by Lee Billings
Laughlin shot a smiling glance my way. “But we do have an option. We can move the Earth.”
A pregnant pause. “Move the Earth?”
“Sure.”
“Like, tow it out of the way?”
“Essentially, yeah. We have more than enough time. Just take some large comets or asteroids from the Kuiper Belt and use them to tap and transfer some of Jupiter’s orbital energy and angular momentum to the Earth over a timescale of hundreds of millions of years. Each time one flew by the Earth, you’d get a small kick, and you’d expand the Earth’s orbit very gradually through those repeated close encounters. You’d need on order of a million close passes, one every several thousand years, but you could move the Earth’s orbit out, close to where Mars is now. This is an idea I worked out ten years back with a couple of friends.”
Another pause, as Ricketts pondered Laughlin’s outlandish proposal. “That’s cool. So it’s a cost-benefit analysis: What would it cost us to tow the Earth out of the way, versus what’s at stake on the benefit side?”
“You might destabilize and lose Earth’s Moon,” Laughlin said. “And you would have to be extremely careful to properly time each flyby so your object didn’t collide with and sterilize the planet down to bacteria. But that intervention could net you billions more years for the biosphere, which is a lot of economic utility, and the cost is very small in comparison, because most of the energy to move the Earth is actually coming from Jupiter, transferred by the comet. You just have to have very subtle control of the comet’s trajectory when it’s way out at the slow, far point of its orbit in the outer solar system. It’s a matter of finesse more than brute force, but it’s just rocket science. The point is, we could start doing this practically today, if we wanted.”
“I’m still hung up on the value being infinite,” said Ricketts, sounding skeptical. “It’s all life on the planet. We know extinction is worth a lot of money to avoid without needing to quantify just how worthwhile it is.”
“But there are hierarchies of infinity,” I interjected. “Some are greater than others. The value of a commodity increases based on its scarcity, right? We still don’t know how common Earth-like planets might be. Not just Earth-size balls of rock, but planets with water, weather, and living things. Maybe they’re common out there, and life is cheap. But what if we end up taking the galactic census only to find that around the nearest five hundred or thousand stars there are simply—”
“No Earths,” Laughlin finished, nodding. “We’ll have to wait and see.”
• • •
Debates over our planet’s place in the universe and the cosmic worth of our world stretch back into time immemorial, to prehistoric speculations about the relationship between the heavens and the Earth that left their imprints on ancient myths and legends. By comparison, the oldest recorded scientific explanations for our cosmic context are quite young, though they still trace back some twenty-five centuries, to the towns and cities of Ionian Greece, scattered along the shores of the Aegean Sea.
In the sixth-century-B.C city of Miletus, in what is now southwestern Turkey, there lived an Ionian philosopher named Thales. Thales was the scion of a noble Phoenician family, and in his youth spent time in Egypt, where he learned geometry and studied ancient astronomical records. He was known throughout the ancient world for predicting a total solar eclipse that occurred over Central Anatolia on May 28, 585 B.C., but his greatest legacy is what we now call the “scientific method.” Thales rejected the supernatural, teaching instead that rational thought and experimentation were the proper approach to making sense of the world. Thales believed everything in existence to be composed of one or more primeval substances and controlled by interacting forces—a belief that, in its essence, would be shared by any of today’s particle physicists.
It was Thales’s Miletian associate, Anaximander, who used these ideas to form a mechanistic explanation of the heavens. Anaximander believed that the universe was limitless and eternal. Beyond our realm, he said, far past where we could ever hope to see, other worlds endlessly formed and disintegrated within the boundless depths of an infinite void. Yet he also suggested that the Earth was at the center of our visible universe. Anaximander’s Earth was a cylinder or disk fixed at a central point and surrounded by concentric shells that contained the fiery Sun, Moon, and stars. Pythagoras, Thales and Anaximander’s young Ionian contemporary, thought instead that the Earth was a globe floating in space. He expanded Anaximander’s model to include additional concentric shells for the planets, and held that the Sun, Moon, and planets orbited in perfect, harmonious circles about the Earth. If Thales and his disciples were the world’s first true scientists, then Pythagoras and his followers were the world’s first pure mathematicians: Pythagoras espoused the principle that numbers, whole and ideal, constituted the deepest reality, and that reality was best investigated not through the senses but through thought alone. The Pythagoreans’ preference for mysticism and metaphysics was destined to prevail over Thales’s empiricism to profoundly influence the philosophies of Plato and Aristotle some two centuries later.
Plato taught that the terrestrial world was made of four primal elements—earth, fire, air, and water—with a fifth element, the aether, forming the heavens. Aristotle incorporated some of these ideas into a larger cosmology, by stating that earth and water, being heavier than fire and air, fell and settled upon a central point, which became the Earth. There could be no place like our own elsewhere in the heavens, because the heavens were made of an entirely different substance, and were perfect and unchanging. In the Platonic and Aristotelian reckonings, the Earth was privileged, corrupted, singular, and profoundly alone—a view that would dominate and stifle much scientific inquiry within Western civilization for nearly two thousand years.
It all could have turned out quite differently. In Plato’s time, the staunchest defender of Thales’s materialistic philosophy was an Ionian Greek named Democritus. For Democritus, the universe was built not from mystical numbers and geometric forms, but from infinitesimal physical particles moving eternally through the infinite void. Democritus called the particles atoms, the Greek word for “indivisible.” Atoms and void, Democritus argued, were all that existed, and were thus the source of all things—including living beings and their thoughts and sensory perceptions. In a universe infinite in space and time, he said, the endless dance of atoms would inevitably lead to countless other worlds and other lives, all in an eternal process of growth and decay. Not all worlds would be like ours—some would be too inhospitable for life, and others would be even more bountiful than Earth. We should be universally cheerful, Democritus believed, at our fortune to exist in a welcoming world with so many pleasures. His constant mirth at humanity’s tragicomic existence led his contemporaries to call him “the laughing philosopher.”
Looking up at the dark Aegean sky, Democritus speculated that the stars, like everything else, were not made of a special celestial substance, but of atoms. They were simply suns, much farther away than our own, some so distant that in aggregate they formed the Milky Way’s pale glow. Almost a century after Democritus’s death, the idea of stars as distant suns reemerged in the work of another Greek astronomer, Aristarchus, who proposed that the Sun, rather than the Earth, was the center of our planetary system. By studying the size of the Earth’s shadow upon the Moon during a lunar eclipse, Aristarchus surmised that the Sun was very much larger than our world, and felt it only natural that a smaller body should orbit a larger one. Aristarchus further realized that his theory suggested the “fixed” stars were far more distant than most anyone had previously believed, based on a measurement called parallax. Parallax is the apparent motion of an object when viewed from a baseline defined by two separate points. You can see parallax very easily, by holding your finger in front of your face and gazing at it first with your left eye, then with your right, using your eyes as your baseline. Parallax tells you how far away an object is—the closer your finger to your face,
the greater its parallax. Increasing the length of your baseline also increases an object’s parallax—think of the apparent shift in the position of a lamp when viewed from opposite edges of one wall in a room. The fact that Aristarchus could discern no parallax shift among the stars when he viewed the night sky from opposite sides of Earth’s orbit round the Sun told him that they were very far away indeed—light-years away, in fact, though that term had yet to be coined.
Aristarchus was accused of impiety for demoting Earth from its central place in the heavens; Plato reputedly so despised the ideas of Democritus that he wished for all the laughing philosopher’s works to be burned. Ultimately, time alone would suppress what Plato could not, and for thousands of years the atoms of Democritus and the stars of Aristarchus would be largely forgotten. Only one minor work of Aristarchus has survived into the present day. None of Democritus’s writings have passed down to us; we know of him through the writings of those he influenced, such as the later ancient Greek philosopher Epicurus. Epicurus’s work, too, is mostly lost. A good portion of what we do know of his philosophy comes from a single book-length poem, De Rerum Natura (On the Nature of Things), written in Latin hexameter around 50 B.C. by the Roman scholar Lucretius. In the poem, Lucretius praised and summarized Epicurean thought—including the idea of atoms, an infinite universe, and the inevitability of other living worlds. It did not stand to reason, Lucretius wrote, that “this was the only world and heavens created, and that beyond it those many bodies of matter do nothing at all.” Atoms, numberless in the totality of infinite space, would on occasion coalesce to “become the beginnings of great things: of the world, sea, sky, and the race of living creatures. . . . It must be admitted that there are other worlds in other regions, as well as different races of men and breeds of wild beasts. . . . In the sum total of things, there is nothing singular, which is born unique and grows unique and alone; it instead belongs to some class, and there are very many of the same kind.”
Lucretius’s poem would have perhaps been lost as well, if not for a single worm-eaten copy that was found by an Italian book collector in the dusty depths of a German monastery in 1417. Soon after the discovery, the poem was translated into modern languages, printed on Gutenberg presses, and distributed throughout Europe, where it and other ancient rediscovered works helped foment the scientific revival of the European Renaissance. It would be well more than a century, however, before the renewed idea of Earth’s modest place in an infinitude of worlds would make its greatest impact. The revolution began with the 1543 publication of De Revolutionibus Orbium Coelestium (On the Revolutions of Heavenly Orbs) by the Polish cleric Nicolaus Copernicus, laying out a heliocentric model of the solar system. The work consumed the last thirty years of Copernicus’s life, and he received the first printed copy while on his deathbed. Copernicus, like Aristarchus nearly two millennia before him, had shown that the apparent motions of the planets in the sky could be more elegantly explained if the planets were moving around the Sun rather than the Earth.
In 1610, the Italian astronomer Galileo Galilei confirmed the Copernican model after turning the newly invented telescope to the heavens. He glimpsed sunspots on our star, and from their gradual motion deduced that the Sun, like the Earth, was spinning. Jupiter, he found, was circled by a number of smaller moons, confirming that smaller bodies do indeed orbit larger ones, and that not everything orbits only the Earth. When he gazed at Venus over the course of a year, he saw the planet swing through a full set of phases, just like the waning and waxing Moon—evidence that Venus passed both in front of and behind the illuminating Sun. This was the crucial evidence for heliocentrism, as geocentric models predicted that Venus, closer to the Sun but still circling Earth, would consequently always be backlit by the Sun’s radiance and would display only a crescent phase as viewed from our world. Yet the Copernican model was still imperfect: it failed to replicate the exact motions of the planets in the sky. In making his model, Copernicus had implicitly relied upon the old Pythagorean notion, extended by Plato, Aristotle, and others, that planets moved in perfect circles.
Around the same time that Galileo began using his telescope, a German astronomer, Johannes Kepler, announced a discovery that would mark the true beginning of modern astronomy, one that ironically came to him while he worked to create a table for casting more accurate horoscopes. Kepler had been grappling with the orbit of Mars, trying to integrate historical records of the planet’s motions into Copernicus’s heliocentric model. He had painstakingly considered circular orbits, even spiraling orbits, but his results did not align with observations. At last, on a despondent whim, he decided to treat Mars as if it moved in a squashed, elongated oval, an ellipse—an idea so basic that Kepler assumed it had already been investigated by prior generations of astronomers. To his great surprise, the results of his calculations beautifully mirrored observations. He subsequently confirmed that the orbits of the other known planets were elliptical as well, and he went on to use the revelation of elliptical orbits to codify his three laws of planetary motion.
The first law simply stated that each planet moves in an ellipse, with the Sun at one focus. The second law was that planetary orbits sweep out equal areas over equal times. This means that when a planet’s orbit brings it closest to a star, the planet must move faster than it does at the opposite end of its orbit in order to sweep out the same area over the same period of time. The third law stated that the square of a planet’s orbital period is directly proportional to the cube of its average orbital separation from the Sun, establishing a clear relationship between the length of a world’s year and its distance from its star. This explained why Mercury, closest to the Sun, raced across Earth’s skies, and why Jupiter and Saturn, much farther out, moved so sedately. Kepler’s third law allowed him to estimate the proportional distances of the planets: he determined, for instance, that Mars was one and a half times the distance from the Sun that our Earth was, and that Jupiter was more than five times as far, though the actual distance from the Earth to the Sun remained unknown.
The importance of Kepler’s findings cannot be overstated. Near the end of the 1600s, Isaac Newton would use Kepler’s laws to derive the universal laws of gravity. Today, knowledge of Kepler’s laws is what allows mission planners to chart the courses for interplanetary spacecraft, and is how planet hunters determine whether an exoplanet resides in its star’s habitable zone based on the world’s orbital period alone. Kepler in a sense unified the heavens and the Earth, showing without a doubt that they existed within one framework dictated by the same physical laws. He also gave solid theoretical heft to what became the crux of Copernican thought: that, all things being equal, the Earth, and by extension the solar system, could not be assumed to be singular, unusual, or in any way privileged, but rather should be presumed common, mediocre, and average—at least until evidence proved otherwise. This “Copernican Principle” or “principle of mediocrity” has quietly guided physics, cosmology, astronomy, and planetary science ever since, though not always in the right direction. Galileo, viewing the Moon’s patchy, cratered terrain through his telescope, declared it to be a world like ours of lands and seas. Kepler even speculated that the Moon was inhabited, and thought some of its more intelligent creatures might have excavated the circular lunar craters to house their cities. Whether in dreams of jungled, primitive Venus or mirages of an advanced, dying civilization building canals on dried-up Mars, for centuries it was common for learned and reputable scientists to vocally profess that most and perhaps even all worlds were habitable and inhabited.
In 1627, while using his new laws to calculate the future motions of Venus, Kepler surmised that the planet would occasionally cross the face of the Sun as viewed from Earth. He calculated that the next transit of Venus would occur over the course of several hours on December 6, 1631, and that other than a near miss for a transit in late 1639, Venus would not cross the Sun’s face again until some time in 1761. Kepler hoped to witness the 1631 transit, but died
in 1630. The transit of 1631 came and went, apparently unseen. In 1639, scarcely a month before the near miss Kepler had calculated, the young British astronomer Jeremiah Horrocks discovered an error in Kepler’s calculations—Venusian transits actually occurred in pairs separated by 8 years; the interval between each pair oscillated between 121 years and 105 years. Horrocks calculated that on the afternoon of December 4, 1639, the transit of Venus could be seen from his home in northern England. He and a friend, William Crabtree, rushed to plan their observations. On the fated day both men watched an event no human eyes had ever before seen, as the silhouette of Venus, one-thirtieth the apparent diameter of the Sun, glided across the blazing star. They were the only two souls on Earth to witness the 1639 transit. Horrocks’s correction of Kepler’s calculations set the timing for transits in future years. A pair would occur in 1761 and 1769, then in 1874 and 1882, then in the far-off years of 2004 and 2012, continuing on and on in what was thought to be an endless cycle.
Writing in the Proceedings of the Royal Society in 1716, the English astronomer Edmond Halley suggested how Venusian transits could provide an absolute Earthly reference point against which the rest of the universe could be measured. When viewed from different places on Earth, Halley wrote, the path of Venus across the Sun would shift slightly, also shifting the transit’s duration. By precisely timing the transit to distinguish the shift between two widely separated locations, it would be possible to triangulate the distance between the Earth and the Sun. From there, simple math would yield the Sun’s true size and each planet’s orbital distance, revealing the physical breadth of the solar system. In the years leading up to the next transit, that of 1761, states across Europe organized more than a hundred teams to travel to the world’s far corners to attempt Halley’s proposed measurements. It was the first-ever flowering of international, state-sponsored science, and it was a spectacular failure. Astronomers hauled delicate equipment by ship, sled, and horseback into wild areas where the transit would be viewable, often only to find their cargo shattered and warped beyond repair at their destination. Wars, disease, and poor weather scuttled many attempts well before the transit actually occurred. The measurements that did trickle back from far-flung expeditions were too inaccurate and contradictory to be useful.
