فصل 11

11.

Exoplanet Earth

Whether you prefer to sprint, swim, walk, or crawl from one place to another on Earth, you can enjoy close-up views of our planet’s unlimited 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 from the sand. All you have to do is look.

From the window of an ascending jetliner, those surface details rapidly 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 rise into space. From the window of the International Space Station, which orbits at about 250 miles up, you might find Paris, London, New York, and Los Angeles in the daytime, but only because you learned where they are in geography class. At night, their sprawling cityscapes present an obvious glow. By day, contrary to common wisdom, you probably won’t see the Great 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 U.S. interstate highways you can barely see from a transcontinental jet.

From orbit, with the unaided eye, you would have seen smoke plumes rising from the oil-field fires in Kuwait at the end of the first Persian Gulf War in 1991 and smoke from the burning World Trade Center towers in New York City on September 11, 2001. You will also notice the green–brown boundaries between swaths of irrigated and arid land. Beyond that shortlist, there’s not much else made by humans that’s identifiable from hundreds of miles up in the sky. You can see plenty of natural scenery, though, including hurricanes in the Gulf of Mexico, ice floes in the North Atlantic, and volcanic eruptions wherever they occur.

From the Moon, a quarter million miles away, New York, Paris, and the rest of Earth’s urban glitter doesn’t even show up as a twinkle. But from your lunar vantage you can still watch major weather fronts move across the planet. 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 large backyard telescope. Travel out to Neptune, three billion miles away—just down the block on a cosmic scale—and the Sun itself becomes a thousand times dimmer, now occupying a thousandth the area on the daytime sky that it occupies when seen from Earth. And what of Earth itself? It’s a speck no brighter than a dim star, all but lost in the glare of the Sun.

A celebrated photograph taken in 1990 from just beyond Neptune’s orbit by the Voyager 1 spacecraft shows just how underwhelming Earth looks from deep space: a “pale blue dot,” as the American astrophysicist Carl Sagan called it. And that’s generous. Without the help of a caption, you might not even know it’s there.

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 spans nearly 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, then 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 using visible light. So could our planet’s twenty-four-hour rotation, because recognizable landmasses rotate into view at predictable intervals of time. 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. The nearest exoplanet—the nearest planet in orbit around a star that is not the Sun—can be found in our neighbor star system Alpha Centauri, about four light-years from us and visible mostly from our southern hemisphere. Most of the cataloged exoplanets lie from dozens up to hundreds of 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 a visible light telescope. It’s like trying to detect the light of a firefly in the vicinity of a Hollywood searchlight. So if aliens have found us, they are likely looking in wavelengths other than visible light, like infrared, where our brightness relative to the Sun is a bit better than in visible light—or else their engineers are adapting some other strategy altogether.

Maybe they’re doing what some of our own planet-hunters typically do: monitoring stars to see if they jiggle at regular intervals. A star’s periodic jiggle betrays the existence of an orbiting planet that may be too dim to see directly. Contrary to what most people suppose, a planet does not orbit its host star. Instead, both the planet and its host star revolve around their common center of mass. The more massive the planet, the larger the star’s response must be, and the more measurable the jiggle gets when you analyze the star’s light. Unfortunately for planet-hunting aliens, Earth is puny, so the Sun barely budges, which would further challenge alien engineers.

NASA’s Kepler telescope, designed and tuned to discover Earth-like planets around Sun-like stars, invoked yet another method of detection, mightily adding to the exoplanet catalog. Kepler searched for stars whose total brightness drops slightly, and at regular intervals. In these cases, Kepler’s line of sight is just right to see a star get dimmer, by a tiny fraction, due to one of its own planets crossing directly in front of the host star. With this method, you can’t see the planet itself. You can’t even see any features on the star’s surface. Kepler simply tracked changes in a star’s total light, but added thousands of exoplanets to the catalog, including hundreds of multiplanet star systems. From these data, you also learn the size of the exoplanet, its orbital period, and its orbital distance from the host star. You can also make an educated inference on the planet’s mass.

If you’re wondering, when Earth passes in front of the Sun—which is always happening for some line of sight in the galaxy—we block 1/10,000th of the Sun’s surface, thereby briefly dimming the Sun’s total light by 1/10,000th of its normal brightness. Fine as it goes. They’ll discover that Earth exists, but learn nothing about happenings on Earth’s surface.

Radio waves and microwaves might work. Maybe our eavesdropping aliens have something like the 500-meter radio telescope in the Guizhou province of China. If they do, and if they tune to the right frequencies, they’ll certainly notice Earth—or rather, they’ll notice our modern civilization as one of the most luminous sources in the sky. Consider everything we’ve got that generates radio waves and microwaves: not only traditional 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 ablaze in long-frequency waves—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 signals from those of the larger planets in our solar system, all of which are sizable sources of radio waves, especially Jupiter. Maybe they’d think we were 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, Antony 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 recounts, “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 Milky Way galaxy. Bell and her associates realized they’d discovered a new class of cosmic object—a star made entirely of neutrons that pulses with radio waves for every rotation it executes. Hewish and Bell sensibly named them “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. As you might guess, cosmochemistry depends on spectroscopy—the analysis of light by means of a spectrometer. 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. And as already discussed, 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 rich with such features. And if a planet is teeming with flora and fauna, its atmosphere will be rich 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), such rampant evidence will be hard to conceal.

Unless they happen to be born with built-in spectroscopic sensors, our space-snooping aliens would need to build a spectrometer to read our fingerprints. But above all, Earth would have to cross in front of the Sun (or some other 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 abundantly in the universe, whether life is present or not. But other molecules thrive in the presence of life itself. Another readily detected biomarker is Earth’s sustained level of the molecule methane, two-thirds of which is produced by human-related activities such as fuel oil production, rice cultivation, sewage, and the burps and farts of domestic livestock. Natural sources, comprising the remaining third, include decomposing vegetation in wetlands and termite effluences. Meanwhile, in places where free oxygen is scarce, methane does not always require life to form. At this very moment, astrobiologists are arguing over the exact origin of trace methane on Mars and the copious quantities of methane on Saturn’s moon Titan, where cows and termites we presume do not dwell.

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

Oxygen—which, after hydrogen and helium, is the third most abundant element in the cosmos—is chemically active and bonds readily with atoms of hydrogen, carbon, nitrogen, silicon, sulfur, iron, and so on. It even bonds with itself. 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 many bacteria, creates free oxygen in the oceans and in the atmosphere. Free oxygen, in turn, enables the existence of oxygen-metabolizing life, including us and practically every other creature in the animal kingdom.

We Earthlings already know the significance of our planet’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, but as we’ve seen, methane has nonliving agents as well.

If the aliens decide that Earth’s chemical features are sure evidence of 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.

Looking more closely at Earth’s atmospheric fingerprints, human biomarkers will also include sulfuric, carbonic, and nitric acids, and other components of smog from the burning of fossil fuels. If the curious aliens happen to be socially, culturally, and technologically more advanced than we are, then they will surely interpret these biomarkers as convincing evidence for the absence of intelligent life on Earth.

The first exoplanet was discovered in 1995, and, as of this writing, the tally is rising through three thousand, most found in a small pocket of the Milky Way around the solar system. So there’s plenty more where they came from. After all, our galaxy contains more than a hundred billion stars, and the known universe harbors some hundred billion galaxies.

Our search for life in the universe drives the search for exoplanets, some of which resemble Earth—not in detail, of course, but in overall properties. Latest estimates, extrapolating from the current catalogs, suggests as many as forty billion Earth-like planets in the Milky Way alone. Those are the planets our descendants might want to visit someday, by choice, if not by necessity.