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FELLOW WANDERERS

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he cosmos seemed so much simpler in ancient

The word "planet" traces its origins to the Greek word for "wanderer," but the underlying concept goes back much further. When the earliest humans looked up at the lights in the sky, it didn't take them long to notice that most of them formed unchanging patterns like the Big Dipper and the three-star belt of Orion. Other lights, however, changed their position relative to these fixed stars.

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The sun and the moon were the most obvious of these wanderers. Another five points of light—which we now call Mercury, Venus, Mars, Jupiter, and Saturn—moved back and forth through the constellations. The Greeks added these five "wandering stars" to the sun and the moon, making a total of seven planetoi circling Earth.1

That worldview held sway throughout the Roman era and well into the Middle Ages. But the rise of the Renaissance stirred new curiosities, new questions, new tools, and clever new ways to use those tools to provide answers. In the process, the cosmos became more complex.

In the 1500s and early 1600s, Nicolaus Copernicus and Johannes Kepler laid down theories that turned the old worldview around: Instead of having the sun, moon, and planets circling Earth, they laid out a system in which the planets circled the sun. Earth became just another planet, orbited by the moon. It was a dramatic change in perspective that took us humans and our home out of the universe's central position.

The groundwork for our current basic understanding of the solar system was thus put into place well before the milestone year of 1609, when Galileo Galilei, armed only with the spyglass of his own construction, began making his revolutionary observations of the moon and planets.

You could argue that Galileo was the first man in recorded history to claim the discovery of a new planet. Four of them, in fact. When he looked through his telescope at Jupiter, he saw four specks of light that lined up with the planet's disk and seemed to change position from night to night. Showing the political savvy embodied a century earlier by his fellow Florentine, Niccolo Machiavelli, Galileo named his four finds the "Medicean stars" in honor of his most important patron, Grand Duke Cosimo Il de' Medici.

Galileo gushed that the newfound objects never strayed far from Jupiter, the dominant planet in the prince's horoscope. For that reason, the astronomer told Cosimo, "it appears that the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness above all others."2

Ironically, the name didn't stick. Instead, "these new planets" are now known as Jupiter's Galilean satellites, the four biggest moons of our solar system's biggest planet. Using a garden-variety pair of binoculars, you can see them much as Galileo did, particularly from wide-open spaces like the cornfields of Iowa.

Galileo's reports sparked the seventeenth century's grand clash between the medieval view of the heavens, which put Earth in a special place at the center of God's universe, and the revolutionary view that Earth was merely a planet that circled the sun along with other wanderers.

The effect on Galileo's career is well-known: He faced not just one, but two church inquisitions that left him under suspicion of heresy—and under house arrest for the last nine years of his life.3

The Medicean stars had a more salutary influence on other scientists who were already working out the implications of the Copernican worldview. Kepler, for instance, saw Galileo's discovery as confirmation that the planets were on a par with Earth, and he was among the first to recognize a distinction between planets that circle the sun and moons that circle the planets. "These four little moons exist for Jupiter, not for us," Kepler wrote. "Each planet in turn, together with its occupants, is served by its own satellites."4

The line between planets and their satellites became clearer as time went on. In 1655, Dutch astronomer Christiaan Huygens spotted Saturn's largest moon, Titan. Over the three decades that followed, Italy's Giovanni Domenico Cassini found four more Saturnian moons. By the time the eighteenth century dawned, old-fashioned observers might have reckoned the planet count at sixteen, but when most astronomers took stock of the sky they counted six planets and ten moons.

Even today, the definition for a planet's moon is more clear-cut than the definition for the planet itself. Any object in orbit around the planet would be considered its satellite, and if the object is of natural origin, you'd call it a moon.

The planet debate stirred again in 1781, when William Herschel, a musician turned amateur astronomer, spotted what looked like a comet through the seven-foot-long, home-built telescope he set up in the garden behind his house in the English spa city of Bath. After repeated observations, astronomers across Europe competed to calculate the object's orbit—and the Royal Society's president, Joseph Banks, urged

Herschel to make up his mind about what he had initially guessed was "a comet of a new species, very like a fixed star." Wrote Banks, "Some of our astronomers here incline to the opinion that it is a planet and not a comet; if you are of the opinion it should forthwith be provided with a name or our nimble neighbors, the French, will certainly save us the trouble of baptizing it."5

Herschel was persuaded to go with the planetary designation—which was the right choice. Further observations confirmed that his comet was indeed a new planet.

Thanks to his lucky discovery, Herschel became the toast of the scientific world. Britain's King George III, who was grateful for the distraction from his troubles with the American colonies, appointed him his private astronomer. Following Galileo' s lead, Herschel named the newfound world Georgium Sidus, or the Georgian Star.

Like Galileo, Herschel laid it on thick. "As a subject of the best of Kings, who is the liberal protector of every art and science; as a native of the country from whence this Illustrious Family was called to the British throne; and as a person now more immediately under the protection of this excellent Monarch and owing everything to his unlimited bounty; I cannot but wish to take this opportunity of expressing my sense of gratitude, by giving the name Georgium Sidus to a star which (with respect to us) first began to shine under His auspicious reign," he wrote.6

Once again, the fawning name didn't stick. Instead, Prussian astronomer Johann Bode came up with Uranus, a

Greek name that refers to the mythological father of the god Saturn. That name was more in conformity with the classical tradition for planet names—and besides, it carried less political baggage for international use.

Herschel steadfastly refused to refer to the planet as Uranus, out of loyalty to his king and the kingly name he gave his discovery. Eventually, however, even British astronomers came around, giving generations of schoolchildren a pronunciation to giggle over.

William Herschel was also a central figure in the nineteenth century's biggest planet debate—an eerie foreshadowing of the twenty-first-century debate over Pluto and its ilk.

Back in 1766, German astronomer Johann Titius saw a mathematical pattern in the spacing of the six planetary orbits known at that time. Based on that pattern, he and his colleague Johann Bode (the man who ended up naming Uranus) figured that one additional planet should theoretically fill the gap between Mars and Jupiter. They also said yet another planet should be found beyond Saturn's orbit.

The placement of Uranus fit the pattern, seemingly confirming what was known as the Titius-Bode law. That revved up the search for a previously undetected world between Mars and Jupiter. Sure enough, an Italian monk and astronomer, Giuseppe Piazzi, found a prospect in the predicted orbit in 1801. "The first of January I discovered a star, which by its motion strongly appears to be a planet. . . . I would very much like for you to search for it," Piazzi wrote in a letter to Herschel.7 Once the find was confirmed, Piazzi named the planet Ceres Ferdinandea, in honor of Ceres, the Roman goddess of the harvest, as well as the Sicilian king Ferdinand IV. Other astronomers quickly shortened the name.

The discovery of Ceres was a classic example showing how even an unfounded theory can sometimes lead to substantive discoveries. To this day, there is no scientific explanation behind the Titius-Bode law; nevertheless, it pointed nineteenth-century astronomers to a previously unknown celestial body. A whole bunch of them, in fact.

Just a year after Piazzi's discovery, astronomers found a second speck in roughly the same orbit. This sister to Ceres was given the godly name Pallas, and most astronomers accepted it as a planet. But Herschel questioned whether the two newfound worlds really deserved to be put in the same category as Jupiter and the other celestial deities. By his estimate, Ceres and Pallas were so small as to be "beyond all comparison less than planets."

In a letter to a fellow member of the Royal Society, William Watson, Herschel complained that "it appears to me much more poor in language to call them planets than to call a rasor a knife, a cleaver a hatchet

"Now as we already have Planets, Comets, Satellites, pray help me to another dignified name as soon as possible," he told Watson.

Within a month, Herschel came up with another name: asteroids. The term was derived from the Greek word for star,

"aster" and means "starlike." Herschel made up the word because he saw the objects as starlike points of light rather than planetlike disks in his telescope.

In 2003 and 2004, the Hubble Space Telescope took the highest-resolution pictures of Ceres ever made, showing it as a round disk with a mottled, thoroughly planetlike appearance. "If Herschel had seen the disk of Ceres he might not have objected to its planetary status," said Mark Sykes, director of the Arizona-based Planetary Science Institute and a member of the scientific team behind the first NASA mission to Ceres. "If he had seen the smaller asteroids, and their irregular shapes, I suspect that he would have drawn the classical line for planet as those objects observed to have round disks. In this case, Pluto's planetary status would never have been in question."

At first, Herschel's made-up term went over about as well as Georgium Sidus. One astronomer complained about Herschel's "idle fondness for inventing names."8 Other critics suggested that Herschel wanted to distinguish asteroids from planets just to make his own discovery seem more important.9

In the decades that followed, the zone between Mars and Jupiter yielded up still more mini-worlds. Their discoverers dutifully gave them mythological names as well as increasingly elaborate planetary symbols. Gradually, however, the symbols and the lists became unwieldy. Ceres and its scores of siblings were set aside in a separate category of minor planets, and numbers instead of symbols were used as shorthand. (Ceres was given the number 1.)

Eventually, the term that Herschel invented took hold after all.

Today, the International Astronomical Union's Minor Planet Center lists more than 200,000 objects, all numbered. Most of them, like Ceres, are in the main asteroid belt between Mars and Jupiter, or out beyond the planet Neptune. (Even Pluto has a number nowadays.) But some asteroids stray across planetary orbits—including Earth's. One such space rock is thought to have blasted the coast of Mexico's Yucatan Peninsula sixty-five million years ago, setting off a thermonuclear-scale explosion and dooming the dinosaurs.

The IAU lists so many asteroids that there aren't enough gods to go around. Finds have been named after esteemed scientists (Herschel, Einstein, and Hawking), cultural icons (Elvis, Sinatra, and Mister Rogers), places (Latvia, Las Vegas, and Bora-Bora), and even favorite pets (Petrina, Sepprina, and Mr. Spock—in this case, the discoverer's late lamented cat, not the Star Trek character).10

Asteroid discoverers have been known to take requests. For instance, I played a small role in getting a space rock named after science-fiction humorist Douglas Adams, author of The Hitchhiker's Guide to the Galaxy series. The asteroid I suggested for the honor had the provisional designation of 2001 DA42, which included the year of Adams's death (2001), his initials (DA), and his whimsical answer to the ultimate question (the number 42). Astronomer Brian Marsden, who headed the Minor Planet Center at the time and played a key role in the Pluto controversy, was so tickled by my suggestion that he persuaded the LINEAR asteroid search team to go along with the idea.11

Why are there so many space rocks in the asteroid belt? In Herschel's day, astronomers speculated that the asteroids were scattered pieces of a much larger planet that broke up ages before. Today, however, the prevailing view is that Ceres and its smaller siblings were built up through the same process that gave rise to the solar system's far bigger planets.

In the beginning, 4.5 billion years ago, the sun condensed from a huge cloud of dust and gas, with a dusty disk swirling around the infant star's midsection. Thanks to mutual gravitational and electrostatic attraction, grains of dust and ice clumped together into bigger and bigger balls of ice and dust. Over time, radiation from the sun burned off the surrounding dusty haze as if it were a morning fog, leaving behind dirty snowballs (or perhaps gassy dirtballs).

The lumpy disk thus became a snowball shooting gallery. Some of the balls became large enough to gobble up smaller ones, or slingshot them out of the solar system altogether.12 The major planets were the big winners in the snowball fight.

The gasball now known as Jupiter was particularly prone to throw its weight around, which was bad news for the asteroid belt. Jupiter's gravitational effect stripped away space rocks, removing some of the raw material that might have clumped up into a large planet. Ceres was the biggest of the rocks left behind. Today, it accounts for as much as a third of the asteroid belt's total mass.

Was Ceres a loser in the planet-forming process, or a survivor? That's like asking whether the glass is two-thirds empty or a third full. It's a cosmic pimple even tinier than Pluto, but it's big enough to have taken on a round shape—and scientists now say it seems to have a crust, mantle, and core, just as Earth does.13

Scientists had no way of knowing all that in the nineteenth century, and still it took decades for the planet versus asteroid question to play itself out. In the 1850s, Ceres and a few other asteroids were included along with the major planets in the astronomical catalogs of the time, while other, smaller asteroids came to be listed in the back of the book as minor planets.

In addition to those little planets, the Berlin Astronomical Yearbook's back-of-the-book list for 1854 included a mysterious new world that eventually got a promotion to match Ceres's demotion: Neptune, the planet that set the precedent for Pluto.14

Neptune was found because something didn't add up about Uranus. In the decades after Herschel discovered his Georgian Star, astronomers gathered more and more data about the newfound planet's orbit—including sightings that were recorded even before people realized Uranus was a planet. Using the formulas put forward by Isaac Newton, astronomers calculated how the gravitational influence of the six other planets should affect Uranus's course around the sun.

The problem was that Uranus's observed orbit didn't match up with the mathematical calculations. Either Newton was wrong, or the observations were inaccurate, or the astronomers were missing something big.

In 1821, French mathematician Alexis Bouvard suggested that the discrepancy was due to "some extraneous and unknown influence which may have acted on the planet."15 Over the two decades that followed, more and more astronomers wondered whether the gravitational pull of yet another planet was affecting Uranus's orbit. The mystery planet's extra mass would explain the slight, puzzling changes in Uranus's orbital speed and position. If you could work out the right mathematical solution to the orbital problem, it just might point you to the mystery planet's location in the sky.

But finding a solution wouldn't be easy. You'd have to start out with a hypothetical planet, and then repeatedly fiddle with the orbit and the mass until you found a solution that fit the data. This is exactly what rival teams of planet hunters did in the mid-1840s. In France, mathematician Urbain Jean-Joseph Le Verrier publicly presented a series of reports narrowing down the range of planetary predictions, while mathematicians and astronomers in England, guided by John Couch Adams's calculations, secretly pursued their own quest.

Le Verrier tried to get French astronomers to look for the planet, but the astronomers were reluctant to invest all that time and effort in what they saw as a highly speculative and suspicious mathematical exercise. Finally, in September 1846, with the British closing in on the prize, a frustrated Le Verrier sent a letter to a German astronomer of his acquaintance, Johann Gottfried Galle at the Berlin Observatory. "I would like to find a persistent observer, who would be willing to devote some time to an examination of a part of the sky in which there may be a planet to discover," Le Verrier wrote.

Galle took the hint. Just hours after he received Le Verrier's letter, he and an astronomy student named Heinrich d'Arrest turned the observatory's telescope to the area of the sky that Le Verrier specified, and checked what they saw against a detailed star atlas. One faint point of light stood out. "That star is not on the map!" d'Arrest declared.

Two days after getting the tip, Galle sent a letter back to Le Verrier. "Sir," he wrote, "the planet whose position you have pointed out actually exists."16

The British had been beaten, and most of the glory went not to Germany's Galle, but to France's Le Verrier. French astronomer François Arago—no impartial observer—hailed him as the first man to discover a planet with "the point of his pen."17 Arago wanted to call the newfound world "Le Verrier," and for a while the French followed his lead. (To be consistent, they also called Uranus "Planet Herschel.")

Those names didn' t stick. Le Verrier himself insisted on calling his planet Neptune, after the Roman god of the sea. And that's how it's known today. Later on, astronomers and historians determined that Galileo had observed Neptune in 1612 and 1613, but assumed it was a fixed star.18

The discoveries of the past four hundred years all go to show that the course of the planet debate never ran smooth, even before Pluto came onto the scene. Figuring out our complex cosmos can be a messy process, with plenty of room for miscalculation and misclassification, for clashing politics and clashing egos. Even the best of us can get it wrong, at least temporarily.

But even wrongheaded science can sometimes produce the right results—and few examples demonstrate that more clearly than the discovery of Pluto.

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