BUNSEN AND KIRCHHOFF'S GIFT
Today, along the hallways from Alex Dalgarno's office at the Harvard-Smithsonian Center for Astrophysics, are the offices of a new generation of bright-eyed astronomy professors and ambitious graduate students who've come of age in a different cosmos from Dalgarno's—and none more so than tall, energetic, and affable Harvard exoplanet astronomer David Charbonneau. As a teenager, Charbonneau recalls sitting in his driveway in suburban Ottawa, Canada, reading Stephen Hawking's A Brief History of Time and being inspired to pursue a career in theoretical astrophysics. When he arrived at Harvard to begin his PhD in 1996, it was at a moment in the Stardust Revolution when astrophysicists were seeing the history of time with new eyes. The previous year marked the discovery of the first exoplanet, and this changed everything. Charbonneau and his generation came of intellectual age in an era when exoplanets were the stuff of textbooks, courses, and careers, rather than of speculation and fruitless striving, as had been the case for a previous generation of astronomers. Their inheritance is a new cosmos, one populated with boundless other worlds. Unburdened from wondering if there are exoplanets, the energy and time of a new generation of astronomers is dedicated to exploring what exoplanets are like and how they form. They are a generation who senses that George Ellery Hale's vision is within their grasp: to understand the astrophysical conditions that lead to Earth-like planets. Not just Earth-sized or rocky planets, but living planets—the great nexus of astronomy and biology.
“What drives us is the hope that we can be part of some big change in how we think about the universe,” says Charbonneau, a member of Harvard's Origins of Life Initiative, who in 2000 forged his place in exoplanet history by discovering the first transiting exoplanet. “For the first time in human history we don't just have to speculate about whether there's life elsewhere in the universe. Now we have the technology to actually look for Earth-like planets, and when we find them we'll be able to study them for signs of life.”
Today in the Stardust Revolution, the search for other cosmic life has shifted from the late twentieth-century hope of discovering some extreme—by Earth standards—life eking out a living on some other body in our Solar System, to that of finding another living planet, even a verdant cosmos. The names of the proposed next generation of space-based telescopes tell the story of their intent: Darwin, Gaia, and the less prosaically named Terrestrial Planet Finder. These are telescopes whose names speak as much to biology as to astronomy. Their goal isn't to search for something “other” and different but to look for something very similar to us. We stand potentially on the edge of a great era of cosmic comparative biology. Charles Darwin's revolutionary insights into the nature of life on Earth were inspired by his circumnavigation of the world as the naturalist aboard the HMS Beagle. Now a new generation of telescopes could enable us to travel the inlets and islands of the Milky Way in search of other worlds with which we can compare our own. It will be through seeing ourselves reflected elsewhere, as if the night sky is a great puddle into which we can look and not just see our reflection but also compare it with other views of life, that we will better know ourselves.
The first signs of life on a distant exoplanet won't be grainy images of alien life flashing across our television screens. The stories carried by light from an exoplanet's atmosphere will be our finger on the pulse of a world light-years away. It's in these exoplanets’ atmospheres, the oh-so-distant film of gas incubating the planet, that stardust scientists are exploring for the first signs of another planet with a heartbeat. A century and a half after Robert Bunsen and Gustav Kirchhoff realized that an element's light fingerprint was the same on Earth as it is in the stars, spectroscopy—the gift they gave humanity—might well carry the first news of another living planet. Starlight passing through an exoplanet's atmosphere will enable the spectroscopic identification of the gases, including “biosignature gases,” a mix that's characteristic of life. Long before we directly glimpse an exoplanet's surface, we will know it by its atmosphere.
While from ground level we think of the evidence for life as blooming, hopping, or swimming in forests, fields, and seas, astronomers have long looked up to a planet's atmosphere for signs of life. Indeed, the Gaia hypothesis—the notion that life doesn't actually live on a planet but that Earth, or any other living planet, is an interactive, integrated complex system of forces that are life—was inspired by James Lovelock's efforts to develop remote planetary life–detection systems. In the mid-1960s, Lovelock worked at the bioscience division at NASA's Jet Propulsion Lab in Pasadena, California, where his job was to figure out how to determine whether other planets in our Solar System, particularly Mars, harbored life, without having to go there to search at ground level. Lovelock realized that a planet's atmosphere shapes, and is shaped by, life. He made the space-age leap that, in looking for alien life, rather than limiting the notion of “life” to cellular biochemistry, it was possible to identify a living planet by the composition of its atmosphere.
In a way similar to the broadening chemical vision of life's essential nature, Lovelock extended an understanding of what life does in a planetary context. One of the things it does is exchange gases. The key to spotting a living planet, said Lovelock and his colleague Dian Hitchcock, was to see a chemical disequilibrium among the mix of gases that make up a planet's atmosphere. By this they meant a mix of gases that, if left alone, would either react with one another or be broken apart by their star's light, but that in either case would disappear. Something, or someone, had to be maintaining this chemical disequilibrium. In the Earth's case, Lovelock argued, an alien civilization would be able to tell that Earth was living based on the imbalance of oxygen and methane in our atmosphere, though it wouldn't be clear what kind of life was off-gassing in this way. By volume, Earth's current atmosphere is about one-fifth oxygen, an amount that is an estimated eight times greater than what would exist if green plants were to stop pumping it out. A living planet has an atmosphere that says this must be so.
That might be the case in theory, but once the first exoplanet was discovered, few astronomers believed it would ever be possible to actually capture the tenuous light from the atmosphere of an exoplanet dozens or hundreds of light-years distant. That changed in 2002, when David Charbonneau first captured light from an exoplanet's atmosphere, peering for the first time into the sky of a world around another sun. He did this by pioneering a clever technique that's become the bread and butter of exoplanet-atmospheric scientists. When an exoplanet transits its star, passing in front of the star as viewed from Earth, the exoplanet's presence is inferred from the characteristic temporary dip in the brightness of the starlight. However, some of the star's light isn't blocked but rather shines through the exoplanet's atmosphere, picking up the exoplanet atmosphere's light fingerprint. At this point, the light tells the tale of both the chemical makeup of the star and the exoplanet's atmosphere. To extract only the exoplanet's atmospheric signature, the astronomer subtracts the star's spectrum (captured when the exoplanet is behind its star) from that of the combined star and exoplanet. Charbonneau used the Hubble Space Telescope to capture the first view of a Jupiter-sized exoplanet's atmosphere. What he recorded were the telltale doublet absorption lines in the yellow part of the spectrum, the light fingerprint of element number 11, sodium. These were the same lines that had first enthralled Robert Bunsen in his Heidelberg laboratory, and the first that Gustav Kirchhoff identified in the Sun's atmosphere. And here, once again, was sodium—the sister element of chlorine that forms the salt used at every table on Earth—shining forth from high in the sky of a faraway world, one that suddenly wasn't so alien but was joined with us in its salty essence.
In the decade since that first exoplanet atmospheric identification, we've moved from the realm of the implausible to the particular and detailed. Now the study of exoplanet atmospheres is its own realm of exploration, with specialists and a wave of upcoming younger researchers imagining how to look into alien skies. Using space
-based telescopes and viewing from Earth, exoplanet sky watchers have looked into more than two dozen exoplanet atmospheres, measuring the presence of familiar molecules, including water, carbon dioxide, carbon monoxide, methane, ammonia, and helium. More than just taking an atmospheric inventory, this research is providing the first insights into the amazing realms of exoplanet weather, climate, and even geology, as well as giving us the ability to deduce an exoplanet's surface geology from its atmospheric mix. The ultimate goal of all this atmospheric searching isn't to discover distant geology but to find hints of the life on another world. As exoplanet scientists finesse their art, it's in the hope that one day soon they'll be able to capture light from a planet in its star's habitable zone, a planet with liquid water, and to probe that atmosphere for biosignatures—the telltale gases of life.
In preparing for this, we're undertaking one of the most intriguing aspects of the Stardust Revolution: thinking of Earth as an exoplanet. It is the final Copernican step, to relinquish our hold on the notion of Earth as the only living planet and to move toward seeing ourselves as being on one kind of living planet. Exoplanet scientists have gotten a good glimpse of what Earth's atmosphere would look like to observers from another solar system by examining “Earthshine.” During the sliver of a crescent Moon, sunlight reflected from our home planet creates Earthshine, a glowing illumination visible to the naked eye on the larger, darker part of the Moon. In this way, the lunar surface becomes a mirror to our nature. When astronomers dissect Earthshine's infrared spectrum, in the valleys and peaks of the light curve, Earth's atmospheric mix of oxygen and ozone stands out as a beacon of what's happening below. Using a variant of this technique that looks at polarized light, astronomers have even been able to identify the presence of Earth's cloud cover, oceans, and the presence of vegetation. From Earth's perspective, oxygen and ozone as biosignatures are particularly strong indicators. We don't know of any abiotic, or nonliving, sources of oxygen in this quantity. And ozone, created when an oxygen pair is split by the Sun's ultraviolet radiation and recombines in an oxygen triplet, requires a steady source of oxygen to sustain its presence, since the same forces that create it will eventually destroy it.
However, the search for the biosignatures of another living planet isn't limited to our love affair with oxygen. Millions of species of bacteria on Earth are anaerobic—in your gut, in your compost heap, and in hot springs, they breathe gases other than oxygen. Indeed, for the majority of Earth's history, most life on Earth was anaerobic. Thus it's entirely possible that the first sister Earth we discover will be a cosmic sibling with a difference—anaerobic all the way down. In this case, such planets’ distinctive atmospheric biosignatures won't be oxygen but might be a variety of anaerobically produced sulfur gases, including dimethyl sulfide, a component of cooked-cabbage and stinky-fish smell. We'll know when we've found our cosmic cousins by what they smell like.
“In the coming ten to twenty years we should have the first list of potentially habitable planets in the Sun's neighborhood,” says Swiss astronomer Michel Mayor. “Making such a list is essential before future experiments can search for possible spectroscopic signatures of life in exoplanet atmospheres.” However, at present, taking this spectroscopic pulse of a potentially living exoplanet is largely beyond our reach. Although astronomers have made major gains in using ground-based telescopes to capture impressive direct images of giant exoplanets, most agree that getting a high-resolution light fingerprint of an exoplanet equivalent of our small blue dot will require a new generation of specialized space-based telescopes. These telescopes will be able to block out a star's intense light in order to see the million-times dimmer reflected light of a small, rocky planet in the star's habitable zone. Yet all these proposed telescopes—the European Space Agency's Darwin and Gaia missions and NASA's Terrestrial Planet Finder—are either on hold or have been canceled. As with Hubble, Spitzer, and Kepler, a new generation of space-based telescopes designed to discover and explore living exoplanets will lead to yet another new way of seeing the cosmos both outside and within ourselves.
Presidents have talked of going to the Moon and to Mars, but there's yet to be a president or prime minister who's talked passionately about embracing our cosmic genealogical search. In the United States, the culture wars over terrestrial evolution still stifle public acknowledgment of NASA and American scientists’ leading twenty-first-century role in piecing together the story of cosmic evolution. At a seminal moment in the Stardust Revolution, we stand as if before a box containing old family letters—and we hesitate. The science of the Stardust Revolution has changed our view of the cosmos, but perhaps we stand unwilling to change, and to embrace, a new view of ourselves.
AN ANCIENT VIEW WITH STARDUST EYES
I get a different view of humanity's relationship to the stars when I find myself standing on the roof of the University of Guanajuato in central Mexico, peering out at the stars sparkling in the inky sky above this bowl-shaped city that spills up the surrounding hillsides. Guanajuato is best known for another kind of sparkling white light. It's referred to as the city that silver built because of the local silver mines—among the world's richest—which over the past four centuries have produced an astounding one-twentieth of all the world's silver. On this night, I look out over the city's twinkling, vibrant nightscape with stardust eyes and know that Guanajuato is ultimately the city that supernovas built. Every atom of every one of the millions of ounces of precious metal hauled back-breakingly from the area's labyrinthine underground mines was forged before the formation of the Earth in the seconds-long death throes of a giant star, a star whose light is long gone but whose body remains here today, glinting underground in the light from miners’ helmets.
Through a small telescope, at eighty times magnification, the staff astronomer takes me on a visual tour of the heavens that traces the great arc of human discovery. We start with a bright point of light to the west, one that hangs midway up the inverted bowl of sky. As I focus the view, this point of light resolves into what is clearly a disk—not a point, but a planet: Venus. As I watch, Venus gradually moves from the center of vision toward the edge—an illusion, of course; it's not Venus that's moving so quickly, but me. I’m experiencing the Earth's rotation, my planet's rotation, as I watch another planet. Then my celestial guide aims the telescope at a point of light farther up the sky, at another planet, Jupiter. I am seeing not just planets but a solar system. Around Jupiter, like a cluster of dancers forming a single plane of orbit, are three of this giant planet's moons. This is the same view that for Galileo solidified the Copernican Revolution: if one looks with greater acuity, neither the Earth nor the Sun is the center of all, but we see that moons orbit other bodies. My guide adjusts the focus and asks me to again look closely at Jupiter. I see pencil-thin lines, the stripes of Jupiter's atmosphere. I’m looking not just up through our sky but into another, just as astronomers now plan to take the light fingerprint of a distant Earth-like planet's atmosphere.
My guide turns the telescope to the west and up toward three stars in a horizontal line that form Orion's belt, which holds up invisible clothes across the waist of the celestial hunter. Above the belt is a bright red star, Betelgeuse. I tell my teenage son and daughter, whom I’ve coaxed to come along, that what appears as a point of red light is in fact a near-death supergiant star, so large that, were it in the place of our Sun, its pulsing outer atmosphere would swallow us where we stood. When it eventually explodes as a supernova, it will seed the Milky Way with silver and gold atoms that might one day be dug up as cosmic treasure by some future Milky Way civilization on a planet yet to be born. My kids both shrug and turn to look down at the brighter, more inviting lights of the luminous city below. I think of Mount Wilson astronomer Paul Merrill—the man who first glimpsed that stars are the sites of the origin of the elements—and his comment that, with stars, “the contrast between the apparent and the real is the most stupendous in all human experience.”
I tur
n again to look through the telescope, which is now positioned to look not at a dying star but at fiery, compact newborn stars. Below Orion's belt is a vertical line of three points of light that form Orion's sword; locally also known as the Three Marys, for the three Marys who, according to the Four Gospels, accompanied Jesus's mother to his grave. It's the middle point of light that my guide wants me to see: part of the Orion Nebula. Through the eyepiece I see not just stars but also something so unlike what we usually see in the star-filled sky: a hazy cloud. In a pocket of the cloud are four clustered blue-white stars, youngsters whose nascent energy is blowing away their natal cocoon of cosmic gas and dust. I think about William Herschel and other nineteenth-century astronomers first seeing these great clouds, or nebulae, and wondering how they fit into the scheme of the eternal heavens. I think about Charles Townes, who in the late 1960s first saw these clouds for what they are: water heavy.
Finally, the telescope is turned nearly straight up, and before looking I ask my guide what I’ll be seeing. He does not reply. As I put my eye to the eyepiece, I just glimpse his smile. The light is so bright that I need to squint: the Moon. The focus is along the border of light between the half-Moon's lit and shadowed sections. What I see makes me think not of astronomy but of geography. From a central Mexican rooftop, I see a distant terrain of massive, circular craters, their central ejecta dimples clearly visible, some craters with smaller ones in turn dimpling their interiors. Here in the Moon's pockmarked face is the story of the tumultuous, collisional formation of our Solar System, the great bombardment erased from Earth's surface by eons of tectonic recycling but still there for anyone to see on a clear night. A bombardment that brought the products of stars like Betelgeuse to a new-formed Earth.
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