the Milky Way, with an eye toward possibilities and dangers for alien biology. In this case, the crucial factor is not the presence of water; it is the proximity of violent, massive stars.
The galaxy’s brightest, hottest, heaviest stars turn out to be crucial for both planets and biology. They are the universe’s only source of crucial heavy elements like silicon (which makes up more than a quarter of Earth’s crust), potassium (essential for the action of cells), and iron (which carries oxygen in our blood). These elements are forged in the stars’ fiery nuclear furnaces. Massive stars end their lives with supernova explosions that spray the heavy elements into space, where they are incorporated into the next generation of stars and help seed the formation of planets.
In thinking about the galactic habitable zone, Lineweaver made the presence of heavy elements his prime criterion. The rate at which massive stars form drops sharply as you venture outward from the Milky Way’s center, and the abundance of heavy elements falls with them. Lineweaver calculates that when the sun formed 4 billion years ago, the outer third of the galaxy lacked enough heavy elements to support life. Since then the elements have become more widely distributed, and now only the galaxy’s outer rim is too undernourished to form Earths easily. Our location, about two-thirds of the way toward the Milky Way’s stellar rim, lies at the center of the currently life-friendly region of the galaxy; the inner part of the galaxy turns out to be hostile to life too.
Massive stars give, but they also take away—and that puts the inner limit on the galactic habitable zone. The supernova explosions that create and spread heavy elements also unleash a torrent of high-energy radiation: gamma rays, X-rays, and ultraviolet light. Those stellar explosions can have lethal effects on planets orbiting stars even tens of light-years away. In the crowded central regions of the galaxy, home to large numbers of massive stars, supernovas are so common that the evolution of complex life-forms might be difficult if not impossible.
The big question is how bad the supernova effect is. Lineweaver and his colleagues calculate that radiation poisoning could exclude the inner 20 percent of the Milky Way, which encompasses about half of all the stars in the galaxy. “You are looking for that sweet spot,” says Fred Adams of the University of Michigan, “where you are not so close to the center that conditions are hostile and not so far out that the metal abundance is too low.” But the Milky Way is huge, so Adams suggests putting things in perspective. “At worst the amount of galactic real estate favorable to life is reduced by a factor of two or three,” he says.
The amount of real estate that is off-limits depends heavily on how life responds to strong doses of radiation. Remarkably, we may already have good information about that locked away in the fossil record right here on Earth.
Every 62 million years, something bad happens to Earth’s biodiversity,” says Adrian Melott of the University of Kansas. “Paleontologists have built up large data sets of all the animals in the fossil record. With these data you can look to see how biodiversity changed with time.” His provocative studies, backed by the work of other groups, show that drops in biodiversity—sometimes indicating mass extinctions—seem to follow a periodic cycle.
Melott links the changes in biodiversity to the motion of the sun and planets through our galaxy. “As the sun orbits the Milky Way, it also bobs up and down, rising above the plane of the disk and then diving below it,” he says. “Every time the sun rises up and pokes out of the ‘north’ side of the galaxy’s disk, our biodiversity goes way, way down.” He notes that the Milky Way’s north side points toward the Virgo cluster, an enormous nearby gathering of galaxies. Our galaxy (and, by extension, our planet and ourselves) is falling toward Virgo at about 120 miles per second.
According to Melott, as the Milky Way plows through intergalactic material, a powerful shock wave forms ahead of it. Shock waves create energetic subatomic particles called cosmic rays, which can tear apart biomolecules and damage DNA beyond repair. Normally the galaxy’s magnetic fields protect us from that radiation. Every 62 million years, though, the sun bobs up above the disk into the danger zone, Melott finds. “When the sun pokes up above the galaxy’s plane on the north side,” he says, “the entire planet gets a giant dose of cosmic rays.”
All stars follow a similar bobbing motion as they move through the galaxy, but ones in the inner regions do so at a faster pace, which may bolster Lineweaver’s view that those regions are less likely to contain complex life. Then again, a certain amount of radiation is a part of life—in fact, an essential part. Radiation helps drive mutation, and mass extinctions clear the way for evolutionary change. That view tends to bolster Adams’s optimistic outlook. “We want enough radiation to pose a challenge and spur development of new life-forms but not so much as to sterilize the whole planet,” Melott concludes.
The Temporal Habitable ZoneMelott’s hypothesis about mass extinctions shows how habitable zones may be measured not just in space but also in time. It turns out that “when” is just as important as “where” for the existence of life.
Supernovas come into play here, too. When the universe emerged from the Big Bang, it consisted almost entirely of hydrogen and helium. Good luck trying to make a planet, much less a person, out of that. Carbon, oxygen, iron, and the like had to wait for stars—especially the massive ones—to form and create heavier elements via nuclear fusion. Those processed elements escaped in stellar winds or supernova explosions and then got picked up by subsequent generations of stars. Building up the elements needed for life this way takes billions of years. The entire universe was, therefore, a nonhabitable zone for perhaps the first few billion years of its 13.7-billion-year history.
Once the universe is full of heavy elements, the tables turn and the mortal nature of stars becomes a limitation. The sun, a medium-size star, is about halfway into its total lifetime of 10 billion years. In another 5 billion years it will swell into a red giant and either consume our planet or bake its surface to concrete. Even sooner, in as little as a billion years, the sun’s gradually increasing luminosity may make Earth unbearable for life. Brighter, more massive stars, which guzzle their nuclear fuel more quickly, may burn out too quickly to allow complex life to evolve.
Fortunately, the realization that dim red dwarf stars could potentially support Earth-like planets greatly stretches out the temporal habitable zone. The dimmest, most economical of those stars might live 10 trillion years, a thousand times as long as the sun. Then again, current studies suggest that the universe will probably expand forever. If so, the cosmos as we know it—full of stars and, maybe, full of life—will be a fleeting moment in an endless duration of cold, dark nothingness.
Feeling grim again? Don’t worry; the latest physics theories point to yet another habitable zone that would allow life to go on long after the last star has expired.
The Multiverse Habitable ZoneThese days, the largest habitable domain to consider is no longer our universe but the hypothetical universe of universes, what cosmologists call the multiverse. After our universe has gone black, perhaps another (or many others) will carry on life’s flame.
The idea that our universe—everything we can observe, including the laws of physics that shape it—is just one among a vast ensemble may seem the stuff of science fiction, but cosmologists build multiverse models using a theory called inflation. Inflationary cosmology, currently the dominant model of the early universe, holds that the entire observable cosmos began as a speck within a far larger (perhaps infinite) existence emerging from the Big Bang. Within 10^-30 second after the moment of creation, this speck underwent a period of hyper-rapid expansion—hence “inflation”—becoming everything we see today. As bizarre as this model sounds, it has some reasonable observational support.
Some cosmologists go further and argue that inflation could also happen in other places and at other times, when these other bits of creation break out, undergo their own inflation, and become separate pocket universes. Physicists call this multiplication of reality “eternal inflation.” It leads to an almost limitless number of separate universes, each with its own laws of physics. (This dovetails with the equally weird predictions from string theory, a model of fundamental physics that suggests there could be something like 10^500 different sets of laws.) “In some of these universes the force of gravity might be stronger or weaker than our own,” Fred Adams says. “In others the electromagnetic force that controls atoms and molecules could be different. The consequences for the formation of life in these different kinds of universes might be dramatic.”
Although there is no evidence for these multiverses, that has not stopped theorists from speculating about them. In our universe the laws of physics seem precisely calibrated to allow the existence of long-lived stars, planets with stable orbits, and molecules that allow complex chemistry. All of these seem to be prerequisites for life. “One of the things people always ask about is the behavior of stars in alternate universes,” Adams says. “If you have universes where stars can’t form, then it’s likely those would be pretty sterile places.”
Adams took this question seriously and began a study of alternative physics and its effect on the existence of stars. “I decided to do an actual calculation,” he says. “Could I get all this speculation down to a well-posed problem?” Each of the four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) has a kind of theoretical knob that can be turned up or down to change its strength. “I decided to calculate a bunch of theoretical stellar models, looking to see what range of forces gave me working stars,” Adams continues. The results surprised a lot of people.
“Many people claim that only a minute fraction of bubble universes would have the right conditions to harbor life,” Adams says. His calculations found instead that functioning stars would be more resilient to variations in physics than anyone expected. Since stars are a prerequisite for life, the findings could indicate far more possibilities for viable habitats. Fully a quarter of his models led to long-lived stars, but with an important caveat. Adams cannot say how probable any given strength of gravity or electromagnetism would be in a randomly chosen pocket. “What you need is to fold what I have done into a probability distribution across the multiverse,” he says. In other words, we need to know the statistics of variation in the laws of physics of pocket universes—and in inflationary cosmology there is no principle that guides the choice of physics in each of them.
Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, has a controversial idea that makes some testable predictions about those other universes. In the process, he makes the case for habitability look even better than Adams inferred.
During the early 1990s Smolin proposed a multiverse model that differs strongly from inflationary cosmology’s pocket universes. His model focuses on the way that black holes warp space and time. Since the 1960s some theorists have floated the idea that when a massive star collapses into a black hole, it gives rise to a new universe. Smolin is building on that concept.
Black-hole-generated universes differ from the ones associated with eternal inflation in an important regard. With inflation there is no connection between the physics of one universe and that of another. The black-hole model, Smolin argues, strongly trends to certain types of physics. “Any universe that produces more black holes will create more daughter universes,” he says, “and its physics will be passed on to those daughters.” As a result, there should be a process analogous to natural selection favoring universes whose physics leads to the formation of more black holes. Such universes should dominate the multiverse.
Smolin’s model has two notable advantages. First, it explains why our universe has the physical laws that it does, since universes like ours that can create the massive stars that produce black holes are strongly selected. Second, it explains why our physical laws allow life to exist: The elements that permit the existence of stars happen to be the same ones that allow the existence of our kind of biology.
Actually, there is a third advantage. Smolin claims his black-hole multiverse hypothesis can be tested. Since universes that give rise to the largest number of black holes have the most offspring, our universe should be optimal for making black holes. Smolin’s predictions, including ideas about cosmological inflation and the mass of the heaviest stable neutron star, have held up so far. “The theory is falsifiable,” he says. “If observations come out contrary to my predictions, then the idea is wrong.”
But if Smolin is correct, we inhabit not just a universe but an entire multiverse that may be teeming with life—a habitable zone unbound.