The Planets (17 page)

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.
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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,

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.

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.
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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.
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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.