Scientists at Kobe University in western Japan believe a Mars-sized planet still waits to be discovered in the outer solar system. Ever since the discovery of Neptune in 1846, scientists have debated whether another planet and its gravity were necessary to account for the observed motions of the other bodies in the solar system. This latest prediction is based on a computer model of the evolution of the Kuiper Belt, that group of thousands of asteroids and mini planets including Pluto.
The composer William Herschel and his sister Caroline in 1781 were the first people to discover a new planet. The idea of finding an unknown planet was so novel at the time that for months the Herschels though they had discovered a comet and were puzzled by its orbit and refusal to develop a tail. When it finally dawned on them what they had discovered, they knew it need a better name than Comet Herschel. They called it George, after the insane king of England.
Understandably, continental astronomers were less than thrilled to accept a name chosen to flatter a political figure. Several of their countries were at war with England at the time in support of the American rebellion. French astronomers graciously pushed for calling the planet Herschel. Johann Bode, a Prussian publisher of ephemeris tables, suggested a compromise. Since all of the other planets had names out of Greco-Roman mythology, why not continue the pattern and name it after a mythological figure? He suggested Uranus, the father of Saturn, as an appropriate name, not realizing how the mere pronunciation of the name would cause English-speaking adolescent boys to fall into fits of giggles.
Bode's suggestion was adopted outside England and France, where astronomers stuck to their own names for another sixty years before finally giving in to the usage of the rest of the world. Bode's name was especially popular among other Germans. In 1789, a Berlin chemist, Martin Klaproth, isolated a new element out of pitchblende ore. Recalling the alchemical traditions of making connections between minerals and planets, Klaproth named his new element after the new planet, calling it Uranium.
Herschel wasn't alone in using discoveries to curry favor with his economic betters. When Galileo discovered the four major moons of Jupiter in 1610, he decided to name them after his former math student Cosimo de'Medici, who had become the powerful Grand Duke of Tuscany. Galileo first thought to name them the Cosmican Stars, but then thought better of it. The name was too close to Cosmic Stars and the significance might be lost on the object of his up sucking. In Sidereus Nuncius, his little book announcing the discovery, he called the moons the Medicean Stars, a name unsubtle enough that even a busy Grand Duke would take notice. The attempt was successful; a few months later, Cosimo offered Galileo a high paying job that the the math teacher quickly accepted.
Four years after Gailileo published his description of the Medicean Stars, a German astronomer, Simon Marius, published a work claiming to have discovered the moons before Galileo. He couldn't offer any convincing proof for his claim, so history has sided with Galileo. Marius' observations were, however, of high quality and he gave us something Galileo did not: individual names for the moons (Io, Europa, Ganymede, and Callisto, all lovers of Jupiter in mythology). The French astronomer Nicolas-Claude Fabri de Peiresc suggested that the for moons be named after the four Medici brothers, something Galileo may also have had in mind, but the suggestion was not taken up by the budding international astronomic community. The mythological names were not, in fact, Marius' first choice. He first thought of an awkward system of naming them after the Sun's planets (i.e. the Mercury of Jupiter, the Venus of Jupiter). At the time there was no reason not to assume that smaller moons might be orbiting the bigger moons and so on. This might have led to names like the Saturn of the Mars of the Mercury of Jupiter. Clearly, a bad idea. Maris humbly credited Kepler with the much better suggestion of classical mythology. Kepler is famous for enough else, so Marius deserves to be remembered for publicizing the suggestion.
In 1655 Christiaan Huygens discovered a moon orbiting Saturn. He cleverly called it Saturn's Moon. When Cassini discovered four more moons around Saturn, he followed Galileo's example and named them Sidera Lodoicea ("the stars of Louis") to honor his employer Louis XIV of France. He did not give his new moons individual names and, oddly, neither did anyone else. For most of the next two centuries, astronomers simply called them by numbers.
Following the Herschels' discovery of Uranus, other astronomers put their telescopes to work seeking out new Georges to name after their own political patrons. In 1801 a Sicilian astronomer, Giuseppe Piazzi, was the first to strike gold. Spotting an object orbiting between Mars and Jupiter and determining it not to be a comet, he announced that he had found a tiny planet, and named it Ceres Ferdinandea. The name seemed to cover all the bases, it had an element from classical mythology (Ceres, the Roman goddess of agriculture) and it sucked up to his king. Unfortunately, Ferdinand of Sicily had recently been overthrown by Napoleon and no one went along with naming a celestial object after a powerless refugee. Other mythological names were suggested, but eventually everyone accepted the Ceres part of Piazzi's suggestion.
As astronomers began looking at the region in which Ceres had been found, they promptly found three more tiny planets. These were named Pallas, Juno, and Vesta. Naming planets after kings had proved to be a non-starter, so the astronomers stuck with classical mythology. On the other hand, naming elements after planets was very popular. Soon after the four tiny planets between Mars and Jupiter, chemists isolating the elements gave us Cerium and Palladium. Juno already had a month named after her, but poor Vesta didn't get squat, which is a shame because Vestanite would be a much cooler name than Rutherfordium or some of the other lame names for the tranuranium elements.
Bode had predicted a planet in the region where the new mini-planets were found based on a pattern he, and other astronomers, perceived in the distances between the planets. This pattern is now called the Titus-Bode Law. However, the tiny new planets in that position bothered astronomers. They were smaller than any of the known moons. William Herschel suggested not letting these insignificant objects into the august club of planets. He coined a new word, "asteroid" (star like), to describe them. The little planets remained in limbo until the 1840s when a new generation of more powerful telescopes led to the discovery of more tiny bodies between Mars and Jupiter. Facing the prospect of dozens or more new planets, the international astronomical community adopted Herschel's suggestion and demoted the asteroids into a separate category apart from the planets.
The Herschels had discovered two moons to go with their new planet. These would later be named, on the suggestion of William's son John, after characters in "A Midsummer Night's Dream." While not strictly classical mythology, Shakespeare's fairies were close enough to satisfy Bode's mythology principle and the names were never seriously challenged. The Herschels also discovered two more moons around Saturn, bringing the known total to seven. Till the 1840s, astronomers had simply refered to the Saturnian satellites by numbers counting out from the planet. This meant the names were subject to change every time a new moon was discovered. The largest moon had already been called Saturn II, IV, and VI. This couldn't continue. John suggested the classical solution of naming the moons after the Titans, the brothers and sisters of Saturn, reserving the name Titan for the first discovered because it was so titanic. The named Saturnian moons are really no more than Titan and the Titans, which might be a decent name for a surf rock band.
At about the same time that the word asteroid and the naming patterns for the moons of Saturn and Uranus were adopted, the search was on for another planet beyond Uranus. Based on a half century of observing Uranus' orbit some astronomers had come to believe that the gravity of another large body must be affecting it, causing it to move faster than expected till 1822 and slower afterwards. By the 1840s astronomers had a rough idea where to look for the mystery planet. In 1846 Urbain Le Verrier calculated and published the exact location and observers in three countries had no problem finding the planet soon after that. British astronomers had calculated the correct location before Le Verrier, but did not publish and were thus denied the glory of being part of the discovery.
Some French astronomers wanted to call the eighth planet Le Verrier, pointing out that naming a planet after its discoverer had a precedent, since they still called Uranus Herschel. Le Verrier at first suggested the name Neptune, after the god of the sea. For a while he also flirted with naming it after himself, but the name Neptune caught on beating out the other classical names Janus and Oceanus. The god of the sea was especially compelling because Neptune is very blue in appearance.
The new planet also got its commemorative element, thought this time it took longer. Neptunium was assembled, not refined, by scientists at Berkeley in 1940. It was the first synthetic element to be built by bombarding Uranium with neutrons. Glenn Seaborg, who led the Berkeley project eventually got an element of his own for his work: Seaborgium.
A mere seventeen days after the location of Neptune was confirmed, William Lassell, an English brewer, announced the discovery of a large moon. Since the astronomical community was busy arguing over the name of the planet, you would think that they would also get hot under the collar over a name for the moon. You would be wrong. The moon carried the dull name Neptune's Moon for over thirty years. In 1880, Camille Flammarion suggested Triton, the name of Neptune's son, for the moon.
In 1919 the International Astronomical Union (IAU) was created uniting various national astronomical societies from around the world. One of its main functions was to be the central authority for assigning names to celestial bodies. In general, certain patterns for naming, such as those John Herschel suggested for moons seventy years earlier are voted on and astronomers are allowed to exercise the discoverer's right on naming within those conventions. The IAU must officially accept an astronomer's name before it goes into international use. A system of numeric designations are used for objects as temporary names prior to the announcement of official names. The IAU came in the nick of time. The ideological conflicts of the twentieth century could easily have been fought out in naming conventions. Each power bloc might have adopted its own name for every discovery and changed their names with every revolution. Imagine St. Petersburg to Petrograd to Leningrad and back to St. Petersburg played out on every comet and crater in the solar system.
In the 1830s, astronomers were convinced that another planet was required to explain Uranus' movements and had begun working on calculations to locate the planet. Even then, some astronomers believed one planet would not be enough. In 1834, a Dutch astronomer, Peter Andreas Hansen, wrote that he was convinced that two planets would be required to explain Uranus' movements. Following the discovery of Neptune, other astronomers agreed, though they did not agree just what was required. By the 1870s, enough data had been collected about Neptune for astronomers the begin making predictions as to where the next planet would be found and how big it should be. Astronomers in various countries began their own searches. None of these predictions matched Le Verrier's and no new planets were found.
Le Verrier himself became involved with the search for a tiny planet between Mercury and the Sun. Mercury's orbit, like Uranus' never quite matched the predictions of astronomers. Beginning in 1859, a number of amateur astronomers claimed to witness the transit of a small body across the sun. Le Verrier examined one such claim and became convinced he had another planet. He announced his discovery to the French Academy and called his second planet Vulcan. Unfortunately, the periodic sightings of a spot on the Sun never resolved into a single planet. After Le Verrier's death Vulcan fell out of fashion and was all but forgotten by the astronomical community. In 1919, the same year that the IAU was founded, Einstein proved the problems with Mercury's orbit were caused by the curving of space so close to the sun and not by the pull of a missing planet. Mysterious dots still are reported from time to time on the face of the sun, but these are usually dismissed as uncharted asteroids, comets, or alien starships, though the latter is decidedly a minority opinion.
In 1894, Percival Lowell burst onto the astronomy scene. Lowell was the product of old an Boston family with lots of old Boston money. Lowell had traveled extensively in Asia, written several books on Asian culture, and served as foreign secretary and counselor for a special Korean diplomatic mission to the United States. In the nineties he turned his attention and considerable enthusiasm to astronomy. Lowell moved to Flagstaff, Arizona and built a world-class observatory in the high, clear, mountain air. At first, Lowell was obsesses with the planet Mars. He was convinced that the "canali" of Mars, as drawn by Italian astronomer Giovanni Schiaparelli, were indication of life and civilization on our red neighbor. Lowell wrote three books and suffered a nervous breakdown before he let go of that idea and moved on to something else.
That something else was the missing planet beyond Neptune. This was a serious problem, recognized by serious astronomers. Though Lowell was thick-skinned about the mockery directed at him over Mars, years of it had begun to wear on the staff. Besides, there was very little more he could do about Mars without a spaceship. Lowell did his own calculations on the Neptune problem and decided a large planet must be lurking in the constellation Gemini. He spent the last eleven years of his life looking for the body he called Planet X, but died without finding it.
After Lowell's death there was a delay of a decade in the search while Lowell's widow, Constance, and the observatory fought over his will. In 1929 with their share of Lowell's wealth assured, the observatory hired a young amateur astronomer from Kansas, Clyde Tombaugh, to take over the search. Tombaugh was an excellent candidate, both hard working and an excellent observer. He carefully went over the calculations for Planet X done by Lowell and by Lowells competitors before deciding on an area to search. On February 18, 1930, after nearly a year of searching, Tombaugh discovered his Planet X.
Naming rights belonged to the observatory. They decided to be democratic and hold a vote. Mrs. Lowell sent suggestions of Zeus, Lowell, and Constance. Mrs. Lowell was not the favorite person at the observatory, having almost stopped their work for a decade. Her names were ignored. The choices on the ballot were Minerva, Cronos, and Pluto. Pluto won unanimously.
While astronomers were excited about the discovery of Pluto, it was clear from the beginning that it was too small to be the longed for Planet X. As time went by, better observations showed that Pluto was even smaller than at first believed--smaller than the Earth's Moon--and that it had an irregular orbit far different that that of any other planet. Pluto, however, had an advantage that Ceres never did in becoming accepted as a planet: mass communication and mass literacy. The discovery of new planet was announced in newspapers and newsreels. The name was suggested by Venetia Burney, an eleven-year-old girl in Oxford, England. Walt Disney introduced a character named Pluto into his Mickey Mouse cartoons later that year. Pluto even got its commemorative element, Plutonium. Like Neptunium, Plutonium was assembled at Berkeley. Pluto wasn't just the business of the astronomical community; Pluto belonged to the masses, particularly to the children.
In the same year that Tombaugh discovered Pluto, Frederick C. Leonard predicted that there was a whole belt of tiny objects beyond Neptune. Sooner or later we would have good enough telescopes to find them and the astronomical community would be faced with the same problem that they had faced with the asteroids: too many and too small to be planets. That day finally came about sixteen years ago. Gerard Kuiper was an astrophysicist, who speculated in 1950 that the region beyond Neptune ought to at one time have contained a belt of debris left over from the formation of the solar system. At the time, when Pluto was still thought to be fairly large, Kuiper believed Pluto would have destroyed the belt. But as estimates of Pluto's size went down, the probability that the debris belt still survived went up. In the late eighties, astronomers began looking for it. One Pluto like object was discovered in 1992. Five more were identified the next year. Today, over 1000 of these Kuiper Belt Objects (KBOs), as they are called, have been discovered.
While thousands more KBOs are expected to lie beyond the orbit of Pluto, very few astronomers expect to find a large planet out there. For one thing, it's no longer needed. Close measurements provided by Voyager 2's 1989 flyby of Neptune allowed astronomers to more accurately measure the mass of Neptune. According to the current measurements of their masses, Uranus and Neptune orbit exactly as they should. Occasionally, astronomers come up with new reasons for a large planet or even a small star to be lurking in the distant reaches of the system, but these no longer have to do with the orbits of the known planets.
This brings us to the Kobe University study. Patryk Lykawka and Tadashi Mukai have determined that a body, Earth sized or just a little smaller, is needed to explain the observed shape of the Kuiper Belt. The rapid discovery of so many KBOs allowed astronomers to map the shape of the belt. To their surprise, the belt abruptly stops at a distance of 50 astronomical units. The belt also appears to have been sorted into several distinct groups of bodies. Lykawka's conclusion is that something fairly large--a new Planet X--was needed to sort and sculpt the belt into the shape we now see.
Close up observation of Saturn's rings have shown that they are herded into shape by complex gravitational forces exerted by Saturn's moons. Lykawka thinks something similar is at work in the Kuiper Belt, but with one difference. In the computer simulations that he and Mukai did, Planet X shapes the belt early in its history and then is thrown into a distant orbit where it has only minor interactions with the belt. After its initial shaping, the main influence on the Kuiper Belt becomes Neptune.
While Lykawka's theory has some sympathetic listeners, it also has some strong critics. Not surprisingly, some of the strongest criticism comes from the proponents of competing theories of the early development of the solar system. The bottom line is that we are just beginning to understand the outer solar system and to come up with plausible scenarios for the evolution of the solar system that account for all of its parts. If Lykawka's theory proves correct and someone finds Planet X, the really important question will be what do we call it. George is still up for grabs.