"It is unnatural in a large field to have only one stalk of wheat, and in the infinite universe only one living world." – Metrodorus of Chios (ca. 350 BCE)
In 2001, Jocelyne DiRuggiero stood on the parched sand of a New Mexico desert, looking up. A rocket had just launched, carrying microorganisms far from their hot springs home to just outside Earth's atmosphere. DiRuggiero, now a Johns Hopkins associate research professor of biology, was at the White Sands Missile Range to test the survivability of various microbes when exposed to the vacuum and extreme UV radiation of space. Much of her previous work had focused on the tiny environs within cells. But as she watched the rocket burn through the clouds, she caught her first glimpse of a much bigger research universe. "It shot into space and came back down, and that got me hooked," says DiRuggiero. "Here I was working with rocket scientists. It was amazing."
Thirteen years later last April, DiRuggiero stood at a lectern at the Space Telescope Science Institute on the Homewood campus. Projected behind her was an ink drawing of the 1633 trial of Galileo Galilei, who was convicted of heresy by the Roman Catholic Church for favoring Nicolaus Copernicus' theory that Earth orbited the sun. This theory "was very bad news for a lot of people who thought that we were at the center of the universe," DiRuggiero told the roomful of researchers just waking up on a Monday morning. "Since then, things have gone downhill." She then flashed another image, the famous Voyager 1 snapshot of Earth taken in 1990—the "Pale Blue Dot." To many, that view of our speck of a planet from 4 billion miles away underscored our inconsequential place in the cosmos.
DiRuggiero and other researchers now ponder our cosmic context via a new prism shaped not only by our lack of centrality but by the discovery of planets light-years away and, on Earth, bizarre sulfur-based organisms that flourish in extreme environments. Scientists in astronomy, biology, astrophysics, geology, planetary science, chemistry, and other disciplines are pursuing a new wave of science that probes a deep and very old question: "Is there life beyond Earth?"
Their research is driving astrobiology, which NASA defines as "the study of the origins, evolution, distribution, and future of life in the universe." Those in DiRuggiero's audience were among 100 researchers who attended a symposium titled "Habitable Worlds Across Time and Space."
"Our solar system is part of a beautiful galaxy, the Milky Way, that is just one among billions of galaxies in the universe," she told them. "The idea that we might not be alone in the universe is one of the most exciting scientific questions of our time."
Astrobiology embodies a paradox: the study of a subject not yet discovered. Does "biology" even exist in our solar system or beyond, be it microbial or complex? How did life originate on Earth, anyway? What is life? The search for answers blends a heady mix of hope, fascination, and science.
Researchers like DiRuggiero credit Galileo and others for helping set today's scientific stage, and for keeping the faith—one has to believe in the likelihood of life elsewhere to spend time looking for it. Speculation surely began as soon as humans first craned their necks to stare at the stars and imagine the heavens. Thales (ca. 600 BCE), considered by many the father of Western philosophy, espoused the concept of "a plurality of worlds." Greek thinkers like Epicurus and Metrodorus also asked whether the world we see with our limited vision is all there is, whether at the atomic level or an otherworldly realm akin to a parallel universe. Early philosophers sometimes paid a high price for their ideas. Giordano Bruno, an Italian Renaissance monk, proposed that stars were distant suns and that other planetary systems might harbor life. For such prescient ponderings and other alleged transgressions, the Roman Inquisition tried Bruno in the early 1590s for heresy, imprisoned him, and burned him at the stake in 1600, when he refused to recant. Later-century astronomers Christiaan Huygens and Johannes Kepler also mused about life on other planets. In his book Cosmostheoros (1698), Huygens wrote "all those Planets that surround that prodigious number of Suns. They must have their plants and animals, nay and their rational creatures too."
DiRuggiero says, "You can have passion and let your imagination go, and at the same time do serious science." (Albert Einstein, after all, elevated imagination above knowledge.) That taps a core concept of the Institute for Planets and Life—founded a few years ago by DiRuggiero and other Johns Hopkins and STScI scientists. Via lecture series, interdisciplinary studies, and federal grant proposals, IPL has strung a web of astrobiological research. There's astrophysics, where one strand is the search and analysis of exoplanets, or planets orbiting stars light-years away. NASA's powerful space-based Kepler telescope has confirmed nearly 2,000 exoplanets over the past few years, including a rocky Earth-like cousin known as Kepler 186f. Another area of IPL research is planetary science, which can analyze atmospheres on other worlds—such as Saturn's moon Titan—as well as the plate tectonics of early Earth to study how planets change. At the heart of the new field is biology, which includes researchers looking at the adaptability of life in extreme environments on Earth, places that might prove analogous to long-ago landscapes on Mars or the present icy surfaces of Jupiter's moons.
A wave of speculation, fueled in part by mass media attention such as Neil deGrasse Tyson's reboot of Carl Sagan's television series Cosmos, has engulfed scientists and the general public alike: Will the question of life elsewhere be answered in our lifetime? Mind-blowing research from all angles makes this a distinct possibility. Topics discussed at the Habitable Worlds symposium, for example, ranged from "Early Earth Across Time and Space" to "Venus and Mars as Failed Biospheres" to "The Habitability of the Milky Way Galaxy." Peter Olson, a Hopkins Earth scientist, discussed early Earth's transition from magma oceans to tectonic plates, science that sheds light on planet habitability. And researcher Ralph Lorenz of the Applied Physics Laboratory covered moons and planets that orbit red giants, stars much different from the golden orb in our sky.
Skygazers also wonder where life could be detected first. Outlying exoplanets—which might offer Earth's true twin? Or microbial communes on closer satellites in our solar system? Either way, astrobiology offers the tantalizing likelihood of human-created technology so unconventional, and extraterrestrial life so weird, that science fiction will soon be made real.
If a life-sustaining planet is detected, it will likely not resemble our modern world. There won't be coffee shops or cellphones on faraway globes. Life on Earth evolved over 3.8 billion years in multiple directions and was reconfigured by at least five mass extinctions long before human latecomers arrived on the scene. Nonetheless, any search for entities elsewhere tends to be a quest for life that could live here, primarily carbon-based life forms. As many scientists observe, they have to start somewhere. With extraordinary telescope technology coming online, researchers will be able to scour the atmospheres of far-flung planets for evidence of life as we know it. At least that's the plan. "In the case of distant stars and other solar systems, it's more complicated," notes Mario Livio, a senior astrophysicist at STScI and one of 30 IPL researchers. "We cannot build a mission to go to those places, certainly not anytime in the foreseeable future. It's more a matter of remote sensing." Yet how do you remotely detect biosignatures that could indicate the presence of living inhabitants? For example, atmospheric gases such as oxygen, methane, ozone, or carbon dioxide, as well as astrobiology's holy grail, liquid water?
Until about two decades ago, the only other planets known to humans were in the solar system. A new generation of telescopes has already expanded our knowledge of exoplanets via a novel, even poetic approach: scanning starlight for the influence of planets. NASA's Kepler Space Telescope, launched in 2009, has monitored nearly 150,000 stars in one region of the Milky Way, looking for dips in their brightness that signal a planet passing between star and telescope, what's known as a transit. Scientists analyze other measurements, such as shifts in a star's spectrum, that might indirectly indicate orbiting bodies. Data from Kepler is archived and studied at STScI. And in 2017, NASA will launch the Transiting Exoplanet Survey Satellite to expand the Kepler experiment across the sky, targeting bright, long-lived stars closer to our solar system and sifting for even better Earth-like candidates.
Jason Kalirai is a research scientist at the Johns Hopkins Center for Astrophysical Sciences, as well as STScI's project scientist for the James Webb Space Telescope, which is set to launch in four years as the successor to the beloved Hubble Space Telescope. As NASA's most powerful telescope, Webb will provide a more sensitive orbiting infrared observatory. Most important for astrobiology, its instruments could measure the atmospheric composition of potentially habitable exoplanets already targeted by TESS. "Webb is powerful enough to detect water vapor, carbon dioxide, and methane, all of which are found here on Earth," Kalirai says. Both TESS and Webb will scan partly for planets within a star's habitable zone, often termed the Goldilocks Zone, where it's not too hot and not too cold.
Finding a habitable world, though, is quite different from finding an inhabited one. "Ultimately, a 'life-finding' telescope will be needed to find Earth 2.0," says Kalirai. For example, an instrument that could scan for spectral signatures of oxygen and other biomarkers in a planet's atmosphere. Scientists at STScI are proponents of such an observatory, possibly with a vast 16-meter mirror. If approved, ATLAST—for Advanced Technology Large-Aperture Space Telescope—would be 2,000 times as sensitive as Hubble and could launch by the late 2020s.
In many ways, this all seems far, far away. "Working on exoplanets is like having a candy I can't reach," DiRuggiero muses. Fortunately, there might be much closer repositories of life in our solar system. Scientists are enthused about the icy moons of Jupiter, Saturn, and Neptune: Europa, Ganymede, Callisto, Enceladus, Titan, or Triton. Various researchers are hankering to get a probe onto the cracked ice of Europa, which orbits Jupiter, or Saturn's moon, Enceladus, where there's evidence of subsurface oceans, organic molecules in atmospheric jets, and tidal heat caused by the gravity of its host planet. "In the solar system, Enceladus ought to be one of the highest priorities for the world's space agencies," says David C. Catling, a professor and researcher for NASA's Astrobiology Institute, in his book Astrobiology: A Very Short Introduction (Oxford University Press, 2013). "Enceladus has a source of internal energy (tidal heating), organic material, and liquid water. That's a textbook-like list of those properties needed for life. Moreover, nature has provided astrobiologists with the ultimate free lunch: jets that spurt Enceladus' organic material into space. Technology certainly exists to build a spacecraft to swing by Enceladus and sample the organics."
In just a couple of months, a NASA spacecraft named Dawn is set to visit Ceres, a Texas-sized asteroid recently reclassified as a dwarf planet in the asteroid belt. Water vapor and organic material have been detected on Ceres, which scientists believe harbors a "thick mantle of ice that, if melted, would amount to more fresh water than is present on all of Earth," according to NASA. While the mission isn't geared to find life on Ceres, the spacecraft's orbital analyses of infrared light emission and reflected sunlight should offer details of the asteroid's surface composition. Some researchers wonder: Could life have existed there in the ancient past, or even now in a sea under all that ice? That's not likely, but the parameters of habitability are being stretched all the time. Water in some form, for example, has shown up in all sorts of unexpected places. In October, NASA's Messenger mission, managed from APL, released the first photos of water ice in shadowed polar craters on the planet Mercury, which orbits so close to the sun, Goldilocks would surely get burned.
Keeping track of all the breaking science is like trying to bottle a meteor shower. By next summer, the New Horizons mission, managed by APL for NASA, will check out frosty Pluto, which is on some scientists' short list of possible life-harboring globes partly because it's heavily endowed with organic (carbon-bearing) molecules. And several weeks ago, APL scientist Louise Prockter was part of a team that published evidence of plate tectonics on Europa, the first sign of "surface-shifting geological activity" on a body other than Earth. Getting boots securely on the ground—or a probe's landing gear, or ChipSats, which are spacecrafts-on-a-chip—could tell so much more. "With the solar system, we can go there," notes DiRuggiero. "We might go to some of these icy satellites in my lifetime."
In many ways, the search for life "out there" begins closer to the hearth—inside the core of planet Earth, even within our own bodies. From geological strata to genomic blueprints, the key to the lock on life's origins might be at, or on, our fingertips. It's all about the 'bio' in astrobiology.
Recent discoveries of extreme life on Earth, known as "extremophiles," have expanded the likelihood that living entities could survive in alien environments that seem hostile to us. "How far can we push 'life as we know it?'" asks DiRuggiero. "By learning more about the extremes of life on Earth, we can determine the best places to look for life in the universe." Take recent research on the extent of life's adaptability in Earth's coldest, hottest, darkest, or driest places. "Some microorganisms couldn't care less about oxygen," says DiRuggiero. "They can use sulfur. They can use iron. They can do amazing things."
Hardy microorganisms known as thermophiles, for example, were discovered in the 1970s in hot springs at Yellowstone National Park. (Some scientists believe life on Earth likely began at superheated hydrothermal vents.) An array of creatures living in deep-sea vents on the Pacific floor depend on chemosynthesis, in which chemicals take the role of sunlight; lanky tubeworms there can grow up to eight feet long. Methane ice worms reside on toxic methane ice mounds along the Gulf of Mexico's floor. And then there are cave-dwelling, acid-producing "snottites," named for their less-than-lovely consistency: a bacterial slime that eats poisonous hydrogen sulfide. Consider also the humble tardigrade, or water bear. These microscopic critters can ride the wind on dust motes and survive being zapped with deadly radiation. Though they prefer to lope around on cushy moss, they can also endure, in deep hibernation, in the most extreme environments on Earth, waiting for better weather. With five body segments and four pairs of tiny clawed legs, they are one of the few species to survive Earth's mass extinctions. Even the vacuum of space doesn't faze them much.
The concept of Weird Life could apply to similar environs on nearby worlds. Take Saturn's moon Titan, where methane monsoons rage and sand dunes shift in the wind, where subsurface water or liquefied natural gas could offer possible solvents for the organic molecules of life. "There's also lots of carbon and lots of nitrogen on Titan, so there's lots of material to make a prebiotic soup," says Lorenz, of APL, co-author of Titan Unveiled: Saturn's Mysterious Moon Explored (Princeton University Press, 2008).
As DiRuggiero notes, "We have to get past the point where we're looking at single organisms because microorganisms are not isolated. They live in a community and interact with each other." Take the tubeworms. "They have a gut, which is made out of a sponge, and microorganisms inside do all the work for them—feeding the worms from the sulfur in the vents. That is so far away from what we can do with our physiology, it's just fascinating." DiRuggiero has visited some of the most extreme environments, taking samples of thermophiles from volcanic craters in Iceland and New Zealand, and scuba diving near hot vents off Italy's Vulcano Island. She currently researches the adaptability and evolution of Archaea (super-tough microorganisms similar to, yet distinct from, bacteria) in one of the driest places on Earth, the high-salt Atacama Desert in Chile.
"We haven't even discovered all life on Earth yet," she says. Upcoming generations of researchers will make some of those discoveries. As part of IPL, DiRuggiero co-teaches a popular undergraduate course titled Planets, Life, and the Universe with Colin Norman, a Johns Hopkins professor of physics and astronomy. Among discussion questions asked by students: "We are carbon-based life forms and use carbon energy. Do sulfur-based life forms use sulfur energy?" (Answer: They are not so finicky.) Astrobiology, per se, is not a major at Johns Hopkins or elsewhere, though some universities offer minors. But as breakthroughs dovetail with increased interest from NASA, the field will likely grow. Hopkins graduate students in physics, planetary science, and biology, for example, are already getting into the game by launching a club known as the Astrobiology Forum.
Some people wouldn't put money down on extraterrestrial life. A "Rare Earth" hypothesis, offered more than a decade ago, posits that Earth just got lucky. The planet's axis is tilted just so, with help from the moon's gravity, stabilizing our climate. Jupiter played the heavy—sweeping up, through its massive gravity, comets that could have hit Earth, destroying species. Earth's plate tectonics recycle atmospheric gases. And we inhabit that solar system sweet spot of a habitable zone. Such a fortunate convergence might at least make us not so common. How often might all those factors align just right for life?
Also, since Earth is relatively young at nearly 4.6 billion years old—the universe's age is about 14 billion—others question why intelligent extraterrestrial beings have not made it here yet (as far as we know, despite Area 51 conspiracy theories). The Fermi Paradox notes the seeming contradiction between a high probability of civilizations in the universe and the lack of contact. "Maybe advanced life is relatively rare," says Livio. Intelligent species might tend to destroy themselves after developing technologies like ours, or before mastering intergalactic space travel. The state of other civilizations "could differ by billions of years," Livio says. "If they are more advanced by a billion years or less advanced by a billion years, the chance of finding them is very, very small. But if life is ubiquitous and there are lots and lots of planets in habitable zones there might be billions of civilizations," he adds. "In that case, you could have the whole spectrum of evolution." It's all a matter of perspective. Notes Livio, "They might be so advanced they know about us but don't care because we are like worms to them."
Even the Fermi Paradox doesn't preclude life in some form. Maybe we just can't see well enough yet. "The universe wants to make life if it can find a nurturing place," DiRuggiero says. "It's just that the universe goes so far, it is hard for us to detect life." The first organisms found off Earth would likely be microbial, scientists predict. If that happens, could we celebrate the discovery of extraterrestrial life? "It comes back to, 'What is life?' And it can be very philosophical," DiRuggiero says. "Yet every living cell is life."
"If we find microbial life, if it's anything that everyone would agree on is life, then yes, that would answer the question,'" says Livio. "[But] if, for example, we were to find life somewhere in the solar system, let's say on Mars, there would always be some nagging suspicion that this maybe represents pollution from Earth." That is, biological material might have reached Mars by way of what is called ballistic panspermia—rocks expelled from Earth with such force they eventually fall to the surface of Mars. Various comets or asteroids also might have seeded multiple worlds with water and organic compounds, offering common origins. (NASA's currently busy Mars rovers, Curiosity and Opportunity, have not found anything alive on the dry, dusty planet—though oxygen and carbon (and evidence of past water) have been detected, offering the possibility the Red Planet may have supported life at one time.)
The continued search there and elsewhere will likely progress in fits and starts. Livio, a scientist by vocation and philosopher by nature, is the author of Brilliant Blunders: From Darwin to Einstein—Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (Simon & Schuster, 2013). At the edge of a possible threshold moment for science and humanity—scientists currently predict there are billions of planets in our galaxy alone—Livio finds inspiration in Europe's Age of Enlightenment. "The idea of life being only here is actually the last bastion of us being special. I am very much in favor of the Copernican principle that says that we are nothing special," he says. "One of my protagonists in Brilliant Blunders is Darwin because in the theory of evolution, humans are nothing special. They just evolved like everything else. That's how things are. We are a mediocre planet around a very mediocre star in a very mediocre galaxy."
In the end, a second Earth that proves habitable might at least make us feel less lonely, and life's signature beyond our planet would mean we are not flying solo—even if our neighbor is a hardy Bacillus. "We are not going to find intelligent life in our solar system, except on Earth," DiRuggiero says. "But I'm sure there is intelligent life somewhere. The universe is so big." That, perhaps, is the crux of astrobiology. "I guess it allows me to dream," DiRuggiero adds. "Even if I am 80 years old, and I am not in science and people find bacteria in an ocean on Europa, I would be so excited. But I would like to be part of it. This is within reach."