The Planets

by Dava Sobel

  The focus in astronomy remained the establishment of the planet’s orbit, no matter what its name. We also wondered how “Uranus” had escaped prior detection, for although William had sighted it through a superb telescope, other astronomers easily found it with inferior instruments once he told them where to look. This suggested that old records might yield helpful early notes on the planet’s past positions, set down innocently by observers who had mistaken it for a star. Herr Bode, perhaps because of his dedication to his own yearbooks full of tables, took up this task and was soon rewarded for his effort. He found a sky chart from 1756 that included a star no longer to be seen at its given coordinates. That spot now stood empty, while the trail of the planet Uranus, as far as anyone had succeeded in describing it, would have touched that exact point in that very year. This proved a most agreeable vindication, and sent Bode scurrying to find more ancient mentions of our new planet. Indeed, William had hardly been the first to see Uranus for what it was NOT! The venerable Mr. Flamsteed had listed it in his star catalog of 1690, in the constellation of Taurus the bull.* This was not as happy a match, however, since no one could make Mr. Flamsteed’s star—now missing—mesh precisely with the path of Uranus as we understood it. Some were tempted to dismiss the information from Greenwich, blaming carelessness or an antiquated telescope for the discrepancy. But I was intimately acquainted with the star catalog of Mr. Flamsteed, a renowned observer in his day and the quintessential Astronomer Royal (perhaps even more perfectionist by nature than his successors), and it seemed unlikely he would have erred in his notations. You can imagine the astronomical dilemma: On the one hand, we desperately needed the old data to aid our calculations, since distant Uranus moved so painfully slowly, and no one relished the thought of spending seventy or eighty years to track its motion once around the Sun! On the other hand, if the ancient observations confounded the best current sense of the orbit’s shape, what help could they provide?

  As our new planet continued its strange peregrinations, William built ever larger telescopes. It was through one of these, on a January night in 1787, when our thermometer registered thirteen degrees Fahrenheit, that William discovered Uranus’s two moons. He never proposed proper names for either body, nor for the two satellites he found two years later at Saturn, but my nephew, who was quite the literary scholar before following his father’s footsteps in astronomy (perhaps you are familiar with John’s translation of the Iliad?), named them all. The nomenclature for the Saturn system rests squarely on the Greco-Roman myths, but John selected the Uranian lunar names from Shakespeare! As a well read librarian, Miss Mitchell, you of course recognize Oberon and Titania as the king and queen of the fairies from A Midsummer Night’s Dream, though I believe the allusion has escaped many another astronomer.

  As years passed in watching William’s planet creep about the sky, the deplorable difficulties with its orbit worsened. The more we accumulated new observations, and the more we extracted ancient sightings from observatory files, the less these could be reconciled. It was proving impossible to predict where Uranus would be even a year or two hence, though most astronomers could assess the future whereabouts of Jupiter or Saturn to within a hair’s breadth till the end of time. Thus the great beauty of the great Newton’s contribution seemed tarnished by the recalcitrant behavior of Uranus.

  Things still stood unsettled, sad to say, when William passed away, forty years after the discovery of his planet. I left England at that point, returning to Hanover to live with my brother Dietrich. Neither of us yet realized that William’s life span had equaled the 83.7-year orbital period of Uranus. (Is that not a remarkable coincidence, Miss Mitchell?!) We knew only that the errors in fitting predictions to observations were growing ever more egregious. The last explanation to reach William before he died suggested that a large comet had struck Uranus just prior to its discovery, and the impact had altered the planet’s course. This supposed collision gave probable cause for the rift between the old data and the new, though it seemed almost too imaginative a solution to be believed—more like some device from Shakespeare’s theater, or the Greek tragedy, where gods descend on contraptions to tidy the loose ends of a drama.

  Surely some astronomers welcomed the impact idea, but, shortly after William’s death, the orbital predictions based on a comet collision ALSO proved incapable of mapping our planet’s route. There was nothing left for the mathematicians, I suppose, but to insist upon another large planet lurking undiscovered in the deep, beyond Uranus, pulling it off course. How William would have applauded the diligence that discerned such a world in the mind’s eye, before it came to light in the sky, purely by harnessing the intellect to paper and pencil! What would he say to the news of the two now celebrated young gentlemen, who both independently discovered the same new planet, without either one’s ever so much as putting his eye to a telescope, or even knowing which end of it to peer through?*

  Only think, Miss Mitchell, of the feats of computation required to build an orbit in the air for a body not known to exist. Think of the bewildering array of possibilities that must first be conjured and then tested one by one to induce this hypothetical body, on its hypothetical travels, to assume responsibility for all of Uranus’s waywardness. I have heard it said M. Leverrier covered ten thousand sheets of paper with his figuring, and not for a moment do I doubt that estimate. Mr. Adams cannot have done less. And yet, after such immense labor, with each man persevering unaware of the other’s toil, both of them had to BEG the chief astronomers of their respective countries to point telescopes toward the heavenly vicinity where the proposed planet could be found.

  That the current Astronomer Royal all but ignored the inexperienced, unpublished Mr. Adams is sad, but not difficult to fathom.* M. Leverrier, on the other hand, was already famously distinguished in Parisian scientific associations, and had PUBLISHED his predicted position for the planet, yet he, too, FAILED to gain the cooperation of his national Observatory. (Were you, Miss Mitchell, per chance among the small cadre of independent astronomers who heeded M. Leverrier’s call to action? I understand that several Americans tried to locate the planet according to his directions.)

  The persistent M. Leverrier ultimately succeeded in circumventing official channels via his written request to young Dr. Galle, a SUBORDINATE at the Berlin Observatory. Galle, fresh from his graduate studies, had had the good fortune to observe Comet Halley in 1835, and then the good sense to send his thesis to Leverrier, so that an affinity had developed between them.* (I bring up these details to urge you, Miss Mitchell, always to announce your findings as soon as you possibly can, not just to garner credit for yourself when credit is due, but because our science thrives on shared information.) Galle surely knew he could lose his post for turning the telescope toward Leverrier’s hunch without permission, and he must have appealed with just the right degree of earnestness and obsequiousness to Prof. Encke. Fortunately for all of them, Encke was hurrying home to his own birthday celebration that evening. Had it not been for his rushing out on account of the party preparations, he might have denied his permission.

  Now picture the scene later that night, when Galle and his assistant arrive, breathless and unannounced, at Encke’s house, to tell him they have actually FOUND Leverrier’s planet!! Meanwhile in England, unbeknownst to all, another pair of astronomers seeks the supposed new planet in a SECRET HUNT, which has at long last been authorized by the Astronomer Royal. And where is the grand personage of the Astronomer Royal the night the new planet makes its entry on the stage of the world? Mr. Airy is here in Germany(!) perhaps only a few miles from the road where Galle rushes through the dark with his extraordinary news! Why, the situation has all the elements of a farce, except that it embodies the finest imaginable testimony to the validity of Newton’s laws.

  Following on the heroic mathematics of Adams and Leverrier, and the stunning confluence of their timing, Galle’s espying of the planet through the telescope could be considered almost anti-climactic. I fee
l certain, however, that it will make his career, and that, whatever else he accomplishes in life, Galle will be known forever as the man who first saw Neptune—or “Oceanus” or “Leverrier,” as its name may be, though here we are already happy with “Neptune.”

  My nephew might well have preceded Galle, and made Uranus and Neptune a pair of father-and-son discoveries(!), for in July of 1830 John’s explorations had taken him to the very neighborhood of the sky—almost to the street and house number, if you will—where Neptune was then residing, though he did not knock at that door. John’s good nature kept him from expressing any personal remorse for his oversight, however, and also helped him quell this past year’s awful national sparring between France and England over claim to the Neptune territory. As my nephew informs me, Mr. Adams had pinpointed the planet at least ten months ahead of M. Leverrier, yet told no one except his superior at Cambridge and the Astronomer Royal at Greenwich. As a result of his cautious silence, Mr. Adams is denied the laurels, and although he graciously accepts second place, his countrymen would rather see him decorated a hero. (And not a few of them would send Mr. Airy to the gallows!)

  Yet no rancor, I am told, divides the two key individuals, for Mr. Adams and M. Leverrier established an immediate rapport when they met last June at Oxford, and grew more friendly during a stay at my nephew’s home in July. I suppose the strength of their common obsession unites them. They are as drawn to each other as their planet and my brother’s planet are bound by the laws of celestial mechanics. For a long time each of these men was ignorant of the other, acting independently of the other, just as Uranus and Neptune seemed unaffected by each other when parted by the vast separations which their orbits allow. But soon after my brother discovered Uranus, his planet neared the environs of Neptune, where the two bodies—one in the limelight, the other behind the scenes—together revealed the full force of their mutual attraction.

  With hindsight, it is easy to understand why Uranus began accelerating at an ever-growing rate from about the time of its discovery in 1781 up until it reached conjunction with the UNSEEN and much SLOWER Neptune in 1822. After Uranus overtook Neptune in that year (the same year that death overtook my William), the gradual deceleration commenced, and precipitated the crisis in prediction that brought Adams and Leverrier, each for his own reasons, to consider THE PROBLEM WITH URANUS, which they proved to be THE EXISTENCE OF NEPTUNE.

  Earlier I remarked how the number of William’s years compared to the period of his planet; surely the slow going of Neptune will exceed the lifetimes of Adams and Leverrier combined, and perhaps that of Galle as well.*

  And now the newfound moon at Neptune obliges our two champion calculators in their continuing considerations. How quickly this body has stepped forward, as though to offer itself as the perfect vehicle for refining their necessarily rough estimates of Neptune’s mass.† Neither Adams nor Leverrier could help but overestimate the mass of the hypothetical Neptune, since they both overestimated its distance from the Sun, but, given the way bodies balance mass against distance under the law of gravity, all’s well that ends well, and the smaller, closer Neptune can wield as much power in reality as ever the larger, farther one did on paper. The new understanding reveals Neptune to be the twin brother of Uranus, at least insofar as their mass is concerned.

  How long do you suppose it may take to uncover more facts of their planetary lives, Miss Mitchell? When will we say what metals they churn, what gases they breathe? No doubt forthcoming discoveries in astronomy will require ever larger, ever more powerful telescopes. Even if we are to see brilliant minds intuit the locations of new planets on the strength of theory and calculation alone, will we not need great tools to pry those deduced worlds from the realm of the invisible? The largest of William’s reflectors reached forty feet of length, with a mirror four feet in diameter, but the huge mirror became so often tarnished that William abandoned it for a smaller, more serviceable instrument. My nephew formally decommissioned the forty-foot behemoth a few years ago at Christmas, when he gathered with Margaret and all their children inside the tube, to sing a ballad he composed for the occasion. But I predict clever artificers will soon step forward, perhaps in your lifetime, and devise larger and bolder new designs capable of reaching far beyond the limit of William’s daring, to collect vast oceans of light from space.

  In anticipation of what we may jointly see, and again with profoundest heartfelt congratulations, I remain,

  Yours most sincerely,

  Caroline Lucretia Herschel.

  POSTSCRIPT

  Miss Herschel and her brother always maintained the discovery of Uranus had been no lucky accident, but the fruit of long years spent building a superior instrument and constantly practicing upon it.

  “To make a person see with such a power,” Sir William wrote, “is nearly the same as if I were asked to make him play one of Handel’s fugues upon the organ.”

  When the planet’s rings turned up unexpectedly two centuries later, that discovery, too, was labeled accidental. But it had taken ten astronomers, packed into the cargo bay of an airborne observatory flying over the Indian Ocean, bent on assessing the exact dimensions of Uranus by chasing its predicted passage in front of a star, to encounter such surprises by such luck.

  About half an hour prior to Uranus’s anticipated eclipse of the star, on March 10, 1977, the star momentarily winked. It winked again, several more times, until Uranus completely obscured its light for twenty-two minutes. After the star re-emerged from behind the planet’s disk, it resumed its winking to repeat the same on-off pattern in reverse, as though it had encountered mirror-image obstacles on the other side. Astonished, the astronomers spoke excitedly among themselves of a possibly ringed Uranus even before their historic flight landed, although caution and disbelief delayed their public announcement of the rings for several days.

  Sir William himself had once reported seeing a ring at the planet he discovered, but later retracted the claim as a mistaken perception. He could not possibly have spied Uranus’s ultra-dark, ultra-thin hoops of tightly packed icy rock and dust, even with the best of his excellent telescopes, for the rings reflect too little visible light. They disclosed their presence only by blocking the light of a star, and they remained nine invisible shadows over the ensuing decade, until they could be visited and imaged close-hand.

  The rings naturally circle the planet’s widest part, at the equator. But Uranus, having been knocked over eons ago by the forceful blow of an extremely large impactor, reclines on its equator. As a result, its rings don’t encircle the planet horizontally, the way Saturn’s do, but stand upright, giving ringed Uranus the semblance of a bull’s-eye target hung on the sky. Through this target, like an arrow on a near-miss trajectory, shot the Voyager 2 spacecraft on its January 1986 flyby of Uranus.

  The spacecraft discovered two additional faint rings around Uranus and ten tiny satellites. Astronomers had predicted a large cast of small moons to support the Uranian rings’ sharp borders, and the sudden bounty of real bodies forced them to brush up their Shakespeare. Cordelia, Juliet, Ophelia, Desdemona, and the like thus joined company with Titania, Oberon, and three other previously known moons. Since 1992, advanced Earth-based and Earth-orbiting telescopes have ferreted out still more minor satellites, duly named for Shakespearean magicians, monsters, and minor characters.

  Most of these moons appear as dark as the rings, as though coated with soot. Perhaps the same collision that upended Uranus long ago shocked the chemistry of its carbon-containing compounds, raising enough black dust to sully all the planet’s companions.

  In contrast to the dingy moons and rings, Uranus itself appears a pale blue-green pearl, light and luminescent. Its near twin, Neptune, reveals a more complex beauty in subtle stripes and spots of royal to navy blue, azure, turquoise, and aquamarine. Both planets frost their upper atmospheres with frozen crystals of methane, which absorb the red wavelengths from incoming Sunlight, and bounce the blues and greens back into space
.

  Under those bluish hydrogen-helium skies, neither Uranus nor Neptune knows any solid surface. Instead, their atmospheric gases give way to interior gases that progressively thicken and compress under the mounting pressures at deeper levels, and terminate at the planets’ rock-ice cores.

  Uranus and Neptune constitute their own class of Solar System objects—the “ice giants.” Each one vastly exceeds the mass of Earth (Uranus by a factor of 15, Neptune 17), yet both are dwarfed in turn by the “gas giants,” Jupiter (318 Earth masses) and Saturn (95). The ice giants might have reached their own greater proportions, if only they hadn’t stood in line behind the gas giants at the feast of planetary accretion.

  The “ices” that characterize the deep atmospheres of Uranus and Neptune comprise water, ammonia, and methane. Planetary scientists call these compounds ices because they solidify at cold temperatures. Pressure-cooked inside Uranus and Neptune, the ices no doubt boil as oceans of water-ammonia-methane broth. The hot soup still counts as “ice” in the parlance of planetary science, however, like the “hot ice and wondrous strange snow” of A Midsummer Night’s Dream.