Fertile Crescent In November 2007, the ESA’s Rosetta spacecraft flew by the Earth en route to a deep space rendezvous with a comet. In this composite image captured from 75,000 kilometers away, sunlight shines on Antarctica’s interior, and lights from human settlements reveal a slumbering Eurasia. Photos courtesy of ESA ©2005 MPS for OSIRIS TEAM MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA.
Some time ago, in the outskirts of a typical spiral galaxy, a wandering spacecraft encountered a promising world. The azure planet orbited third from its yellow star, at just the right distance to allow liquid water at its surface. As the probe approached, gaps in the clouds far below revealed continents scattered amidst a world-girdling ocean. In a vast cosmic desert, this was an oasis. The probe sampled the atmosphere, finding abundant oxygen and traces of methane. Chemistry dictates that the two reactive gases could never coexist for long; something was replenishing them. Analyzing starlight reflected off the land, it saw regions absorbing light at wavelengths corresponding to no known non-biological process. Perhaps this was vegetation. The spacecraft also detected powerful, modulated radio emissions from the surface—almost certainly a sign of substantial technology. There was life on this planet, and at least some of it seemed intelligent. The date was December 8, 1990, and the spacecraft was the Jupiter-bound Galileo probe. The planet, of course, was Earth. For the first time, scientists had proved Earth’s biosphere could be clearly detected from space.
In principle, the only limits in our quest to find extraterrestrial life are the size of the observable universe and our imagination. Beyond sheer luck, any chance of success depends upon how we shape our search. Realizing this, the visionary astronomer Carl Sagan conceived the Galileo observations of Earth with the simple premise that we’d have better chances if we could first demonstrate that we could detect life on our own planet from space. When the Galileo probe successfully identified life here, it proved Sagan’s point that we can best seek that which we know how to find—planets like ours, life like us.
Since then, an astronomical revolution has unfolded. Beginning with the first detections in 1995, astronomers have discovered nearly 300 extrasolar planets, or exoplanets, orbiting other stars. We’ve found most of them by detecting the subtle orbital wobble their masses induce upon their parent stars. Others have revealed themselves as transient shadows crossing the faces of their suns. A handful has been discovered by more exotic means. Out of all those we’ve found, however, not one could harbor life as we know it. None are remotely like Earth, perhaps because we just haven’t had the skills to find those that are. Until now. Several recent groundbreaking studies have demonstrated that we can not only detect Earth-size, potentially Earth-like exoplanets, but also explore them from afar. Everyone now living stands at the precipice of what may be the most profound moment in history: The day we discover we are not alone.
Galileo’s flyby wasn’t the first time Sagan studied the Earth at a distance. Ten months prior, he had orchestrated an observation by the Voyager 1 spacecraft. To cap its epic journey through the solar system, Voyager 1’s controllers swiveled its telescope toward the Sun and captured an incredible, humbling image: our Earth, a pale blue dot glimmering against the void, more than six billion kilometers distant. Reflecting on the image, Sagan wrote that although the color indicated the presence of clement oceans and clouds, he doubted an alien observer could deduce as much about our world from so far away. Our planet seemed too small, too dim, too lost in the radiance of the Sun. Had he lived longer, however, he would surely have changed his mind. Several space telescopes have been designed (but not yet built) that could nullify a star’s glare, allowing us to see light reflected by its orbiting planets. It’s not that those telescopes would produce images of stunning new clarity; at best, their images will span a single pixel. But like William Blake, who wrote of worlds contained in grains of sand, astronomers will perceive entire planets within them.
The proof is close to home, as nearby as the Moon. When the Moon appears as a thin crescent, its darkened portion isn’t really dark at all, but shines with a dim, greenish shimmer. Called earthshine, that light comes to us third hand, having gone from Sun to Earth to Moon and back to Earth again. Earthshine may not seem like the light Voyager 1 captured, but it is. In both earthshine’s green glow and our planet’s pale blue, the light fluctuates in brightness and in color. Hidden within those wavering hues, a wealth of information about our planet awaits discovery. Our own astronomers have already found evidence of Earth’s vegetation, atmosphere, and ocean by their ashen reflection in the mirroring Moon. What might other, more distant eyes see?
Observed at astronomical distances, Earth varies in brightness more than any other planet in our solar system. This is because of our planet’s variegated surface: clouds, ice, snow, and sand reflect more light, while forests, grasslands, and seas reflect less. Oceans are generally dark, but in some circumstances, water (or any other liquid, for that matter) behaves like a mirror—something familiar to anyone who has been dazzled by the glare of a sunlit sea. Darren Williams of Penn State Erie, and Peter McCullough independently at the Space Telescope Science Institute, realized that on cosmic scales, a properly aligned observer would see Earth’s brightness more than double as sunlight fell directly upon portions of our global ocean, then dim again as continents came into view. This would hint at the size of our seas and the spread of our continents. Sara Seager of MIT and Eric Ford of the University of Florida, along with their collaborators, found that over time, Earth’s periodic fluctuations in brightness would reveal the length of its days and the cycles of its seasons. The brightness that a distant observer sees would tell only one part of Earth’s story. The color of the light would reveal much of the rest.
Every substance—each gas in the atmosphere, each mineral on the ground, each pigment in every organism—leaves a unique fingerprint on the light it emits, absorbs, and reflects. Any observers seeing those beams of light, so long as they know as much about chemistry as we do, would be able to isolate those fingerprints by examining our planet’s color. If we can assume that they, like us, seek oxygen-fueled beings of water and carbon, they may very well take the same steps our astronomers would when (and if) an Earth-like planet is found. First they’d look for signs of water, which may be a universal requirement for life. Then they’d search for ozone, which is much easier to detect than its chemical kin, oxygen. The presence of ozone requires the presence of oxygen; to find the former is to find a planet rich in the latter—that is, a planet on which we could breathe. Oxygen itself is so chemically reactive that for it to saturate an atmosphere requires constant renewal, most likely by life. They might also search for life’s other detectable traces, compounds like methane and nitrous oxide.
For the past 2 billion years, our planet has displayed chemical signals of life to the stars; these spectral calling cards long ago washed over the entire Milky Way and spread even now, echoing through intergalactic space, visible to anyone who cares to look. But perhaps it’s premature to speculate on who or what may watch our planet and the things they might discern. After all, there’s no proof that life exists beyond Earth. But it is not too early to consider what we ourselves will find. The first terrestrial exoplanets we scrutinize will likely confuse us, as they’ll represent outcomes of planetary evolution that we’ve scarcely imagined. We’ll develop a census of worlds: ocean planets, desert planets, planets eternally shrouded in clouds or suffocated in airless desolation, planets locked in ice or consumed by fire, planets where life’s flame is freshly kindled, and planets where it sputters near death. We will see that cosmic destruction sows the seeds of creation. Exploding suns spark the formation of new solar systems and spread the building blocks of biology; planetary mass extinctions create new ecological niches for the evolution of further forms of life. Viewing other worlds, we will learn just how cruel or kind Mother Nature really is. Whatever we discover, the implications will surely stagger us.
Consider the Copernican principle, the idea that there’s nothing special about our position in the universe. We may find that our ill-informed ancestors who placed our planet and ourselves atop a cosmic pedestal were, in fact, right. Maybe we’ll find that there’s nothing alive out there at all. Even if our galaxy were teeming with life, it could be that nothing more complex than a microbe exists. It may be that, for intelligence to emerge from primal chaos, life must pass through the eyes of so many needles that the entire universe now lies fallow.
Or maybe we’re special only for the brief period of time in which we humans exist. There may be many circumstances in which intelligence can be born, but each particular instance could be so short-lived that by the time another emerges that could make contact, the previous group is already gone. Across our skies we may find dead worlds, graveyards of civilizations fallen silent beneath the weight of time. If in all our searching we discover life everywhere but never anyone else to talk to, it will be both a blessing and a curse. The entire galaxy could be ours to use as we wish if we reach the stars, but some unspeakable, universal catastrophe may loom in our future that has claimed all cosmic cultures that came before and will annihilate all those yet to exist.
It’s conceivable, too, that our observations will eventually reveal a civilization in its death throes. Imagine witnessing a planet’s light fade beneath gray radioactive soot from a global thermonuclear war, or its color changing as levels of atmospheric carbon dioxide and chlorofluorocarbons soared and its ice caps and protective ozone layer disappeared. We wouldn’t know for certain what lived there, but we’d know its survival just got a lot tougher. Whether this would cause us to change how we live on our world is another matter.
Throughout history, our knowledge has grown through human ambition and curiosity, only to regress beneath human apathy and caprice. The greatest obstacle to efforts to find another Earth, to discover life elsewhere in the universe, isn’t some flaw in our methodology or our technology, but in our will. Most of us alive today are unlikely to see these efforts bear their fullest fruit. Even optimistic young astronomers are uncertain that they will see the light from other living worlds in their careers, or even their lifetimes. But they work as though they will. Whether they see it personally doesn’t matter; what matters is that these other planets be seen someday. In preparation for that day, they continue to send modern-day robotic voyagers, like NASA’s EPOXI spacecraft, to gaze at our own planet from deep space, while still searching, as EPOXI also does, for planets elsewhere.
This is only the beginning of our understanding of the living cosmos, and our place in it. Though we and our planet are truly the best examples we have to guide our search for cosmic context and meaning, it is delusion to pretend any of us look at ourselves, or the stars, dispassionately. Just as pragmatism shapes our search, so too do pride, loneliness, vanity, and fear; we don’t merely seek the familiar—we ache for it. Our fervent desire to find in that great darkness someplace like home will sculpt all we eventually see. And so, reason inseparable from emotion, we gaze outward to the cosmos, hoping to behold our reflections in some distant mirror.
Glint in Darwin’s Eye Taken by NASA’s Mercury-bound spacecraft during a 2005 flyby, these images are stills from a movie capturing a complete 24-hour rotation of the Earth. The bright spot of light in each image is sunlight scattering off Earth’s oceans. In the largest image, the Galapagos Islands can be seen through breaks in the clouds off the west coast of South America. Photos courtesy of NASA/John’s Hopkins University Applied Physics Laboratory/Carnegie Institute of Washington.
Originally published May 21, 2008