The Planets

by Dava Sobel

  In May 1919, the rings were tipped toward Earth to Saturn’s artistic advantage. Approximately once every fifteen years, or twice during Saturn’s 29.5-year orbit of the Sun, the rings turn edgewise to Earthly admirers, and withdraw their flattering light. Even in telescope views, all that can be seen of the rings at such times is a thin shadow line across the planet’s yellowish globe. Such periodic disappearances confounded the rings’ earliest observers.

  Galileo, the first to glimpse bulges alongside Saturn in July 1610, mistook them for a pair of close “companions,” which did not move about like Jupiter’s satellites, but hugged the planet’s flanks to make it appear “triple-bodied.” Monitoring Saturn over the next two years, Galileo confessed astonishment in late autumn of 1612 to find the planet suddenly solitary and round, deserted by its erstwhile supporters. “Now what is to be said about such a strange metamorphosis?” he wrote to a fellow philosopher. Perhaps the planet Saturn, like its mythological counterpart, had “devoured his own children?”

  Galileo predicted the companions would return, and when they did they were much altered. In 1616 he said they resembled a pair of handles on Saturn, and later he likened them to ears, though he never grasped the fantastic nature of their true identity. Not until 1656 did Dutch astronomer Christiaan Huygens lay the changing shape of Saturn to the existence of “a broad, flat ring, nowhere touching, and inclined to the ecliptic.” Huygens published a full explication in his book, Systema Saturnium, in 1659.*

  Christiaan Huygens prepared this diagram for the 1659 publication of Systema Saturnium to show how Saturn’s appearance, over the course of its 29.5-year orbit, alters in the eyes of Earthly admirers.

  Huygens always spoke of “Saturn’s ring” as a single solid entity, and so it was thought to be until 1675, when Jean-Dominique Cassini, director of the Paris Observatory, detected a dark dividing line that split the ring into two concentric lanes, dubbed “A” (for the outer one) and “B” (the inner and brighter). The passage of another two centuries yielded a third segment—the dim interior “C” ring, discovered in 1850—though still no one could ascertain how any of the rings was made. Embattled opinion on ring structure swayed from solid sheets to swarms of small satellites to rivers of orbiting liquid and exhalations of planetary vapors.

  “I have effected several breaches in the solid ring,” young James Clerk Maxwell of Scotland boasted in 1857 from the midst of his mathematical calculations, “and am now splash into the fluid one, amid a clash of symbols truly astounding.” Convinced that Saturn’s gravity would shatter a solid construction of such large dimensions, Maxwell construed the rings as a profusion of individual particles so numerous as to create the far illusion of solidity. Each particle would perforce pursue its own orbit, according to Kepler’s laws, with the particles farthest from Saturn traveling at the slowest speeds, and the nearer ones the more quickly, just as Saturn itself proceeded ponderously about the Sun compared to the rapid pace of Mercury. (What choruses Kepler might have scored for these multitudes!)

  Within the crowded rings, particles constantly jostle their neighbors, bumping each other into wider or narrower orbits by the exchange of energy and momentum. Collisions also fling particles above and below the flat plane of the rings, but such strays are quickly whipped back into line.

  Since 1966, four more rings, designated D through G, have joined Saturn’s classical A, B, and C rings. As a group, they flout the alphabetical order of their discovery in their progression outward from Saturn—D, C, B, A, F, G, E—like notes on a practice scale. Each lettered region distinguishes itself from the others by slight color or brightness variations, or the density of its particles, or its unusual shape. When seen from the privileged perspective of a visiting spacecraft, the lettered segments further resolve into myriad slender ringlets, separated by as many tiny gaplets, and patrolled by embedded moonlets.

  The ring system probably formed from the break-up of an icy moon, or perhaps a captured planetoid, some sixty miles in diameter. That hapless body, destroyed a few hundred million years ago, may still be striving to reassemble itself in Saturn orbit. As its particles gravitationally attract one another and cling together, they form larger aggregates that pull in additional particles to keep on growing bigger, but only up to a point. Any accreting ring body that exceeds certain size limits gets torn apart by Saturn’s tidal forces, and so the scattered bits seem destined never to reunite into a single satellite.

  Earth’s Moon, which passed through a similar stage as a ring of collision debris, nevertheless cobbled itself together because its pieces orbited far enough from our planet to escape the destructive effects of tidal forces. At Saturn, the rings huddle close. They occupy a nearby region of perpetual fragmentation known as the Roche zone, named for the nineteenth-century French astronomer Edouard Roche, who formulated the safe distances for planetary satellites. The larger moons of Saturn all lie well beyond Roche’s limit, outside the perimeter of the rings. However, Saturn’s extended family (more than forty moons at last count) includes many small members in and among the rings that help sculpt their intricacies. The F ring, for example, owes its peculiarly twisted and narrow outlines to the action of two accompanying moonlets, one of which runs rapidly along the inside of the ring while the other laps its outside. Together they work as “shepherd” satellites, herding the flocks of particles between them into clumps, knots, braids, and kinks.

  When the Cassini spacecraft reached Saturn in the summer of 2004, it trumpeted its arrival by soaring up through the gap between the F and G rings, skimming across the broad expanse of the ring plane, and then diving back down through the far side of the same gap, where it emerged unscathed. The relative emptiness of such spaces results from the interplay of Saturn’s satellites with particles in the rings, following the same rules Pythagoras defined in his experiments with strings.

  Pythagoras had shown how the pitch of a string rose an octave when he shortened its length by half. Playing strings of these two lengths together pleased the ear, he said, because their vibrations resonated in the whole-number relationship of 2:1. Other whole-number relationships, or resonances, yielded other felicitous musical intervals, such as thirds, fourths, and fifths. Galileo, commenting on the effects of sympathetic vibrations in his book Two New Sciences, judged the octave “rather too bland and lacks fire,” while the sound of a 3:2 resonance (the musical interval of the fifth) caused “a tickling of the eardrum so that its gentleness is modified by sprightliness, giving the impression simultaneously of a gentle kiss and of a bite.”

  The most notable resonance effect in the rings of Saturn is the Cassini Division—the three-thousand-mile-wide separation between the A and B rings. The Division derives from its 2:1 resonance with the moon Mimas, orbiting more than forty thousand miles away. Ring particles within the Cassini Division travel twice around Saturn to Mimas’s once, and so they repeatedly overtake the slower-moving moon at precisely the same two points in their orbit. There they gravitate toward it. Eventually the pull of the moon, boosted by rhythmic repetition, boots the particles out of the resonant orbit, clearing the gap. A similar but narrower gap near the outer margin of the A ring, called the Encke Division (for Johann Encke, a former director of the Berlin Observatory), shares a 5:3 resonance with Mimas and a 6:5 resonance with another moon. Also the decorative scalloped border on the outer edge of the A ring owes its six petal-like lobes to a 7:6 resonance with two small satellites that occupy a single orbit and may once have been a single object.

  The rings resonate also to the beat of Saturn’s rapidly rotating magnetic field. Generated within the planet’s liquid-metallic-hydrogen interior, the magnetic field spins in time with Saturn’s rotation every 10.2 hours. Particles in the B ring traveling just that fast—or half as fast, or twice as fast—are consequently driven from their orbits.

  Saturn reigned as the lone ring world for three hundred years, until the discoveries of the 1970s and 1980s showed that all the giant planets bear rings of some k
ind. Jupiter has tenuous, transparent “gossamer” rings consisting of dross flaked off the surfaces of several small moons. Uranus owns nine dark, narrow rings with sharply defined borders constrained by shepherd satellites. And Neptune’s five faint, dusty rings are so irregular in thickness that some sections thin almost to nothingness, leaving the impression of partial ring arcs. None of these recently recognized ring systems can really compete with Saturn’s baroque, even rococo rings. Rather, each of the others portrays a single nuance of ring dynamics—some phenomenon present at Saturn as well, but drowned there in the volume of variations and embellishments.

  All the rings undergo constant change through repetitive rounds of build-up and breakdown. From year to year, they are the same yet not the same. As they fray and wear away in the friction of internal collisions, new infusions of moon dust and infalling meteorites replenish the particle supply.

  Each ring system, the product of gravity and harmony, suggests a template for cosmic design. Rings recall the birth of our whole family of planets, which arose from the flat, spinning disk that surrounded the infant Sun five billion years ago. Rings also find frequent echo now in the so-called protoplanetary disks discerned around distant young stars, where the raw materials of gas and dust are uniting in new world syntheses. Saturn’s rings thus link our Solar System to other, extrasolar systems in the making, and the present Solar System to its own ancient past.

  “Music,” as Holst observed in a letter to a friend, “being identical with heaven, isn’t a thing of momentary thrills, or even hourly ones. It’s a condition of eternity.”

  *Paul Hindemith’s 1956–57 opera, Die Harmonie der Welt (The Harmony of the World), dramatizes Kepler’s work on the planetary order.

  *Both Galileo and Huygens were able lute performers and friendly with many composers. Huygens also experimented with a 31-tone equal-temperament scale that influenced the music of the Netherlands into the twentieth century.

  NIGHT AIR

  The Herschels worked a great many years. Sir William Herschel’s papers, published in various scientific journals, stretch through a period of forty years. Sir John Herschel’s reach through a period of fifty-seven years—about twice the average length of life. Sir William Herschel died at eighty-three, Sir John at seventy-eight; and, as if to show that a woman can live and work even longer than a man, Caroline, the sister of Sir William, died at ninety-eight.

  Is it worth while to talk about the unhealthiness of “night air” when that class of people who are most exposed to its influence, whose calling keeps them breathing it, are so long-lived? For the work of the practical astronomer is mainly out-of-doors and in good night air, instead of indoors in bad air. (I think it is Florence Nightingale who asks, What air can any one breathe in the night except night air?)

  —Maria Mitchell, American astronomer (1818–1889)

  Hanover, Germany, November———, 1847

  My Dear Miss Mitchell,

  Please accept my most excited congratulations on your recent discovery. Word of “Miss Mitchell’s Comet” had already reached me from several sources here on the continent before your letter arrived, as well as from my nephew* in London, but how delighted I am to know that you thought of me in your hour of glory, and took the time to share your triumph with an old woman. Indeed, as you say, you and I enjoy a special bond. Even though my own telescope is now the chief ornament of my sitting room, it let me watch many comets come out of the dark, looking dull and plainly clad at first, but growing as they approached, until, nearing the Sun, they sprouted their great fuzzy caps and spread the tails that are the peacocks of the cosmos.

  I am particularly gratified to hear the new comet will keep your name, Miss Mitchell, because such fame will secure your future as nothing else could do. One of my comets took Professor Encke’s name, after he computed its orbit and predicted its return.* That still left me seven other “lady’s comets,” though I had no need of any, what with my brother’s name about me like an aegis, plus a royal pension as his assistant. You, however, are a young woman alone in a young country, and the discovery of your comet surely surpasses your employment at the Nantucket Library, both in terms of assuaging your family’s concerns for your welfare AND rousing the world’s regard for your abilities.

  Just the way your father, bless him, has encouraged your pursuits, so did my brother support me in mine, although I suppose the more correct assessment is that he trained me because he required an adept assistant willing to struggle long hours at his side as no hired, indentured, or enslaved help would do. The irony is that while I became William’s right arm in his astronomical investigations, and kept all the official nightly records, I was ABSENT that particular evening during the week of my birthday when he discovered the “comet” we are now pleased to call the planet Uranus.*

  William was not seeking a planet, of course, for we held it almost an article of faith that only six planets orbited the Sun. When his sweeps of the heavens turned up something blurred or indistinct, something that stood out from the stars’ points of light, he naturally wondered whether he had come upon a new comet he could claim as his own, or someone else’s comet on a return visit, or one of the more mysterious nebulous objects that so engaged his attention.

  You, Miss Mitchell, have savored the promise of first sighting such a possibility, and have passed your own anxious hours till the next cloudless night when you could turn your eye to the same spot of sky, your heart full of hope that your blur had not stayed put where you left it, but rather strayed among the stars, to testify by its movement, “I am a comet, yes, and because you caught me, perhaps I may be yours!”

  Dr. Maskelyne was first to confirm William’s find, though he declared it the oddest comet he ever did see, sans tail, sans coma, and in possession of a disturbingly well-defined disk. I think he suspected even then that William had found a planet and not a comet, which is a remarkable thing for an Astronomer Royal, truly, and the good Dr. Maskelyne was not prone to creative leaps.* Of course, the job of Astronomer Royal does not require imagination; it requires exactitude in the mapping of the heavens, at which Dr. Maskelyne excelled, and yet he seemed ready and willing to jump to a new planet. Who would have guessed it of him?

  It was he who urged William to write a paper for the Royal Society, which William titled simply “Account of a Comet.” One of the members of the Society read it aloud at the April meeting in London, while we remained in Bath, for William was still engaged full-time as organ master at the Octagon Chapel, and moreover the inadvertent discovery of the “comet” had interrupted a busy program of stellar observing and measuring separations between double stars. Though we declined to go to London, soon most of London came to us, to see our little house with its basement workshop and the seven-foot telescope in the garden.*

  Shortly after William’s comet aroused our interest, it took a summer holiday, spending several months out of sight in the daytime sky, so that no one could amass the observations required to establish its orbit. When it returned at the end of August, we—and here William and I were joined by I daresay half the astronomers in Europe, not to mention Russia—all fixated upon it. Night after night we strove to fit our observations along a typical comet’s parabolic path, while the object refused to obey our rules, and would move stubbornly in a circular arc. All through the autumn it failed to brighten for us; it denied us the delight of seeing it flash its tail. By November the truth finally dawned: The comet was a planet at twice the distance of Saturn!

  As I have taken pains to explain to you, Miss Mitchell, our “Eureka!” moment followed the detection of the body by more than half a year. William had uncovered one thing that proved to be quite another. When the magnitude of his feat shone clear, and word spread that he had single-handedly doubled the width of the Solar System with this distant planet, King George offered his official protection, including a handsome stipend of about two-thirds Dr. Maskelyne’s salary. The planet could not have arrived at a more propitious moment, give
n the crown’s recent loss of the Colonies in America.

  William, universally hailed as the first man in history to discover a planet, ceased offering music lessons and performing in concerts to become a full-time astronomer. In France some people campaigned to name his new find “Planet Herschel,” just as yours is “Comet Mitchell.” Men who had never before heard of William conceded that his homemade telescopes had put the tools of every great observatory to shame. Not a visitor left our home unshaken by what William had built with his own hands and at his own expense. Hardly a congratulatory letter arrived that did not include a request for William to sell the writer an instrument.

  None of the adulation turned William’s head, however, and he would hear none of “Planet Herschel.” We both felt that while it was all well and good for comets to carry their discoverers’ names—since this practice had precedent in our field, and the number of comets might be legion—the naming of a planet, being such a much rarer occasion, begged for different criteria.

  William suggested “Georgium Sidus,” to acknowledge the King’s kindness, though it was quickly pointed out that any reference to national allegiance might be misplaced in a heavenly body. Many other names came forward before the idea of “Uranus” occurred to Herr Bode in Berlin, who sought safety in mythology.* Bode published an annual ephemeris, which gave him an influential voice in such decisions, but even so, the planet answered to three names—“Uranus” in most of Europe, “Herschel” in France, and “the Georgian” in England—for SIXTY YEARS before “Uranus” gained currency. In that interim, a prodigious chemical experimenter—another German, named Klaproth—extracted a metal from pitchblende and called it “Uranium.” He told us that alchemists of old had always given planet names to their metals, and he thought the new planet deserved to have one christened in its honor.*