Space Chronicles: Facing the Ultimate Frontier Read online

  If the resolution of their equipment were high enough, the aliens would see more than just a pale blue dot. They would see intricate coastlines, too, strongly suggesting that the water is liquid. And smart aliens would surely know that if a planet has liquid water, the planet’s temperature and atmospheric pressure fall within a well-determined range.

  Earth’s distinctive polar ice caps, which grow and shrink from the seasonal temperature variations, could also be seen optically. So could our planet’s twenty-four-hour rotation, because recognizable landmasses rotate into view at predictable intervals. The aliens would also see major weather systems come and go; with careful study, they could readily distinguish features related to clouds in the atmosphere from features related to the surface of Earth itself.

  Time for a reality check: We live within ten light-years of the nearest known exoplanet—that is, a planet orbiting a star other than the Sun. Most catalogued exoplanets lie more than a hundred light-years away. Earth’s brightness is less than one-billionth that of the Sun, and our planet’s proximity to the Sun would make it extremely hard for anybody to see Earth directly with an optical telescope. So if aliens have found us, they are likely searching in wavelengths other than visible light—or else their engineers are adapting some other strategy altogether.

  Maybe they do what our own planet hunters typically do: monitor stars to see if they jiggle at regular intervals. A star’s periodic jiggle betrays the existence of an orbiting planet that may otherwise be too dim to see directly. The planet and host star both revolve around their common center of mass. The more massive the planet, the larger the star’s orbit around the center of mass must be, and the more apparent the jiggle when you analyze the star’s light. Unfortunately for planet-hunting aliens, Earth is puny, and so the Sun barely budges, posing a further challenge to alien engineers.

  Radio waves might work, though. Maybe our eavesdropping aliens have something like the Arecibo Observatory in Puerto Rico, home of Earth’s largest single-dish radio telescope—which you might have seen in the early location shots of the 1997 movie Contact, based on a novel by Carl Sagan. If they do, and if they tune to the right frequencies, they’ll certainly notice Earth, one of the “loudest” radio sources in the sky. Consider everything we’ve got that generates radio waves: not only radio itself but also broadcast television, mobile phones, microwave ovens, garage-door openers, car-door unlockers, commercial radar, military radar, and communications satellites. We’re just blazing—spectacular evidence that something unusual is going on here, because in their natural state, small rocky planets emit hardly any radio waves at all.

  So if those alien eavesdroppers turn their own version of a radio telescope in our direction, they might infer that our planet hosts technology. One complication, though: other interpretations are possible. Maybe they wouldn’t be able to distinguish Earth’s signal from those of the larger planets in our solar system, all of which are sizable sources of radio waves. Maybe they would think we’re a new kind of odd, radio-intensive planet. Maybe they wouldn’t be able to distinguish Earth’s radio emissions from those of the Sun, forcing them to conclude that the Sun is a new kind of odd, radio-intensive star.

  Astrophysicists right here on Earth, at the University of Cambridge in England, were similarly stumped back in 1967. While surveying the skies with a radio telescope for any source of strong radio waves, Anthony Hewish and his team discovered something extremely odd: an object pulsing at precise, repeating intervals of slightly more than a second. Jocelyn Bell, a graduate student of Hewish’s at the time, was the first to notice it.

  Soon Bell’s colleagues established that the pulses came from a great distance. The thought that the signal was technological—another culture beaming evidence of its activities across space—was irresistible. As Bell recounted in an after-dinner speech in 1976, “We had no proof that it was an entirely natural radio emission. . . . Here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us.” Within a few days, however, she discovered other repeating signals coming from other places in our galaxy. Bell and her associates realized they’d discovered a new class of cosmic object—pulsing stars—which they cleverly, and sensibly, called pulsars.

  Turns out, intercepting radio waves isn’t the only way to be snoopy. There’s also cosmochemistry. The chemical analysis of planetary atmospheres has become a lively field of modern astrophysics. Cosmochemistry depends on spectroscopy—the analysis of light by means of a spectrometer, which breaks up light, rainbow style, into its component colors. By exploiting the tools and tactics of spectroscopists, cosmochemists can infer the presence of life on an exoplanet, regardless of whether that life has sentience, intelligence, or technology.

  The method works because every element, every molecule—no matter where it exists in the universe—absorbs, emits, reflects, and scatters light in a unique way. Pass that light through a spectrometer, and you’ll find features that can rightly be called chemical fingerprints. The most visible fingerprints are made by the chemicals most excited by the pressure and temperature of their environment. Planetary atmospheres are crammed with such features. And if a planet is teeming with flora and fauna, its atmosphere will be crammed with biomarkers—spectral evidence of life. Whether biogenic (produced by any or all life-forms), anthropogenic (produced by the widespread species Homo sapiens), or technogenic (produced only by technology), this rampant evidence will be hard to conceal.

  Unless they happen to be born with built-in spectroscopic sensors, space-snooping aliens would need to build a spectrometer to read our fingerprints. But above all, Earth would have to eclipse its host star (or some other light source), permitting light to pass through our atmosphere and continue on to the aliens. That way, the chemicals in Earth’s atmosphere could interact with the light, leaving their marks for all to see.

  Some molecules—ammonia, carbon dioxide, water—show up everywhere in the universe, whether life is present or not. But others pop up especially in the presence of life itself. Among the biomarkers in Earth’s atmosphere are ozone-destroying chlorofluorocarbons from aerosol sprays, vapor from mineral solvents, escaped coolants from refrigerators and air conditioners, and smog from the burning of fossil fuels. No other way to read that list: sure signs of the absence of intelligence. Another readily detected biomarker is Earth’s substantial and sustained level of the molecule methane, more than half of which is produced by human-related activities such as fuel-oil production, rice cultivation, sewage, and the burps of domesticated livestock.

  And if the aliens track our nighttime side while we orbit our host star, they might notice a surge of sodium from the sodium-vapor streetlights that switch on at dusk. Most telling, however, would be all our free-floating oxygen, which constitutes a full fifth of our atmosphere.

  Oxygen—the third most abundant element in the cosmos, after hydrogen and helium—is chemically active, bonding readily with atoms of hydrogen, carbon, nitrogen, silicon, sulfur, iron, and so on. Thus, for oxygen to exist in a steady state, something must be liberating it as fast as it’s being consumed. Here on Earth, the liberation is traceable to life. Photosynthesis, carried out by plants and select bacteria, creates free oxygen in the oceans and in the atmosphere. Free oxygen, in turn, enables the existence of oxygen-metabolizing creatures, including us and practically every other creature in the animal kingdom.

  We earthlings already know the significance of Earth’s distinctive chemical fingerprints. But distant aliens who come upon us will have to interpret their findings and test their assumptions. Must the periodic appearance of sodium be technogenic? Free oxygen is surely biogenic. How about methane? It, too, is chemically unstable, and yes, some of it is anthropogenic. The rest comes from bacteria, cows, permafrost, soils, termites, wetlands, and other living and nonliving agents. In fact, at this very moment, astrobiologists are arguing about the exact origin of trace amounts of methane on Ma
rs and the copious quantities of methane detected on Saturn’s moon Titan, where (we presume) cows and termites surely do not dwell.

  If the aliens decide that Earth’s chemical features are strong evidence for life, maybe they’ll wonder if the life is intelligent. Presumably the aliens communicate with one another, and perhaps they’ll presume that other intelligent life-forms communicate too. Maybe that’s when they’ll decide to eavesdrop on Earth with their radio telescopes to see what part of the electromagnetic spectrum its inhabitants have mastered. So, whether the aliens explore with chemistry or with radio waves, they might come to the same conclusion: a planet where there’s advanced technology must be populated with intelligent life-forms, who may occupy themselves discovering how the universe works and how to apply its laws for personal or public gain.

  Our catalogue of exoplanets is growing apace. After all, the known universe harbors a hundred billion galaxies, each with hundreds of billions of stars.

  The search for life drives the search for exoplanets, some of which probably look like Earth—not in detail, of course, but in overall properties. Those are the planets our descendants might want to visit someday, by choice or by necessity. So far, though, nearly all the exoplanets detected by the planet hunters are much larger than Earth. Most are at least as massive as Jupiter, which is more than three hundred times Earth’s mass. Nevertheless, as astrophysicists design hardware that can detect smaller and smaller jiggles of a host star, the ability to find punier and punier planets will grow.

  In spite of our impressive tally, planet hunting by earthlings is still in its horse-and-buggy stage, and only the most basic questions can be answered: Is the thing a planet? How massive is it? How long does it take to orbit its host star? No one knows for sure what all those exoplanets are made of, and only a few of them eclipse their host stars, permitting cosmochemists to peek at their atmospheres.

  But abstract measurements of chemical properties do not feed the imagination of either poets or scientists. Only through images that capture surface detail do our minds transform exoplanets into “worlds.” Those orbs must occupy more than just a few pixels in the family portrait to qualify, and a Web surfer should not need a caption to find the planet in the photo. We have to do better than the pale blue dot.

  Only then will we be able to conjure what a faraway planet looks like when seen from the edge of its own star system—or perhaps from the planet’s surface itself. For that, we will need spaceborne telescopes with stupendous light-gathering power.

  Nope. We’re not there yet. But perhaps the aliens are.

  • • • CHAPTER THREE

  EXTRATERRESTRIAL LIFE*

  The first half-dozen or so confirmed discoveries of planets around stars other than the Sun—dating to the late 1980s and early 1990s—triggered tremendous public interest. Attention was generated not so much by the discovery of exoplanets but by the prospect of their hosting intelligent life. In any case, the media frenzy that followed was somewhat out of proportion to the events.

  Why? Because planets cannot be all that rare in the universe if the Sun happens to have eight of them. Also, the first round of newly discovered planets were all oversize gas giants that resemble Jupiter, which means they have no convenient surface upon which life as we know it could exist. And even if the planets were teeming with buoyant aliens, the odds against these life-forms being intelligent are astronomical.

  Ordinarily, there is no riskier step that a scientist (or anyone) can take than to make a sweeping generalization from just one example. At the moment, life on Earth is the only known life in the universe, but compelling arguments suggest that we are not alone. Indeed, nearly all astrophysicists accept the high probability of life elsewhere. The reasoning is easy: if our solar system is not unusual, then the number of planets in the universe would, for example, outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet in the universe with life would be inexcusably big-headed of us.

  Many generations of thinkers, both religious and scientific, have been led astray by anthropocentric assumptions and simple ignorance. In the absence of dogma and data, it is safer to be guided by the notion that we are not special, which is generally known as the Copernican principle. It was the Polish astronomer Nicolaus Copernicus who, in the mid-1500s, put the Sun back in the middle of our solar system where it belongs. In spite of a third-century B.C. account of a Sun-centered universe (proposed by the Greek philosopher Aristarchus), the Earth-centered universe has been by far the most popular view for most of the past two thousand years. In the West, it was codified by the teachings of Aristotle and Ptolemy and later by the preachings of the Roman Catholic Church. That Earth was the center of all motion was self-evident: it not only looked that way, but God surely made it so.

  The Copernican principle comes with no guarantees that it will guide us correctly for all scientific discoveries yet to come. But it has revealed itself in our humble realization that Earth is not in the center of the solar system, the solar system is not in the center of the Milky Way galaxy, and the Milky Way galaxy is not in the center of the universe. And in case you are one of those people who think that the edge may be a special place, we are not at the edge of anything either.

  A wise contemporary posture would be to assume that life on Earth is not immune to the Copernican principle. How, then, can the appearance or the chemistry of life on Earth provide clues to what life might be like elsewhere in the universe?

  I do not know whether biologists walk around every day awestruck by the diversity of life. I certainly do. On our planet, there coexist (among countless other life-forms) algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to one another in size-place. If you didn’t know better, you would be hard pressed to believe that they all came from the same universe, much less the same planet. And by the way, try describing a snake to somebody who has never seen one: “You gotta believe me! There’s this animal on Earth that (1) can stalk its prey with infrared detectors, (2) can swallow whole, live animals several times bigger than its head, (3) has no arms or legs or any other appendage, and yet (4) can travel along the ground at a speed of two feet per second!”

  Nearly every Hollywood space movie includes some encounter between humans and alien life-forms, whether from Mars or an unknown planet in a faraway galaxy. The astrophysics in these films serves as the ladder to what people really care about: whether we are alone in the universe. If the person seated next to me on a long airplane flight finds out I’m an astrophysicist, nine times out of ten she’ll query me about life in the universe. I know of no other discipline that triggers such consistent enthusiasm from the public.

  Given the diversity of life on Earth, one might expect diversity among Hollywood aliens. But I am consistently amazed by the film industry’s lack of creativity. With a few notable exceptions—such as the life-forms in The Blob (1958) and 2001: A Space Odyssey (1968)—Hollywood’s aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet—and they can walk. Anatomically, these creatures are practically indistinguishable from humans, yet they are supposed to have come from another planet. If anything is certain, it is that life elsewhere in the universe, intelligent or otherwise, will look at least as exotic to us as some of Earth’s own life-forms do.

  Space Tweets #3 & #4

  Just drove by the huge, 30-ft tall L-A-X letters near the airport – surely visible from orbit. Is LA an alien space port?

  Jan 23, 2010 9:06 AM

  Last day in LA. Like the big LAX letters at airport, the HOLLYWOOD sign is huge. Visible from space? Must be where aliens land

  Jan 28, 2010 2:16 PM

  The chemical composition of Earth-based life is primarily derived from a select few ingredients. The elements hydrogen, oxygen,
and carbon account for more than 95 percent of the atoms in the human body and in all other known life. Of the three, it is carbon whose chemical structure allows it to bond most readily and strongly with itself and with many other elements in many different ways—which is why we say life on Earth is carbon-based, and why the study of molecules that contain carbon is generally known as “organic” chemistry. Curiously, the study of life elsewhere in the universe is known as exobiology, one of the few disciplines that attempt to function, at least for now, in the complete absence of firsthand data.

  Is life chemically special? The Copernican principle suggests that it probably isn’t. Aliens need not look like us to resemble us in more fundamental ways. Consider that the four most common elements in the universe are hydrogen, helium, carbon, and oxygen. Helium is inert. So the three most abundant, chemically active ingredients in the cosmos are also the top three ingredients of life on Earth. For this reason, you can bet that if life is found on another planet, it will be made of a similar mix of elements. Conversely, if life on Earth were composed primarily of manganese and molybdenum, then we would have excellent reason to suspect we’re something special in the universe.

  Appealing once again to the Copernican principle, we can assume that an alien organism is not likely to be ridiculously large compared with life as we know it. There are cogent structural reasons why you would not expect to find a life-form the size of the Empire State Building strutting around a planet. Even if we ignore the engineering limitations of biological matter, we approach another, more fundamental limit. If we assume that an alien has control of its own appendages, or more generally, if we assume the organism functions coherently as a system, then its size would ultimately be constrained by its ability to send signals within itself at the speed of light—the fastest allowable speed in the universe. For an admittedly extreme example, if an organism were as big as the orbit of Neptune (about ten light-hours across), and if it wanted to scratch its head, then this simple act would take no less than ten hours to accomplish. Subslothlike behavior such as this would be evolutionarily self-limiting, because the time since the beginning of the universe might well be insufficient for the creature to have evolved from smaller forms.