The Planets

by Dava Sobel

  The steadily increasing census of the Kuiper Belt counts Quaoar, Varuna, and Ixion, all discovered in 2001 and 2002, among its larger constituents. Their names reflect a modern ethic of ethnic awareness: Quaoar is the creation force recognized by the Tonga tribe, the original inhabitants of what is now Los Angeles.

  Pluto, the premier object in the Kuiper Belt, follows a steeply inclined and highly elliptical orbit. Over a period of 248 years, Pluto alternately soars above the plane of the Solar System and dives below it, strays out to almost twice Neptune’s distance from the Sun at one extreme and ducks inside the orbit of Neptune at the other.* This wandering path, so different from that of any other planet, helped brand Pluto as an oddball from its earliest days. By the standards of the Kuiper Belt, however, the orbit appears common. Some 150 other Kuiper Belt objects trace the same course, and they all avoid collision with Neptune thanks to the resonance agreement among them: Neptune circles the Sun three times in the time it takes Pluto and company to go around twice. When Pluto trespasses into Neptune’s orbit, it does so always at the height of its swing, leaving Neptune far below and at least a quarter-turn away.

  Pluto spins around its own axis once every six days, rotating the dim splotches of its vague landscape in and out of view. Like Uranus, Pluto lies on its side, the victim of a prior collision. Indeed, planetary scientists believe that a single impactor knocked down Pluto and chipped off its moon Charon in one blow.

  Pluto and Charon, only about twelve thousand miles apart, lock each other in orbit around a point partway between them. They both rotate at the same pace while jointly circling this point, so that each keeps the same face always turned toward the other. The uniqueness of their orbital engagement has recast Pluto and Charon as “Pluto-Charon,” the first known example of a true “double” or “binary planet.”

  Less than a decade after Charon’s discovery, Pluto and Charon oriented themselves in space so as to take turns eclipsing one another, as viewed from Earth. Such a fortuitous arrangement can occur only twice during Pluto’s orbit, or once every 124 years. Beginning in 1985, astronomers took advantage of the numerous mutual occultations to derive the best possible approximations of the two bodies’ mass, diameter, and density. At about twice the density of water, both Pluto and Charon are more dense than any of their gaseous giant neighbors, though not half so dense as the iron-rich terrestrial planets Mercury, Venus, and Earth.

  Perhaps two-thirds to three-quarters of Pluto consists of rock, and the rest ice. Above Pluto’s bedrock of water ice, patches of frozen nitrogen, methane, and carbon monoxide have been identified from afar. When Pluto warms itself inside Neptune’s orbit for two decades every two centuries during its nearest approach to the Sun, ices on the planet’s surface partially evaporate to form a puffy, rarefied atmosphere. Later, as Pluto recedes from the Sun and its temperature drops back to a frigid normal (about two hundred below zero Centigrade), the atmosphere falls down and coats the ground, especially around the poles, with fresh, exotic snow. In this regard Pluto behaves somewhat like a comet (which would also heat up and blow off icy gas upon nearing the Sun), though it remains too distant to create any great display.

  By the time the Sun’s light reaches Pluto, distance has dimmed it a thousand-fold, so that the Sunlit planet in daytime resembles a winter evening by Moonlight. On Pluto’s reflective landscape, bright surface frosts coexist with dark areas that may represent rock outcrops or deposits of organic compounds extorted from the ice by the Sun’s ultraviolet light. Polymers in carbon-rich colors—pink, red, orange, black—probably proliferate on Pluto.

  Despite the Pluto-Charon similarity in composition and the pair’s shared common origin, the moon’s smaller mass and lower gravity cause it to lose its grip on gases. Molecules vaporized from Charon’s surface do not hover aboveground waiting to return as snowflakes; they simply escape into space. As a result, Charon reflects considerably less light than Pluto, and its surface will most likely appear dull-neutral in photographs when the binary worlds of Pluto-Charon are eventually visualized by a visiting spacecraft.

  All past attempts to mount a mission to Pluto failed at the funding stage—before any craft could reach the launch pad, much less begin the long journey. Now, after the disappointing cancellations of projects such as “Pluto Express” and “Pluto Fast Flyby,” Plutophiles finally have a scout being readied for the Kuiper Belt. NASA’s minimalist “New Horizons,” equipped to map and image Pluto, Charon, and at least one other KBO at close range, should see its promised lands in 2015. By then, the number of known KBOs may have increased exponentially, from the several identified to date, to the hundreds of thousands more anticipated.

  Already the demographics of the Kuiper Belt hint at great waves of migration that characterized early Solar System history. All the KBOs, it seems, were exiled to their present locations, from positions closer to the Sun, at the time the giant planets were completing their own accretion. Jupiter and Saturn swallowed some small planetesimals in their vicinity and accelerated many more with such force that the bodies were banished from the Solar System. While Uranus and Neptune also participated in this planetesimal diaspora, they lacked the power to hurl objects entirely beyond the Sun’s reach, and relegated them instead to the Kuiper Belt.

  As a result of these displacements, Jupiter lost some of its orbital energy and moved in closer to the Sun. Saturn, Uranus, and Neptune, in contrast, gained energy and edged farther away. Pluto, which is thought to have occupied a round, regular orbit at this early stage, was shoved outward by the gravitational influence of Neptune. Over tens of millions of years, Neptune forced Pluto, the ultimate expatriate, to follow an ever more tilted, more elliptical course.

  Pluto and the other Kuiper Belt residents have thus been worked over by events in the Solar System. Although scientists had hoped the Kuiper Belt might preserve pristine material, unchanged since the formation of the Sun, they now see it as a war zone where bodies have been deposited and left to fray each other. The true, untainted genealogical roots of the solar family must be pursued at a still further remove.

  Today, ever more distant worldlets are swimming into view beyond the Kuiper Belt. The planetoid Sedna, discovered in 2003 and named for the Inuit goddess of the icy sea, is currently the coldest, most distant known member of the Solar System. About half the size of Earth’s Moon, Sedna seems to ply an orbit that reaches to nine hundred times Earth’s distance from the Sun, and that takes ten thousand years to complete.

  Farther on, between the dim body of Sedna and the bright spectacle of the distant stars, astronomers expect to encounter a spherical swarm of trillions more small objects surrounding the Solar System. Among these frozen leftovers of creation lie perhaps the profoundest answers to the question of where we came from.

  The outlying ancient debris distributes itself over such a distended area that the Solar System’s periphery is transparent as a crystal ball. Through the bubble of its outer boundary we can see forever—across the Milky Way home of our Sun, into the other galaxies that twirl like pinwheels strewn across the Universe, their many billion stars frothing with planets.

  Sometimes the stupefying view into deep space can send me burrowing like a small animal into the warm safety of Earth’s nest. But just as often I feel the Universe pull me by the heart, offering, in all its other Earths elsewhere, some larger community to belong to.

  *Like “planet,” the word “life” poses similar difficulties for astrobiologists: A wildfire, for example, exhibits lifelike behavior as it takes in oxygen, grows, moves, consumes, even generates new fires with its own sparks, but it is not “alive.”

  *Pluto last dipped within the orbit of Neptune in 1979 and emerged in 1999. At perihelion, in 1989, Pluto lay approximately one billion miles closer to Earth than it had been when discovered in 1930.

  PLANETEERS

  There was a big party at Andy Ingersoll’s house in Pasadena the night after the Cassini spacecraft flawlessly inserted itself into orbit around
Saturn in the summer of 2004. The music and dancing, the food and drink, the camaraderie were really intended for the scientists and engineers whose years of work had led to such happy cause for celebration, but a few outsiders standing in the right place at propitious moments had also been invited.

  When I arrived, too early, I found our host, a senior and much venerated planetary scientist at the nearby Jet Propulsion Laboratory, fabricating a model Saturn to hang at the driveway as a location marker for the couple hundred guests. He had an old red tether ball with the cord still attached to it, and there on the cleared kitchen table he was cutting poster-board rings of the proper proportion to tape in place around it. A colleague came in through a back door and casually began offering technical advice, as though the current prank were some new research challenge. Within minutes, they had Saturn on a string, dangling from a branch.

  Ingersoll, tall and bony, excels at modeling planetary atmospheres. He works the data points collected by telescopes and spacecraft—temperature readings, gas abundances, fluid pressures, wind speeds, cloud patterns—into sophisticated weather analyses. His journal publications have titles such as “The runaway greenhouse: A history of water on Venus,” “Dynamics of Jupiter’s cloud bands,” and “Seasonal buffering of atmospheric pressure on Mars.” He could likely match wits with any of the most famous astronomers in history, but he is unlikely to dominate the future, the way a Cassini or a Huygens persists today, because the nature of science itself has changed, from a field for lone geniuses to a collaborative effort.

  The ebullient early-bird volleyball game in the Ingersolls’ backyard ended about half an hour later, when caterers came to lay out the long buffet and set up tables and folding chairs around and under the trees. In the group I happened to sit with, half the people were speaking Italian, and the other half a British-accented English. The party grew steadily more multinational because the Cassini spacecraft is global in every way. As the joint project of NASA, ESA (European Space Agency), and ASI (Agenzia Spaziale Italiana), Cassini represents seventeen countries and the pooled talents of some five thousand individuals, including a team of seamstresses who custom-tailored the spacecraft’s golden lamé thermal suit, to protect its instruments from dust-sized micrometeoroids and the extreme cold of the near-Saturn environment.

  Each wave of latecomers to the party brought fresh bulletins from the lab. Some of them hadn’t slept in days, and looked it, but they relished the cause of their exhaustion. The news from Cassini, chattering into the Deep Space Network’s receivers in Spain, Australia, and California, was all good. Ideal, in fact. The craft’s first close-up pictures of Saturn’s rings exposed such depth of exquisite detail that one astronomer had accused another, farther up in the data stream, of doctoring the images as a practical joke.

  The adrenaline rush that most of these men and women had experienced the previous evening during Cassini’s passage through Saturn’s rings now mellowed into a general euphoria, a veritable Saturnalia. As the revelers toasted the present success, they also hailed the mission’s next major phase—the delivery, six months hence, of Cassini’s robotic passenger, the Huygens probe, to Saturn’s largest moon, Titan. That grand satellite, a body bigger than Mercury or Pluto, and possessed of a thick orange atmosphere as rich in nitrogen as our own air, had long intrigued scientists for its promised insights into conditions on the early Earth before life began. No one yet knew what lay on the smog-obscured surface of Titan, but many scientists were willing to wager great lakes filled with chill liquid methane and other hydrocarbons.

  “I dream of landing in an ocean,” Huygens project scientist Jean-Pierre Lebreton had said the day before the party at a press briefing. “To go to Titan now is like going back in time to Earth four billion years ago.”

  From the moment Christiaan Huygens first saw Titan from The Hague in 1655, he called it simply “Saturn’s moon.” Jean Dominique Cassini, who found four other Saturnian moons between 1672 and 1684, was content to refer to them by number. And when Sir William Herschel sighted the next two in 1789, he, too, applied numerical designations. But Sir William’s son, Sir John Herschel, chose names for them all from Greek mythology, beginning with “Titan,” an ancient race of giants, the youngest of whom was Saturn.*

  * * *

  IN DECEMBER 2004, on schedule, Cassini released the Huygens probe it had carried on its seven-year journey from Cape Canaveral, and nudged it toward Titan. For the next three weeks Huygens, still asleep, obediently coasted to its rendezvous, while Cassini executed another long loop around Saturn and returned in time for the planned excitement.

  On January 14, 2005, Huygens’s internal alarm woke its systems to prepare for action at Titan. The probe hit the atmosphere with its heat shield forward, decelerated in the friction of the thick air, and parachuted to a perfect landing. It sampled the clouds and haze all during its two-and-a-half-hour descent, and when it got close enough to the moon’s frigid surface (about thirty miles, as measured by the onboard radar) it photographed that, too, then relayed its findings to Cassini, and Cassini forwarded them to Earth.

  On Titan, Huygens saw sights as familiar as clouds changing shape, as strange as the novel landscapes of an alien world, too unusual to be parsed.

  The fact that Huygens survived touchdown and continued to broadcast evidence of its own robust health for several hours upset the widespread expectation of its drowning in a methane sea. However, the great dark expanse where Huygens laid itself to rest, now called Xanadu, cannot be viewed as the site of a failed prediction. Rather, it is the embarkation point for another new way of imagining the content of the Solar System, and of other solar systems as well. In July 2006, Cassini located several of the long-sought hydrocarbon lakes near Titan’s north pole.

  I wish I could tell you what happens next, how the interpretation of the Huygens data will play out, what Cassini will encounter as it sweeps by this or that Saturnian satellite—Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus—on the busy itinerary of its ongoing exploration. But what book can keep abreast of current events in an active field of study? If reading these pages has helped someone befriend the planets, recognizing in them the stalwarts of centuries of popular culture and the inspiration for much high-minded human endeavor, then I have accomplished what I set out to do.

  For myself, I confess that none of the truly staggering facts I have been privileged to share here has altered the planets’ fundamental appeal to me as an assortment of magic beans or precious gems in a little private cabinet of wonder—portable, evocative, and swirled in beauty.

  *Later astronomers followed suit up through Pan, the eighteenth satellite of Saturn, discovered in 1990. The next twelve moons, including Mundilfari and Ymir, received names from broader cultural contexts. Since Cassini arrived at Saturn, at least sixteen additional satellites have been found.

  Acknowledgments

  Thank you to all the scientists and advisors who gave me such generous portions of their time, or enthusiasm, or both: Diane Ackerman, Kaare Aksnes, Claudia Alexander, Mara Alper, Will Andrewes, William Ashworth, Victoria Barnsley, Jim Bell, Bob Berman, Rick Binzel, Bruce Bradley, William Brewer, Joseph Burns, Donald Campbell, John Casani, Clark Chapman, K. C. Cole, Guy Consolmagno, Lynette Cook, Kathryn Court, Dave Crisp, Jeff Cuzzi, David Douglas, Frank Drake, Jim Elliot, Larry Esposito, Tony Fantozzi, Timothy Ferris, Jeffrey Frank, Lou Friedman, Maressa Gershowitz, George Gibson, Owen Gingerich, Tommy Gold (died 2004), Dan Goldin, Peter Goldreich, Donald Goldsmith, David Grinspoon, Heidi Hammel, Fred Hess, Susan Hobson, Ludger Ikas, Torrence Johnson, Isaac and Zoe Klein, E. C. Krupp, Nathania and Orin Kurtz, Barbara Lebkeucher, Sanjay Limaye, Jack Lissauer, Rosaly Lopes, M. G. Lord, Stephen Maran, Melissa McGrath, Ellis Miner, Philip Morrison (died 2005), Michael Mumma, Bruce Murray, Keith Noll, Doug Offenhartz, Donald Olson, Jay Pasachoff, Nicholas Pearson, Elaine Peterson, David Pieri, Carolyn Porco, Christopher Potter, Byron Preiss, Pilar Queen, Kate Rubin, Vera Rubin, Carl Sagan (died 1996), Lydia Salant, Carolyn Sc
herr, Steven Soter, Steve Squyres, Rob Staehle, Alan Stern, Dick Teresi, Rich Terrile, Peter Thomas, John Trauger, Scott Tremaine, Alfonso Triggiani, Neil deGrasse Tyson, Joseph Veverka, Alexis Washam, Stacy Weinstein, Joy Wulke, Paolo Zaninoni, and Wendy Zomparelli.

  Two people truly championed this project and guided it to its present form: Michael Carlisle of InkWell Management, my wonderful agent, by wanting to know the difference between the Solar System and the Milky Way, and likewise between the galaxy and the universe; and Jane von Mehren, former editor-in-chief and associate publisher at Penguin Books, who responded to my manuscript with dozens of astute questions and hundreds of helpful suggestions, all tendered with patience and wisdom. Michael and Jane would not have considered themselves “planeteers” at the outset, but now that we have made this journey together, they both look to the night sky much more often than before.

  Glossary

  Apogee—the greatest distance from Earth reached by the Moon in its monthly orbit, or by an artificial satellite circling our planet.

  Apparent Magnitude—the brightness of a heavenly body as viewed from the vantage point of Earth, expressed as a number; the lower the number, the brighter the object appears. (The Sun, with an apparent magnitude of –27, is the brightest object in Earth’s sky, though if it were judged according to its intrinsic brightness, or absolute magnitude, it would pale in comparison to larger stars.)