by Dava Sobel
Within the liquid turmoil of these planetary mantles, where boiling ices mix with bits of molten rock, the turning of Uranus and Neptune bestirs electric currents that generate global magnetic fields around both worlds.
Uranus and Neptune spin at similar rotation rates (seventeen hours and sixteen hours, respectively), but their days pass in nothing like similar fashion, because the unusual prone posture of Uranus confounds the meaning of days as seasons change. Lying on its side, and taking nearly eighty-four Earth years to complete a single revolution, Uranus spends twenty years of each orbit with its south pole facing Sunward, and later another twenty years with its north pole toward the Sun. At such times, the planet’s rapid rotation fails to produce a cycle of light and darkness, so the “days” (and “nights”) last a full two decades. During the two twenty-year periods when the Sun strikes Uranus on the equator, however, the days dwindle to about eight hours, followed by nights of equal length.
Neptune’s 29-degree tilt—about on a par with Earth, Mars, or Saturn—keeps days of more consistent length all through its inordinately long years, each the equivalent of 163.7 Earth years, or nearly double those of Uranus.
Very little light or warmth from the Sun reaches across the two billion miles to Uranus, and even less arrives at Neptune, another billion miles farther away. Yet the high atmospheres of both planets register the same low temperature, and this likeness exposes an important difference between them: The more distant Neptune generates considerably more heat from within.
Neptune’s heat powers active weather patterns, with dark storms and white clouds borne across the planet’s blue expanses on swift winds. Some such tempests resemble the size and shape of Jupiter’s Great Red Spot, though they seem to freely change shape as they swirl. They also roam from one latitude to another, dissipating as they go, instead of persisting confined in any specific zone.
Before Voyager 2 flew by Neptune in 1989, the planet had just two known moons. The larger one, first observed by William Lassell in 1846 and later named Triton (for Neptune’s sea-god son), amazed its discoverer by orbiting the planet backward. Neptune probably captured this moon—a body the size of the planet Pluto—and forced it into orbital submission. The second moon, Nereid (a sea nymph), was discovered and named by Gerard Kuiper in 1949.*
Voyager 2 found six small, dark satellites orbiting near and among Neptune’s dim, dusty, icy rings. These moons—Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus (all named for sea deities)—cause the ring particles to bunch up in disorderly clumps. From a distance, silhouetted against the backdrop of stars, the rings create the illusion of fragmentary arcs because they block starlight on one side of Neptune or the other, but not both. Only on close inspection do the partial curves link up, along thin connecting bridges of material, into complete rings.
Although no spacecraft has visited either ice-giant planet since the 1980s, the pace of discovery at Uranus and Neptune has picked up of late, thanks to observations made from Earth and near-Earth via infrared radiation—the very region of the electromagnetic spectrum that Sir William Herschel discovered in 1800.
Experimenting with thermometers and a prism, Sir William had taken the temperature of Sunlight’s various colors, noting how the mercury rose from violet through red, and continued rising in what he called the “invisible light” or “calorific rays” beyond the red. But he never could apply this important discovery to his own astronomical researches, because water vapor in Earth’s atmosphere—the same feared enemy that made Sir William rub his skin with an onion to ward off the ague while he braved the damp night air—blocks out most infrared emissions from planets and stars.
Orbiting telescopes, however, transcend the interference of atmospheric moisture. From a high perch, 375 miles above Earth’s surface, the Hubble Space Telescope’s infrared camera has followed recent changes on the ice giants. Large ground-based telescopes, too, specially equipped and set at high altitude in Hawaii and Chile, can now collect and amplify the few wavelengths of infrared radiation that do penetrate Earth’s atmosphere. Detailed new time-lapse pictures show a dark hood spreading over bland Uranus’s south pole, as summer slowly draws to a close there, while large bright clouds gather in the northern hemisphere. As the planet progresses to a new season, it turns its thin rings to face Earth edge-on. (Had they not already been discovered in 1977, the rings would avoid discovery now.) On Neptune, the current build-up of bright new clouds over the southern hemisphere progressively lightens the color of the sky.
The planet Neptune, fished from the pool of space as the answer to a dynamical puzzle, repaid the favor of its discovery by posing a new dynamical problem. Early in the twentieth century, the conviction that Neptune alone could not account for all of Uranus’s orbital vagaries (not to mention a few vagaries of Neptune’s own) fomented a “Search for Planet X,” which culminated in the discovery of Pluto.* Recent recalculations, however, prove the mass of Neptune to be sufficient after all. Voyager 2, the only spacecraft to visit Jupiter, Saturn, Uranus, and Neptune, provided precise measurements of the pull that each giant planet exerted on the craft’s own small body. These results forced an upward revision of the mass estimate for Neptune, amounting to one-half of 1 percent, or just enough to render Pluto irrelevant in shaping Uranus’s orbit. As in Miss Herschel’s time, the wanderings of Uranus can still be laid to the presence of Neptune.
But if Uranus begs no further explanation from the Solar System’s outer limits, the moons of Neptune do. The strange orbital patterns of Triton and Nereid point accusingly to origins in the outer depths. Out there, far beyond the precinct of the major planets, and lying just below the current threshold of detection, untold numbers of objects still await discovery.
*Caroline Herschel’s nephew, Sir John Herschel (1792–1871), was president of the Royal Astronomical Society and son of renowned astronomer Sir William Herschel (1738–1822), who discovered the planet Uranus in 1781.
*The comet named for Johann Franz Encke (1791–1865), who became director of the Berlin Observatory in 1825, returns every 3.3 years.
*Sir William first noted what proved to be Uranus on March 13, 1781, three nights before his sister’s thirty-first birthday. On the 17th he confirmed the object’s motion.
*The Reverend Doctor Neville Maskelyne (1732–1811) served as England’s fifth Astronomer Royal, from 1765 until his death. Acknowledging one of Miss Herschel’s several comet discoveries, he called her “My worthy sister in astronomy.”
*The Herschels’ small Georgian house, at 19 New King Street, Bath, England, is now open to the public as The William Herschel Museum. The seven-foot “Uranus telescope,” with its six-inch mirror, resides at the Science Museum in London.
*Johann Elert Bode (1747–1826), editor of the Berliner Astronomisches Jahrbuch, became director of the Berlin Observatory in 1786.
*Analytical chemist Martin Heinrich Klaproth (1743–1817) isolated and named uranium in 1789.
*John Flamsteed (1646–1719) became England’s first Astronomer Royal in 1675, the year the Royal Observatory opened in Greenwich Park.
*In 1845, theoreticians Urbain Jean-Joseph Leverrier (1811–1877) and John Couch Adams (1819–1892) successfully completed their separate calculations showing that a large, exterior planet could account for the irregularities of Uranus’s motion.
*The seventh Astronomer Royal, Sir George Biddel Airy (1801–1892), is remembered for his autocratic direction of the Royal Observatory at Greenwich, and for allegedly depriving England of primacy in the discovery of Neptune.
*Johann Gottfried Galle (1812–1910) later succeeded Encke as observatory director, and lived long enough to witness Halley’s Comet a second time, in 1910.
*Neptune takes 164 years to complete a single orbit—longer than the 66 years of Leverrier’s life added to the 73 of Adams’s, but Galle’s 98 years tip the balance.
†Within weeks of Neptune’s first sighting (on September 23, 1846), amateur astronomer William Lassell (1799�
�1880) of Liverpool discovered its largest moon, Triton, on October 10. Other astronomers confirmed the find the following July.
*Dutch-American astronomer Gerard Peter Kuiper (1905–1973) is generally considered the father of modern planetary science.
*Pluto was discovered by American astronomer Clyde W. Tombaugh (1906–1997), and the finding made public on March 13, 1930.
UFO
My Grandpa Dave, a teen-aged alien, arrived at Ellis Island alone in a crowd, then worked a private eon of man-hours—sewing buttonholes, delivering soda water—to bring his mother, father, and younger brothers light-years over the ocean in his wake. “Mama!” he screamed to her across the packed immigration hall, where health officers had detained her for an eye infection more foreign and unwanted than she. Deportation seemed imminent, but the officials, moved by the emotion of the mother-and-son reunion, instead welcomed Malka Gruber to America.
My mother could never tell this story without crying, as though she had witnessed its embraces, or suffered the threatened exclusion. Even when she became a very old woman, the retelling of that moment long before her own birth would catch in her throat. I, too, yet another generation removed, can turn weepy over it—an empathic response that predisposes me, a recent psychological study has shown, to the creation of false memories, such as the recollection, now prevalent among an estimated three million Americans, of having interacted with visitors from another planet.
The idea that aliens might hail from other planets—as opposed to the “old country” my grandparents and other immigrants left behind—gained credence in 1896. In that year Percival Lowell, scion of the wealthy and privileged Boston Lowells, woke the public to the plight of pitiable Martians who had all but exhausted their global water supply and were husbanding what remained via canals crisscrossing their world.
Lowell had spent his early manhood traveling in Europe, the Middle East, and the Far East, proving his facility with language and flair for explaining foreign ways to Yankees. In preparation for the 1894 close approach of Mars, Lowell indulged his passion for astronomy by establishing a private observatory in Flagstaff, Arizona, free from the control of any academic, military, or government authority. The thirty-nine-year-old Lowell so overextended himself—building, staffing, and equipping the site on “Mars Hill,” then observing the planet from May of ’94 through April ’95, collecting his thoughts and nine hundred drawings into his popular book, Mars, and addressing numerous general audiences during a lengthy lecture tour before hastening to Mexico in ’97 to catch the next Mars opposition—that he collapsed. Lowell’s attack, diagnosed as “severe nervous exhaustion,” disabled him for four years.
When he returned to Flagstaff from Boston in 1901, he found his staff demoralized by the fuss over the canals. Lowell’s sensational conclusions and rush to publication had made Mars Hill a laughing stock among professional astronomers. Although immune to criticism himself, Lowell, who had excelled in mathematics at Harvard, determined to restore his observatory’s reputation by calculating the whereabouts of a ninth planet. Enough discrepancy still disturbed the orbit of Uranus to suggest that the spectacular feats of Adams and Leverrier in the previous century could be repeated, on American soil, to yield a new world beyond Neptune.
Lowell called his quarry “Planet X.” He pursued it enthusiastically, albeit unsuccessfully, until his death in 1916. For the next ten years, Lowell’s widow hamstrung all observatory operations by disputing the intent of his will. The planet search finally resumed in 1929, with a new dedicated telescope situated in a new dome on Mars Hill, and a raw youth—an amateur with only a high school education—hired through the mail to man it.
Clyde Tombaugh, perhaps the most upstanding, hardworking, unimpeachably decent young man ever to leave the wheat fields of Kansas for the astronomical high ground of Arizona, traded his life’s savings for a one-way train ticket to Flagstaff. On an impulse, he had sent his drawings of Jupiter and Mars, as seen through his own homemade telescope, to the Lowell Observatory. The director, impressed, had written back to him, inquired after his health, then offered him the difficult, low-paying job of methodically searching the heavens, inch by inch.
In comparison to Johann Galle, who homed in on Neptune after only an hour’s guided effort, Clyde Tombaugh spent ten months of cold nights in the open dome on Mars Hill, photographing the sky in a meticulous series of hours-long exposures. After he processed the plates, he examined and compared them by pairs with a microscope, scrutinizing the many thousand points of light to see which, if any, had shifted position from one image to the next. Through this tedious process, he located Lowell’s Planet X in mid-February of 1930. The planet was traveling among the stars of Gemini, at a rate that suggested it lay a billion miles past the orbit of Neptune—and just about at the coordinates Lowell had predicted.
Tombaugh’s cautious older colleagues made him confirm and re-confirm his discovery for three weeks before they released an official announcement, with all due protocol, by mailing a detailed circular to every observatory and astronomy department they could name. The world went wild. The Associated Press carried the news by wire, and when the story reached The Tiller and Toiler, the weekly paper of Pawnee County, Kansas, the editor phoned Muron and Adella Tombaugh on their farm in Burdett to ask, “Did you know your son discovered a planet?”
Clyde was twenty-four years old. Having made history, he took a leave of absence from the Observatory to attend Kansas University and earn his degree in astronomy.
A spate of telegrams hit Flagstaff in response to the Pluto news, followed by sacks of mail and soon hundreds of visitors every day. Reporters clamored for photos, but the discovery images no doubt disappointed most expectations. They looked like a pair of ink spatters, differing from each other by the placement of a single spot no bigger than the dot of an “i.”
The best available instruments strained for better views of Pluto, but few could resolve the dim dot into a planet-like disk, let alone discern features on its surface. Indeed, Pluto is so small and so far away that even today, the most detailed portraits obtained with the Hubble Space Telescope reveal merely a bleary sphere in shades of gray, as unsatisfying and lacking in detail as a faked photo of a UFO.
Doubting astronomers in 1930 challenged the claim that Lowell’s Planet X had been found. That planet had promised to exceed the mass of Earth several times over, to be big enough to sway Uranus and Neptune. The newly discovered planet, however, seemed much too insubstantial to tug giants.
Since the 1930s, Pluto has shrunk ever smaller with each new improvement in measuring techniques. Its mass dropped from the original estimate of ten times the mass of Earth to one-tenth the Earth’s mass, to one-hundredth, to about two-thousandths. Meanwhile Pluto’s diameter diminished from an Earthlike 8,000 miles to a mere 1,500 at most. Pluto turns out to be smaller than the planet Mercury, and smaller also than seven Solar System satellites, including Earth’s Moon. Pluto’s own moon, Charon, discovered in 1978, measures half the width of Pluto itself, while most other moons’ diameters are only one-hundredth that of their parent planets.
Pluto’s precipitous size decline over the fifty years following its discovery prompted two planetary astronomers to publish a whimsical graph in 1980, depicting the diminution of Pluto as a function of time, and predicting the planet would soon disappear!
Shriveled and ridiculed, Pluto was altogether stripped of its reason for being after Voyager 2 passed Neptune in 1989. The need for a ninth planet vanished then in the realization that Neptune and Uranus balanced each other’s orbital anomalies. The calculations that had led Lowell to the prediction of Planet X apparently held no more water than his Martian canals. Pluto had entered into popular awareness as the answer to a meaningless question.
In 1992, a small new Pluto-like body turned up on the fringes of the Solar System, followed in 1993 by another five like it, and over the next few years by several hundred more. This outlying population offered Pluto a new identity—if
not the last planet, then the first citizen of a distant teeming shore.
Pluto seemed to be reliving the history of the first asteroid, Ceres. Hunted, like Pluto, on mathematical grounds, Ceres was greeted as the “missing planet” between Mars and Jupiter at the start of the nineteenth century. When continued observation proved Ceres too puny and its type too numerous to rank with the major worlds, astronomers reclassified the lot as “asteroids” in 1802, and later as “minor planets.”
No public outcry attended the application of those lesser terms to Ceres, Pallas, and their companions. Pluto, in contrast, retains an emotional hold on planethood. People love Pluto. Children identify with its smallness. Adults relate to its inadequacy, its marginal existence as a misfit. Anyone accustomed to a quota of nine planets—anyone averse to changes in the status quo—balks at disqualifying Pluto on a technicality.
Even within the six-hundred-member fraternity of planetary astronomers, opinions on Pluto stand angrily divided. Is it a planet or isn’t it? Unfortunately, the word “planet,” coined long before science demanded much specificity of definition, cannot support the many possible gradations of meaning implied by recent discoveries.*
The campaign to drop Pluto from the planet registry, although widely perceived as a shameful demotion, in fact salutes the greater diversity of an expanded Solar System. Pluto and its ilk fill a donut-shaped “third zone” that extends outward from Neptune to at least fifty times the Earth-Sun distance. Since all the objects in this territory differ fundamentally from the terrestrial worlds in the first zone or the gas and ice giants in the second, they have been given their own new designation of “ice dwarfs,” or “Kuiper Belt objects (KBOs).”
The eponymous Gerard Kuiper first conceived of these bodies in 1950. Born and educated in the Netherlands, Kuiper emigrated to the United States in 1933 and became the country’s major proponent of planetary studies, with discoveries ranging from the atmosphere of Saturn’s largest moon, Titan, to new satellites for Uranus and Neptune. Looking to the future, Kuiper forecast that Pluto, the lone outcast of the Solar System, would be found to have hundreds or thousands of fellow travelers. Half a century later, when Kuiper’s myriads began to materialize in the trans-Neptunian deep, astronomers recognized them as his hypothesis come real.
