Saturn's icy moon Enceladus seems to be
warm and wet inside.
NASA/JPL
See Adam Frank's recent book, The Constant Fire: Beyond the Science vs. Religion Debate, and the companion blog to the book.
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 Zone
One 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 Zone
Astrobiologists’ 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.
Our galaxy, the Milky Way, is harshly irradiated near the center and nearly barren at the edges. Even so, it may contain billions of habitable planets.
2Mass/J. Carpenter, M. Skrutskie, R. Hurt
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.
