Five Billion Years of Solitude

by Lee Billings

  There are, however, significant uncertainties in Kasting’s considerations, such that science cannot yet entirely dismiss the possibility of a man-made moist stratosphere leading to a premature runaway greenhouse on Earth. Other greenhouse gases besides CO2 and water vapor play a role in Earth’s climate, and could potentially have significant future effects that are unaccounted for in Kasting’s models. And no one presently knows the exact amount of fossil fuels locked away within the Earth, or how much of that guesstimated total could be effectively extracted and burned based on future market conditions and potential technological development. Most fundamentally, no one fully understands how wide variations in temperature and pressure can subtly affect water vapor’s absorption of thermal-infrared radiation. Nowhere is this haziness more evident than in considering the problem of clouds.

  To the average person, clouds are simple things, pieces of cottony fluff in blue skies or ominous gray sheets portending dismal weather. To a climate modeler like Kasting, clouds are the most mercurial and beguiling form of water vapor, fickle creatures almost alive in their fiendish complexity. Depending on a cloud layer’s extent, altitude, and composition, it may either warm or cool a planet. A blanket of dense, low clouds can reflect a good portion of sunlight into space, potentially reducing temperatures. But throw a layer of thin clouds high above the low, dense ones, and much of that cooling effect will be undone, as the translucent upper layer now allows sunlight to stream down but traps the heat that subsequently tries to escape. What everyone agrees on is that as a planet like Earth warms, more water vapor steams into the air to form more clouds. But there is no consensus on where exactly those clouds would form and linger in the atmosphere, or the limits of their feedback effects. Both global-warming deniers and publicity-hungry planet hunters have found refuge in the resulting nebulosity: water-vapor clouds could, in theory, save an otherwise habitable planet from runaway global warming, whether induced by an overabundance of greenhouse gases or by the too-bright light of a nearby star. Farther out away from a star, where temperatures drop low enough for CO2 to condense into ice, an insulating blanket of dry-ice clouds could in some circumstances warm a planet enough to preserve liquid water at its surface. In 1993, Kasting conservatively estimated the habitable zone’s outer edge to lie slightly beyond the orbit of Mars at 1.65 AU, but it could in fact extend out much farther, depending in large part on the uncertainties associated with CO2 clouds.

  There are two divergent strategies for numerically approximating clouds. One is to model them as accurately as possible in extremely detailed three-dimensional simulations. This approach requires reams of data from Earth-observing satellites as well as state-of-the-art supercomputers, and risks losing the distinction between cause and effect in a flurry of variables and feedbacks. The other strategy is to model clouds much more simply in fewer dimensions, which carries the risk of overlooking vital behaviors that only emerge through complex interactions beyond the model’s boundaries. Kasting prefers simplicity. His models are one-dimensional, approximating the entirety of a planet’s atmosphere with a single linear sounding, something like measuring the average temperature and salinity of an ocean by sampling seawater through a very long seabed-to-surface soda straw.

  “Clouds are pretty arbitrary in 1-D—you can get any effect you like in a 1-D model by playing with how you represent them. The ideal scenario for a 1-D model is a cloudless sky, which is obviously a huge weakness,” Kasting acknowledged when we discussed his models. “I try to get around it by basically painting the clouds on the ground, approximating their effect by tuning the surface albedo until it reproduces the average temperature of whatever planet I’m trying to look at—Earth, for example, or Mars. Some people don’t like that, and exactly what my method actually means in terms of real clouds is complicated, but I think of it as minimizing any cloud feedbacks that may occur as a planet’s temperature changes. To do any better than that, you have to go to 3-D, which is a very big step, and even there, clouds remain the biggest uncertainty—the 3-D guys don’t know how to do them, either.”

  Owing to its simplicity, a 1-D model is also much faster than any 3-D counterpart. A state-of-the-art 3-D climate model might take a week on a very expensive dedicated computing cluster to arrive at the conclusion that doubling Earth’s present atmospheric CO2 levels would raise the average temperature somewhere between 2 and 5 degrees Celsius. Kasting’s 1-D climate model calculates the results of a CO2 doubling in less than a minute on a run-of-the-mill desktop computer, and arrives at an answer of 2.5 degrees. “With a 1-D model, I’m limited by how fast I can think, not how fast my computer can,” Kasting said. “So over the course of a week, while a 3-D model may be processing a single iteration, I can pretty well explore the entire parameter space. That’s what this is about—exploring the limits of what appears possible, and challenging others to build on that numerically or to look deeper empirically.”

  • • •

  The more we talked, the clearer it became that Kasting had grown increasingly jaded in recent years toward press releases claiming progress toward finding exoplanetary twins of Earth. Much of the early furor had centered around the planetary system of the red dwarf star Gliese 581, some 20 light-years from Earth. First came Gliese 581c, a super-Earth skirting the inner edge of Kasting’s habitable zone that, for a few months in 2007, was thought to perhaps be clement. Simple calculations by Kasting and others, however, revealed that regardless of atmospheric composition, the planet is bathed in 30 percent more starlight than Venus. Attention then shifted to its farther-out companion, the super-Earth Gliese 581d, which brushes the habitable zone’s outer edge. Its habitability could not be as easily ruled out as c’s, but Kasting hastened to point out that the world receives 10 percent less starlight than Mars. Additionally, c and d are so massive, each more than five times the bulk of our own world, that they could well be gas-shrouded shrunken Neptunes rather than rocky super-size Earths. Then in 2010 came the announcement of Gliese 581g, Zarmina’s World, orbiting in the middle of the habitable zone, and, with an estimated mass a bit more than three times that of Earth, almost certainly terrestrial. That one had excited Kasting, at least until other astronomers began questioning the planet’s existence.

  Some months before our meeting, a European team had announced the discovery of another potentially habitable super-Earth, HD 85512b. Kasting thought “potentially” was an overly generous descriptor—the planet roasts in only slightly less starlight than Venus. “They wrote that a ton of cloud coverage could reflect all that light and make things okay,” he recalled, referring to the European team’s discovery paper. “But clouds sure didn’t save Venus, did they?”

  News of more potentially habitable planets had by then become a fairly regular occurrence, with each world’s fortunes cresting and subsiding on the shifting tides of public interest and scientific opinion. Each discovery followed a similar cycle, first announced in academic journals, the primary producers upon which much else feeds. Pure empirical measurements of masses, orbits, and stellar fluxes then filtered down to the hazy murk of news reports, which processed them into rampant speculation. An infectious cocktail of certain fact and wild conjecture about each promising world then fanned out to mutate within the darkness and confusion at the base of so much human discourse. Before long, bizarre blog and forum postings would appear, wondering when NASA would send a probe, or, better yet, colonists, and whether when we arrived we’d find the builders of the Egyptian pyramids, or perhaps the home planet of the cattle-mutilating, human-abducting Grays, or maybe even Jesus Christ, on yet another pit stop midway through his universal tour of salvation. Again and again, the handful of known facts for each planet became buried beneath the familiar fictions that so many people construct for themselves.

  Seeing the same pattern play out for each successive world, Kasting and his peers sometimes felt like soothsayers, obediently displaying tea leaves, yarrow stalks, chicken entrails, and other crude omens to an audience eag
er to imbue them with subjective meaning. One researcher once told me, with pained exasperation, that provided with only a planet’s mass, radius, and orbit around any given star, the best way to determine the world’s actual surface temperature would be to grab a newspaper and consult the horoscopes.

  Kasting was less extreme, but equally dismissive. “None of these announcements of planets in or near the habitable zone should be big news by themselves,” he told me with an edge of frustration. “They are in a way meaningless, because we presently can’t follow up on the initial act of discovery. The big news will only come when we are able to actually look at one of these planets to discern whether or not it is actually habitable and see if there is evidence of life, right? And if we do that—excuse me, when we do it—the real revolution will begin.”

  To that end, for most of the past two decades Kasting had devoted his time and effort to two intertwined tasks: how to distinguish whether any terrestrial planet is a living world or a lifeless rock based on only a faint smear of starlight reflected off its atmosphere, and how to design a space telescope capable of making those observations. He had worked tirelessly, serving on a multitude of planning committees, panels, and task forces for NASA, the NSF, and the National Academy of Sciences. He helped churn out a mountain of reports defining the observational criteria to which armies of engineers and mission planners would eventually aspire, and it is no exaggeration to say that for a time nearly every definitive paper on the subject bore his name as a coauthor. The telescopes Kasting wished to build were called Terrestrial Planet Finders—TPFs for short.

  In the years leading up to the turn of the millennium, as the pace of exoplanet discovery quickened, American federal coffers, bursting with surplus, had generously funded all manner of space science. The quest for exoplanetary life, like the nation itself, had seemed set on an unstoppable upward trajectory; telescopes to gather evidence for or against other nearby living worlds would be in hand, Kasting and his peers had told themselves, within perhaps a decade. Instead, a series of catastrophes had soured the nation’s fortunes and slowed meaningful progress to a virtual standstill. The terrorist attacks of 9/11, the ensuing ruinous wars and unbalanced federal budgets, the collapse of the housing-securities bubble, and the onset of the Great Recession all could be said to have played a role, but much of the blame for TPF’s failure was due to territorial infighting between competing communities of astronomers scrabbling for dwindling federal funding.

  “It’s one of the few things I get upset about, because I used to hope that something like the TPFs would happen in my career,” Kasting had confessed when he was halfway through his margarita the night before. “I no longer hope that. I now just hope it happens while I’m still alive, because I want to know the answer. But my personal time is running out, and it seems to be receding further and further into the future. There’s a good chance a TPF-style mission won’t happen until after I’m dead.”

  When he talked about looking for life-bearing exoplanets, Kasting’s words sometimes took on martial tones. He would “fall on his sword” to keep NASA missions like Kepler operating as long as necessary to find Earth-size planets in habitable zones, and would “fight to the end” for bigger, better space telescopes to search for signs of life. The money required for a TPF, he pointed out with rancor, was large on astronomy’s scales but trivial by national and international standards: five or ten billion dollars for the chance to find out if humans were not alone in the universe, equivalent to a few weeks’ worth of war in the Middle East, less than a year of Americans’ expenditures on pets. The astronomers were being kicked around by NASA, and NASA was being kicked around by a monumentally dysfunctional Congress. Not that astronomers were blameless: Kasting took a dim view of senior space scientists who still viewed the exoplanet boom with disdain. Discussing them, Kasting’s once-fiery words suddenly were rimed with frost: “They are mostly old cosmologists, and ten years from now, a lot of them will have died. The young kids all surging into exoplanets now should eventually dominate the decision making. Statistically speaking, the opposition will be buried just by numbers.”

  For Kasting, if the search for habitable exoplanets was not something to die for, it certainly was worth the remainder of his days. And in that calculation, any difference became blurred. He didn’t consciously dwell on his own mortality as he mechanically swam, ran, and lifted weights in his morning workout sessions, but in the back of his mind each stroke, stride, and bench press became an extension of life, a flint-spark struck against the onrushing night, propelling him incrementally further forward in time toward the elusive light of other living worlds. It wasn’t selfishness that drove him, but fear—fear that when faced with the possible discovery of potential signs of life on an alien world, planet hunters would botch the call.

  “I hate to say it, but most astronomers I’ve talked with have shown no evidence that they really know anything at all about planets,” he had told me the previous evening as we finished our dinner. “If I’m still around when we get a potential hit, I can help determine if it’s real, and if I’m not, hopefully my ideas will be.” Kasting had hedged his bet on personal longevity by condensing and dumping his acquired knowledge into an instruction manual that would certainly outlive him: How to Find a Habitable Planet, which was published in 2010 by Princeton University Press.

  Swigging melted ice from his now-empty margarita glass, Kasting excused himself and said it was time for him to get home: it was almost 11:00 p.m., but he planned to hole up in his study to prepare a lesson for an undergraduate class the following day, as well as a presentation for an upcoming meeting of NASA’s Exoplanet Exploration Program Analysis Group, a top-level planning committee he chaired and viewed as perhaps his last chance to steer the agency’s course toward his hoped-for TPF space telescopes.

  Four months later, Kasting would step down from his chairmanship, driven out by critics who said any mission like TPF had slipped too far into the future to be worth seriously considering.

  • • •

  Proposals to look for chemical signs of life on other planets—“biosignatures”—first emerged in the summer of 1965, via two separate papers, both published in the journal Nature, a month apart. Both papers primarily concerned the search for life on Mars. The first was authored by Joshua Lederberg, the Nobel laureate chemist who had mused about the prevalence of extraterrestrial intelligence at Frank Drake’s Green Bank meeting four years earlier. In his paper, Lederberg laid out several guiding principles, among them the idea that life could be detected by its indirect thermodynamic effects upon a planet’s environment. Any conceivable organism must metabolize to survive—that is, must draw energy from and eject waste into its environment to grow, reproduce, and maintain orderly structure. Life on Earth, and presumably all life based on chemicals, drives its metabolism using chemical-energy gradients that chemists call “redox reactions,” in which electrons are transferred between substances. (If a substance gains electrons, it is counterintuitively said to be “reduced.” If a substance loses electrons, a chemist would say it is “oxidized,” even if no oxygen was involved in the reaction, because oxygen is one of the most voracious electron acceptors known. Confusing vagaries of nomenclature such as these are in large part why many science journalists avoid writing about chemistry.) Lederberg noted that metabolic processes, regardless of their biochemistry, should create extreme thermodynamic disequilibrium upon a planet. These global-scale chemical imbalances would be made by organisms locking away energy and vital molecules in biomass and ejecting degraded waste products. He wrote that searchers might generally look for a biosignature of “chemically unstable [molecules] which should reach equilibrium with coexistent oxidant,” a thermodynamic miracle akin to finding an unblemished, flame-licked log at the heart of a roaring bonfire.

  The second paper, by the British scientist James Lovelock, honed Lederberg’s broad assertions into a far sharper criterion for life: a planet’s atmosphere, Lovelock suggested, w
ould be the best target to examine for signs of thermodynamic disequilibrium. In particular, the search should look for “the presence of compounds in the planet’s atmosphere which are incompatible on a long-term basis.” Lovelock cited the Earth’s atmosphere as an example, since oxygen and methane both exist there in chemically implausible concentrations. If left alone in a sealed vessel at room temperature and pressure, oxygen will react with methane to form carbon dioxide and water. Yet in the Earth’s atmosphere, which is composed of just over 20 percent oxygen by volume, methane somehow persists at just under 2 parts per million—the two gases are out of equilibrium by nearly 30 orders of magnitude. The sole explanation for this lingering thermodynamic imbalance is that methane is being constantly replenished.

  Almost all of Earth’s methane comes from our planet’s ancient Archean refugees, the anaerobic methanogens, though a very small fraction is also produced abiotically by hydrothermal vents on the ocean floor. Even without the methane, Earth’s abundant oxygen is by itself far out of equilibrium and exceedingly peculiar, as oxygen prefers to bond with rocks and minerals rather than linger in the air. It clearly must be replenished, too. Our world’s oxygen, of course, primarily comes from photosynthetic bacteria and plants, though, as with methane, small amounts can be produced abiotically—in oxygen’s case, when ultraviolet starlight photolyzes water vapor. Since both oxygen and methane have possible abiotic production routes, the existence of one without the other cannot necessarily be taken as a certain, foolproof biosignature. But when they appear together, their presence constitutes the most potent evidence astrobiologists can recognize for life beyond the solar system, short of a SETI-style radio transmission or a flying saucer landing on the White House lawn.