Space Chronicles: Facing the Ultimate Frontier (5 page)

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CHAPTER TWO

EXOPLANET EARTH
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W
hether you prefer to crawl, sprint, swim, or walk from one place to another, you can enjoy close-up views of Earth’s inexhaustible supply of things to notice. You might see a vein of pink limestone on the wall of a canyon, a ladybug eating an aphid on the stem of a rose, a clamshell poking out of the sand. All you have to do is look.

Board a jetliner crossing a continent, though, and those surface details soon disappear. No aphid appetizers. No curious clams. Reach cruising altitude, around seven miles up, and identifying major roadways becomes a challenge.

Detail continues to vanish as you ascend to space. From the window of the International Space Station, which orbits at about 225 miles up, you might find London, Los Angeles, New York, or Paris in the daytime, not because you can see them but because you learned where they are in geography class. At night their brilliant city lights present only the faintest glow. By day, contrary to common wisdom, with the unaided eye you probably won’t see the pyramids at Giza, and you certainly won’t see the Great Wall of China. Their obscurity is partly the result of having been made from the soil and stone of the surrounding landscape. And although the Great Wall is thousands of miles long, it’s only about twenty feet wide—much narrower than the US interstate highways you can barely see from a transcontinental jet.

Indeed, from Earth orbit—apart from the smoke plumes rising from the oil-field fires in Kuwait at the end of the first Gulf War in 1991, and the green-brown borders between swaths of irrigated and arid land—the unaided eye cannot see much else that’s made by humans. Plenty of natural scenery is visible, though: hurricanes in the Gulf of Mexico, ice floes in the North Atlantic, volcanic eruptions wherever they occur.

From the Moon, a quarter-million miles away, New York, Paris, and the rest of Earth’s urban glitter don’t even show up as a twinkle. But from your lunar vantage you can still watch major weather fronts move across the planet. Viewed from Mars at its closest, some thirty-five million miles away, massive snow-capped mountain chains and the edges of Earth’s continents would be visible through a good backyard telescope. Travel out to Neptune, 2.7 billion miles away—just down the block on a cosmic scale—and the Sun itself becomes embarrassingly dim, now occupying a thousandth the area on the daytime sky that it occupies when seen from Earth. And what of Earth itself? A speck no brighter than a dim star, all but lost in the glare of the Sun.

A
celebrated photograph taken in 1990 from the edge of the solar system by the Voyager 1 spacecraft shows how underwhelming Earth looks from deep space: a “pale blue dot,” as the American astronomer Carl Sagan called it. And that’s generous. Without the help of a picture caption, you might not find it at all.

What would happen if some big-brained aliens from the great beyond scanned the skies with their naturally superb visual organs, further aided by alien state-of-the-art optical accessories? What visible features of planet Earth might they detect?

Blueness would be first and foremost. Water covers more than two-thirds of Earth’s surface; the Pacific Ocean alone makes up an entire side of the planet. Any beings with enough equipment and expertise to detect our planet’s color would surely infer the presence of water, the third most abundant molecule in the universe.

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.

T
ime 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.

S
o 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.

T
urns 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.

O
xygen—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 Mars and the copious quantities of methane detected on Saturn’s moon Titan, where (we presume) cows and termites surely do not dwell.