SEVEN
PLANET PARADE
In the study of the cosmos, it’s hard to come up with a better tale than the centuries-long history of attempts to understand the planets—those sky wanderers that make their rounds against the backdrop of stars. Of the eight objects in our solar system that are indisputably planets, five are readily visible to the unaided eye and were known to the ancients, as well as observant troglodytes. Each of the five—Mercury, Venus, Mars, Jupiter, and Saturn—was endowed with the personality of the god for which it was named. For example, Mercury, which moves the fastest against the background stars, was named for the Roman messenger god—the fellow usually depicted with small and aerodynamically useless wings on his heels or his hat. And Mars, the only one of the classic wanderers (the Greek word planete means “wanderer”) with a reddish hue, was named for the Roman god of war and bloodshed. Earth, of course, is also visible to the unaided eye. Just look down. But terra firma was not identified as one of the gang of planets until after 1543, when Nicolaus Copernicus advanced his Sun-centered model of the universe.
To the telescopically challenged, the planets were, and are, just points of light that happen to move across the sky. Not until the seventeenth century, with the proliferation of telescopes, did astronomers discover that planets were orbs. Not until the twentieth century were the planets scrutinized at close range with space probes. And not until later in the twenty-first century will people be likely to visit them.
Humanity had its first telescopic encounter with the celestial wanderers during the winter of 1609–10. After merely hearing of the 1608 Dutch invention, Galileo Galilei manufactured an excellent telescope of his own design, through which he saw the planets as orbs, perhaps even other worlds. One of them, brilliant Venus, went through phases just like the Moon’s: crescent Venus, gibbous Venus, full Venus. Another planet, Jupiter, had moons all of its own, and Galileo discovered the four largest: Ganymede, Callisto, Io, and Europa, all named for assorted characters in the life and times of Jupiter’s Greek counterpart, Zeus.
The simplest way to explain the phases of Venus, as well as other features of its motion on the sky, was to assert that the planets revolve around the Sun, not Earth. Indeed, Galileo’s observations strongly supported the universe as envisioned and theorized by Copernicus.
Jupiter’s moons took the Copernican universe a step further: although Galileo’s 20-power telescope could not resolve the moons into anything larger than pinpoints of light, no one had ever seen a celestial object revolve around anything other than Earth. An honest, simple observation of the cosmos, except that the Roman Catholic Church and “common” sense would have none of it. Galileo discovered with his telescope a contradiction to the dogma that Earth occupied the central position in the cosmos—the spot around which all objects revolve. Galileo reported his persuasive findings in early 1610, in a short but seminal work he titled Sidereus Nuncius (“the Starry Messenger”).
ONCE THE COPERNICAN model became widely accepted, the arrangement of the heavens could legitimately be called a solar system, and Earth could take its proper place as one among six known planets. Nobody imagined there could be more than six. Not even the English astronomer Sir William Herschel, who discovered a seventh in 1781.
Actually, the credit for the first recorded sighting of the seventh planet goes to the English astronomer John Flamsteed, the first British Astronomer Royal. But in 1690, when Flamsteed noted the object, he didn’t see it move. He assumed it was just another star in the sky, and named it 34 Tauri. When Herschel saw Flamsteed’s “star” drift against the background stars, he announced—operating under the unwitting assumption that planets were not on the list of things one might discover—that he had discovered a comet. Comets, after all, were known to move and to be discoverable. Herschel planned to call the newfound object Georgium Sidus (“Star of George”), after his benefactor, King George III of England. If the astronomical community had respected these wishes, the roster of our solar system would now include Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George. In a blow to sycophancy the object was ultimately called Uranus, in keeping with its classically named brethren—though some French and American astronomers kept calling it “Herschel’s planet” until 1850, several years after the eighth planet, Neptune, was discovered.
Over time, telescopes kept getting bigger and sharper, but the detail that astronomers could discern on the planets did not much improve. Because every telescope, no matter the size, viewed the planets through Earth’s turbulent atmosphere, the best pictures were still a bit fuzzy. But that didn’t keep intrepid observers from discovering things like Jupiter’s Great Red Spot, Saturn’s rings, Martian polar ice caps, and dozens of planetary moons. Still, our knowledge of the planets was meager, and where ignorance lurks, so too do the frontiers of discovery and imagination.
CONSIDER THE CASE of Percival Lowell, the highly imaginative and wealthy American businessman and astronomer, whose endeavors took place at the end of the nineteenth century and the early years of the twentieth. Lowell’s name is forever linked with the “canals” of Mars, the “spokes” of Venus, the search for Planet X, and of course the Lowell Observatory in Flagstaff, Arizona. Like so many investigators around the world, Lowell picked up on the late-nineteenth-century proposition by the Italian astronomer Giovanni Schiaparelli that linear markings visible on the Martian surface were canali.
The problem was that the word means “channels,” but Lowell chose to translate the word badly as “canals” because the markings were thought to be similar in scale to the major public-works projects on Earth. Lowell’s imagination ran amok, and he dedicated himself to the observation and mapping of the Red Planet’s network of waterways, surely (or so he fervently believed) constructed by advanced Martians. He believed that the Martian cities, having exhausted their local water supply, needed to dig canals to transport water from the planet’s well-known polar ice caps to the more populous equatorial zones. The story was appealing, and it helped generate plenty of vivid writing.
Lowell was also fascinated by Venus, whose ever-present and highly reflective clouds make it one of the brightest objects in the night sky. Venus orbits relatively near the Sun, so as soon as the Sun sets—or just before the Sun rises—there’s Venus, hanging gloriously in the twilight. And because the twilight sky can be quite colorful, there’s no end of 9-1-1 calls reporting a glowing, light-adorned UFO hovering on the horizon.
Lowell maintained that Venus sported a network of massive, mostly radial spokes (more canali) emanating from a central hub. The spokes he saw remained a puzzle. In fact nobody could ever confirm what he saw on either Mars or Venus. This didn’t much bother other astronomers because everyone knew that Lowell’s mountaintop observatory was one of the finest in the world. So if you weren’t seeing Martian activity the way Percival was, it was surely because your telescope and your mountain were not as good as his.
Of course, even after telescopes got better, nobody could duplicate Lowell’s findings. And the episode is today remembered as one where the urge to believe undermined the need to obtain accurate and responsible data. And curiously, it was not until the twenty-first century that anybody could explain what was going on at the Lowell Observatory.
An optometrist from Saint Paul, Minnesota, named Sherman Schultz wrote a letter in response to an article in the July 2002 issue of Sky and Telescope magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients’ eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell’s own retina by his ocular blood vessels. When you compare Lowell’s diagram of the spokes with a diagram of the eye, the two match up, canal for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension—which shows up clearly in the vessels of the eyeballs—with his will to believe, it’s no surpri
se that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants.
Alas, Lowell fared only slightly better with his search for Planet X, a planet thought to lie beyond Neptune. Planet X does not exist, as the astronomer E. Myles Standish Jr. demonstrated decisively in the mid-1990s. But Pluto, discovered at the Lowell Observatory in February 1930, some 13 years after Lowell’s death, did serve as a fair approximation for a while. Within weeks of the observatory’s big announcement, though, some astronomers had begun debating whether it should be classified as the ninth planet. Given our decision to display Pluto as a comet rather than as a planet in the Rose Center for Earth and Space, I’ve become an unwitting part of that debate myself, and I can assure you, it hasn’t let up yet. Asteroid, planetoid, planetesimal, large planetesimal, icy planetesimal, minor planet, dwarf planet, giant comet, Kuiper Belt object, trans-Neptunian object, methane snowball, Mickey’s dim-witted bloodhound—anything but number nine, we naysayers argue. Pluto is just too small, too lightweight, too icy, too eccentric in its orbit, too misbehaved. And by the way, we say the same about the recent high-profile contenders including the three or four objects discovered beyond Pluto that rival Pluto in size and in table manners.
TIME AND TECHNOLOGY moved on. Come the 1950s, radio-wave observations and better photography revealed fascinating facts about the planets. By the 1960s, people and robots had left Earth to take family photos of the planets. And with each new fact and photograph the curtain of ignorance lifted a bit higher.
Venus, named after the goddess of beauty and love, turns out to have a thick, almost opaque atmosphere, made up mostly of carbon dioxide, bearing down at nearly 100 times the sea level pressure on Earth. Worse yet, the surface air temperature nears 900 degrees Fahrenheit. On Venus you could cook a 16-inch pepperoni pizza in seven seconds, just by holding it out to the air. (Yes, I did the math.) Such extreme conditions pose great challenges to space exploration, because practically anything you can imagine sending to Venus will, within a moment or two, get crushed, melted, or vaporized. So you must be heatproof or just plain quick if you’re collecting data from the surface of this forsaken place.
It’s no accident, by the way, that Venus is hot. It suffers from a runaway greenhouse effect, induced by the carbon dioxide in its atmosphere, which traps infrared energy. So even though the tops of Venus’s clouds reflect most of the Sun’s incoming visible light, rocks and soils on the ground absorb the little bit that makes its way through. This same terrain then reradiates the visible light as infrared, which builds and builds in the air, eventually creating—and now sustaining—a remarkable pizza oven.
By the way, were we to find life-forms on Venus, we would probably call them Venutians, just as people from Mars would be Martians. But according to rules of Latin genitives, to be “of Venus” ought to make you a Venereal. Unfortunately, medical doctors reached that word before astronomers did. Can’t blame them, I suppose. Venereal disease long predates astronomy, which itself stands as only the second oldest profession.
The rest of the solar system continues to become more familiar by the day. The first spacecraft to fly past Mars was Mariner 4, in 1965, and it sent back the first-ever close-ups of the Red Planet. Lowell’s lunacies notwithstanding, before 1965 nobody knew what the Martian surface looked like, other than that it was reddish, had polar ice caps, and showed darker and lighter patches. Nobody knew it had mountains, or a canyon system vastly wider, deeper, and longer than the Grand Canyon. Nobody knew it had volcanoes vastly bigger than the largest volcano on Earth—Mauna Kea in Hawaii—even when you measure its height from the bottom of the ocean.
Nor is there any shortage of evidence that liquid water once flowed on the Martian surface: the planet has (dry) meandering riverbeds as long and wide as the Amazon, webs of (dry) tributaries, (dry) river deltas, and (dry) floodplains. The Mars exploration rovers, inching their way across the dusty rock-strewn surface, confirmed the presence of surface minerals that form only in the presence of water. Yes, signs of water everywhere, but not a drop to drink.
Something bad happened on both Mars and Venus. Could something bad happen on Earth too? Our species currently turns row upon row of environmental knobs, without much regard to long-term consequences. Who even knew to ask these questions of Earth before the study of Mars and Venus, our nearest neighbors in space, forced us to look back on ourselves?
TO GET A better view of the more distant planets requires space probes. The first spacecraft to leave the solar system were Pioneer 10, launched in 1972, and its twin Pioneer 11, launched in 1973. Both passed by Jupiter two years later, executing a grand tour along the way. They’ll soon pass 10 billion miles from Earth, more than twice the distance to Pluto.
When they were launched, however, Pioneer 10 and 11 weren’t supplied with enough energy to go much beyond Jupiter. How do you get a spacecraft to go farther than its energy supply will carry it? You aim it, fire the rockets, and then just let it coast to its destination, falling along the streams of gravitational forces set up by everything in the solar system. And because astrophysicists map trajectories with precision, probes can gain energy from multiple slingshot-style maneuvers that rob orbital energy from the planets they visit. Orbital dynamicists have gotten so good at these gravity assists that they make pool sharks jealous.
Pioneer 10 and 11 sent back better pictures of Jupiter and Saturn than had ever been possible from Earth’s surfce. But it was the twin spacecraft Voyager 1 and 2—launched in 1977 and equipped with a suite of scientific experiments and imagers—that turned the outer planets into icons. Voyager 1 and 2 brought the solar system into the living rooms of an entire generation of world citizens. One of the windfalls of those journeys was the revelation that the moons of the outer planets are just as different from one another, and just as fascinating, as the planets themselves. Hence those planetary satellites graduated from boring points of light to worlds worthy of our attention and affection.
As I write, NASA’s Cassini orbiter continues to orbit Saturn, in deep study of the planet itself, its striking ring system, and its many moons. Having reached Saturn’s neighborhood after a “four-cushion” gravity assist, Cassini successfully deployed a daughter probe named Huygens, designed by the European Space Agency and named for Christiaan Huygens the Dutch astronomer who first identified Saturn’s rings. The probe descended into the atmosphere of Saturn’s largest satellite, Titan—the only moon in the solar system known to have a dense atmosphere. Titan’s surface chemistry, rich in organic molecules, may be the best analog we have for the early prebiotic Earth. Other complex NASA missions are now being planned that will do the same for Jupiter, allowing a sustained study of the planet and its 70-plus moons.
IN 1584, in his book On the Infinite Universe and Worlds, the Italian monk and philosopher Giordano Bruno proposed the existence of “innumerable suns” and “innumerable Earths [that] revolve about these suns.” Moreover, he claimed, working from the premise of a Creator both glorious and omnipotent, that each of those Earths has living inhabitants. For these and related blasphemous transgressions, the Catholic Church had Bruno burned at the stake.
Yet Bruno was neither the first nor the last person to posit some version of those ideas. His predecessors range from the fifth-century B.C. Greek philosopher Democritus to the fifteenth-century cardinal Nicholas of Cusa. His successors include such personages as the eighteenth-century philosopher Immanuel Kant and the nineteenth-century novelist Honoré de Balzac. Bruno was just unlucky to be born at a time when you could get executed for such thoughts.
During the twentieth century, astronomers figured that life could exist on other planets, as it does on Earth, only if those planets orbited their host star within the “habitable zone”—a swath of space neither too close, because water would evaporate, nor too far, because water would freeze. No doubt that life as we know it requires liquid water, but everyone had just assumed that life also required starlight as its ultimate source of
energy.
Then came the discovery that Jupiter’s moons Io and Europa, among other objects in the outer solar system, are heated by energy sources other than the Sun. Io is the most volcanically active place in the solar system, belching sulfurous gases into its atmosphere and spilling lava left and right. Europa almost surely has a deep billion-year-old ocean of liquid water beneath its icy crust. In both cases, the stress of Jupiter’s tides on the solid moons pumps energy to their interiors, melting ice and giving rise to environments that might sustain life independent of solar energy.
Even right here on Earth, new categories of organisms, collectively called extremophiles, thrive in conditions inimical to human beings. The concept of a habitable zone incorporated an initial bias that room temperature is just right for life. But some organisms just love several-hundred-degree hot tubs and find room temperature downright hostile. To them, we are the extremophiles. Many places on Earth, previously presumed to be unlivable, such creatures call home: the bottom of Death Valley, the mouths of hot vents at the bottom of the ocean, and nuclear waste sites, to name just a few.
Armed with the knowledge that life can appear in places vastly more diverse than previously imagined, astrobiologists have broadened the earlier, and more restricted, concept of a habitable zone. Today we know that such a zone must encompass the newfound hardiness of microbial life as well as the range of energy sources that can sustain it. And, just as Bruno and others had suspected, the roster of confirmed exosolar planets continues to grow by leaps and bounds. That number has now risen past 150—all discovered in the past decade or so.
Once again we resurrect the idea that life might be everywhere, just as our ancestors had imagined. But today, we do so without risk of being immolated, and with the newfound knowledge that life is hardy and that the habitable zone may be as large as the universe itself.
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