One of Percival Lowell’s drawings of the Mars “canal” system.
(From Mars as the Abode of Life, Lowell 1908)
Serious astronomers were aghast at Lowell’s certainty. Prestigious scientific journals refused to publish his findings. William Wallace Campbell, then director of the Lick Observatory (the Lowell Observatory’s chief competitor), called Lowell “a trial to sane astronomers.” Many other observers were not seeing the same Martian features, and with good reason. “From Earth,” University of New Mexico geographer K. Maria Lane has noted, “the surface of Mars was (and still is) notoriously difficult to make out. Even under excellent conditions for ‘seeing,’ Mars shimmered tantalizingly, allowing only fleeting glimpses of its surface.” Lowell had clunkily collated his overall map from dozens of sketches of individual Martian regions, each glimpsed in a flash. A new method of planetary photography, which his observatory introduced in 1905, didn’t help his case; a few dark markings were seen, but not a globe-spanning canal system.
The public and the popular press, however, reveled in Lowell’s story—so much so that by 1907 the Wall Street Journal reported that evidence for the existence of Martian folk surpassed that year’s financial panic as the news story of the year.
That media endorsement, though, was Lowell’s last hurrah. Within a few years, making further observations with larger telescopes, astronomers generally concurred that Lowell’s canals were merely an optical illusion—the eye imposing linearity upon an array of smaller, irregular details. The Boston Brahmin’s exotic imaginings lingered long after his death in 1916 at the age of sixty-one but were finally put to rest (once and for all!) when a series of Mariner missions, launched by NASA in 1965 and 1969, showed Mars to be a completely barren world, pitted with craters.
Intriguingly, though, Mars orbiters later photographed ancient riverbeds with tributaries and erosion patterns that appeared to be carved by catastrophic flooding episodes. Probes now roaming over the Martian landscape confirmed those observations. Perhaps Lowell would have been elated that there were Martian channels after all. But these passages were forged by water flowing naturally, and in Mars’s distant past rather than in the present day. In the end, to Lowell’s likely dismay, there were no little green men digging trenches.
CHAPTER FIVE
Rings, Rings, Rings
Finding that planets could have rings
TO see this amazing celestial feature in person, hop a spaceship and rocket toward the Scorpius and Centaurus constellations. After traveling a distance of some four hundred light-years, you’ll come upon an astounding sight—a ringed planet that makes Saturn’s rings look scrawny by comparison.
Several years ago, astronomers had observed this planet’s sun, known simply as J1407, undergo a complex series of eclipses. Over the course of fifty-six days, the star’s light brightened and dimmed erratically. What could be causing such fluctuations? Astronomers from both the University of Rochester and Leiden Observatory in the Netherlands have suggested that those repeated eclipses were due to the transit of a giant ringed planet orbiting the star.
And not just any ringed planet. According to their model, this exoplanet’s rings extend outward for some 56,000,000 miles (90,000,000 kilometers). Such a disk would be quite a sight if it resided in our solar neighborhood, as its radius is more than half the distance from the Sun to the Earth. Saturn’s most prominent rings reach out a mere 175,000 miles (282,000 kilometers) from the planet’s equator. This colossal ring system is one of the first suspected to reside outside our solar system. And its discovery was announced nearly four centuries after Saturn’s planetary hula hoop was first recognized for what it was.
What J1407b’s ring system would look like
at dusk in the skies above Leiden University in the
Netherlands, if it were in Saturn’s orbit.
(M. Kenworthy/Leiden)
As with so many seminal moments in astronomy, the long path toward understanding that a planet could even be surrounded by a ring began with Galileo. With his publication of Sidereus nuncius, the “Starry Messenger,” in March 1610, Galileo first announced to the world the cosmos-shattering revelations spied through his homemade telescope: that the lunar landscape was filled with mountains and craters; that a multitude of stars blended together to form the Milky Way’s luminous white band; and that the planet Jupiter, like some mini-solar system, was repeatedly circled by a set of moons.
But that was just the start. Four months later, once Saturn became visible in the nighttime sky, Galileo turned his telescope to what was then the farthest known planet. And what he encountered he called a “very strange wonder.” While keeping his discovery secret from fellow scientists for several months, Galileo swiftly notified the secretary of his Medici patron, the Grand Duke of Tuscany. “The star of Saturn is not a single star,” disclosed Galileo, “but is a composite of three, which almost touch each other.”
With the poor quality of his rudimentary telescope, Galileo was, of course, erroneously seeing Saturn’s ring system as two small blobs, perched on either side of the bigger central planet. The seventeenth-century Venetian poet Giulio Stroz-zi, in an ode to the great astronomer, lyrically described the sight as “in three minor knots divided.”
Likely thinking of Saturn’s appendages as separate moons, much like Jupiter’s, Galileo aimed to keep track of how they orbited the planet. But, to Galileo’s great surprise, Saturn’s telescopic image instead underwent “a strange metamorphosis,” changing back and forth over the years. Johann Locher, an astronomy student in Bavaria, made this cyclic transformation the subject of his dissertation in 1614. “Saturn deceives or really mocks the astronomers out of hatred or malice. For [the planet] has projected various appearances,” he wrote. “Sometimes he is seen single and sometimes triple; at one time elongated and at other times round.” By 1616, Saturn looked as if it had handles. All these variations were due to how Saturn’s rings were positioned with respect to the Earth, although astronomers didn’t know that yet.
By 1650, according to astronomy historian Albert Van Helden, “the problem of Saturn’s appearances had become a celebrated puzzle.” Astronomers were wondering whether Saturn was round, egg-shaped, or composed of three bodies.
It’s easy to assume that better telescopes eventually solved the mystery, but that wasn’t fully the case. There was also some clever thinking involved. The inventive Dutch astronomer Christiaan Huygens had built a fifty-powered telescope that allowed him in 1655 to discover Saturn’s first moon, Titan. Saturn itself, as Huygens described it, then had “arms extended on both sides in a straight line, as though the planet were pierced through the middle by a kind of axis.” By the start of 1656 these arms had vanished altogether. Despite this disappearing act, Huygens still reasoned that Saturn’s chameleonic changes could be explained by the planet being “surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic.” First keeping this knowledge secret, needing more time to flesh out his theory and observe the ring with an even better telescope, Huygens finally made it public in his Systema Saturnium, published in 1659.
His fellow astronomers, however, did not greet the ring hypothesis with open arms. An accomplished observer in Rome, Honoré Fabri, declared it “pure fiction.” He preferred to think that Saturn was merely accompanied by several satellites. But within a decade, as telescopes improved, even Huygens’s harshest critics came to accept his explanation.
From the start Huygens imagined the ring as solid, like some kind of celestial phonograph record. But that assumption was considerably undermined in 1675 when Giovanni Cassini, director of the Paris Observatory, discovered that Saturn’s ring had a prominent gap, now known as the Cassini division. Cassini suspected that the ring was composed of small celestial bodies, a notion spurned by most astronomers. But a century later, the French mathematician Pierre-Simon Laplace offered a further argument against the solid-ring idea. He demonstrated mathematically that a solid structure would be high
ly unstable.
It was not until the nineteenth century that both theory and observation at last resolved the makeup of Saturn’s rings once and for all. In a prize-winning 1856 essay, the Scottish physicist James Clerk Maxwell (who several years later went on to develop his historic theory of electromagnetism) lucidly proved that the ring had to be composed of innumerable particles, each orbiting Saturn like a minuscule moon. It was the only configuration that remained durable against gravitational and centrifugal forces. All doubts were erased in 1895 when James E. Keeler, then director of the Allegheny Observatory in Pennsylvania, pegged the velocity of Saturn’s rings. Newton’s law of gravity predicted that the tiny chunks circulating in the outer part of the ring would travel more slowly than those closer in—just as Neptune, far from the Sun, orbits at a lower velocity than the solar system’s inner planets. And that’s exactly what Keeler measured. Within days of his observation, he sent a report to the Astrophysical Journal (“A Spectroscopic Proof of the Meteoric Constitution of Saturn’s Rings”), triggering a torrent of magazine and newspaper articles around the world.
Saturn’s ring material, composed largely of ice and dust, ranges in size from grains to boulders the size of a house and larger. This material may have originated when an ancient ice-cloaked Saturnian moon was either ripped apart by tidal forces or shattered by an incoming comet. Or possibly it is simply material left over from the nebular disk out of which Saturn itself formed.
Saturn lost its special status as our solar system’s sole ringed planet in the 1970s and 1980s, when both telescopic observations and spacecraft flybys of the other gas giants—Jupiter, Uranus, and Neptune—spotted rings around them as well. It took longer to find these ring systems, as they are far less substantial, hence fainter and difficult to see.
That wouldn’t be the case for Saturn Giganticus, or J1407b, as the exoplanet is officially known. If it replaced Saturn within our solar system, the rings would appear many times larger than the width of the full Moon, and with our eyes alone we’d be able to marvel at their beauty during a long, dark night.
CHAPTER SIX
The Baffling White Dwarf Star
Discovering this star opened up a whole
can of cosmic worms
IN 1862 the first hint arrived that the stellar universe was far stranger than anyone imagined—or could imagine. It came with the knowledge that a faint companion slowly circles Sirius, the brightest star in the nighttime sky.
Astronomers at the time didn’t recognize what they had uncovered. It would take decades—until the 1910s—for them to fully realize that Sirius B, as the tiny companion came to be known, was a star like no other seen before. Once its nature was revealed, though, it didn’t take long for theorists to conceive of other bizarre creatures that might be residing in the stellar zoo.
The story begins, not in 1862, but actually two decades earlier. For a number of years, the noted German astronomer Friedrich Wilhelm Bessel, director of the Königsberg Observatory, had been going through old stellar catalogs, as well as making his own measurements, to track how the stars Sirius and Procyon were moving across the celestial sky over time. By 1844 he had enough data to announce that Sirius and Procyon were not traveling smoothly, as expected; instead, each star displayed a slight but distinct wobble—up and down, up and down. With great cleverness, Bessel deduced that each star’s quivering walk meant it was being pulled on by a dark, invisible companion circling it. Sirius’s unseen companion, he estimated, completed one orbit every fifty years.
Bessel was clearly excited by his find; in his communication to Great Britain’s Royal Astronomical Society, he wrote, “The subject . . . seems to me so important for the whole of practical astronomy, that I think it worthy of having your attention directed to it.”
Astronomers did take notice, and some tried to discern Sirius’s companion through their telescopes. Unfortunately, at the time Bessel reported his discovery, Sirius B was at its closest to gleaming Sirius, from the point of view of an observer on Earth, and thus lost in the glare. But even years later, no one was successful in spotting the companion.
That all changed on January 31, 1862. That night in Cambridgeport, Massachusetts, Alvan Clark, the best telescope manufacturer in the United States, and his younger son, Alvan Graham Clark, were testing the optics for a new refractor they had been building for the University of Mississippi. It was going to be the biggest refracting telescope in the world. Looking at notable stars to carry out a color test of their 18.5-inch (47-centimeter) lens, the son observed a faint star very close to Sirius.
This momentous sighting might have gone unrecorded. But fortunately, the father was an avid double-star observer and possibly encouraged his son to report the discovery to the nearby Harvard College Observatory. In fact, according to historian Barbara Welther, rather than its being an accidental discovery, as long asserted in astronomy books, “there might have been a [prearranged] connection between the elder Clark and someone at Harvard” to look for Sirius’s companion.
Whatever the case, George Bond, the observatory’s director, confirmed the find a week later, and he soon wrote up two papers, one submitted to a German journal of astronomy, the other to the American Journal of Science. One question was uppermost on Bond’s mind: “It remains to be seen,” he wrote, “whether this will prove to be the hitherto invisible body disturbing the motions of Sirius.” The newfound star seemed to be in the right place to explain the direction of Sirius’s wavelike motions, but its luminosity was extremely feeble—so dim, in fact, that it suggested at the time a star too small to have enough mass to account for the wobble. Here was the first clue to Sirius B’s uniqueness. For revealing Sirius’s dark companion, Alvan Graham Clark in 1862 garnered the prestigious Lalande Prize, presented by the French Academy of Sciences for the year’s most outstanding achievement.
As astronomers around the globe continued over the years to observe the orbital dance of Sirius and its partner, they eventually determined that the companion was hefty enough (a solar mass) to pull on Sirius, though with a light output less than a hundredth of our Sun’s. But no one worried about this disparity at first. They just figured it was a sunlike star cooling off at the end of its life. At this point, no one had yet secured a spectrum of the light emanating from Sirius B, a difficult task owing to the overwhelming brightness of the binary’s primary star. Astronomers just assumed it had to be yellow or red, like other dim and cooler stars. Astronomy had a general rule at the time: the hotter the star, the brighter. The brightest stars’ colors were white, blue-white, or blue.
But in 1910, Princeton astronomer Henry Norris Russell noticed something in a past observation that cast doubt on that rule. On a Harvard College Observatory photographic plate, a faint companion of the star 40 Eridani—a companion known since 1789—was labeled as blue-white. How could that be? Russell doubted that such a classification could be correct for such a faint star. But in 1914, Walter Adams at the Mount Wilson Observatory in California confirmed the spectrum. The star was indeed white-hot, yet dim. “I was flabbergasted,” recalled Russell. “I was really baffled trying to make out what it meant.” Then, in 1915, Adams determined that Sirius’s faint companion, too, displayed the spectral features of a blazing blue-white star.
Soon, theorists, such as the British astrophysicist Arthur S. Eddington, figured out what was going on. If a star is both white and hotter than our Sun, it must be emitting more light over each square inch of its surface. But since Sirius B is so faint, that could only mean it had less surface area than our Sun—in other words, it is far smaller, roughly the size of the Earth. Such stars came to be called “white dwarfs.”
But how does a Sun’s worth of mass get squeezed into such a tiny volume? As Eddington later remarked mischievously, “The message of the companion of Sirius when it was decoded ran: ‘I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a matchbox.’
What reply can one make to such a message? The reply which most of us made . . . was—‘Shut up. Don’t talk nonsense.’”
An artist’s impression of the blue-white star Sirius A (left) and its tiny white-dwarf companion Sirius B (right) as they might appear to an interstellar visitor.
(NASA/ESA/G. Bacon [STSci])
It took quantum mechanics, under development in the 1920s, to solve the puzzle. By 1926 British theorist Ralph Fowler finally figured out that the density inside the compact dwarf star becomes so extreme that all its atomic nuclei, like droves of little marbles, are packed into the smallest volume possible, while its free electrons generate an internal energy and pressure that keeps it from collapsing even further. This creates an ultraconcentrated material impossible to assemble on Earth. Astronomers later learned that this is the end stage for a star like our Sun. The white dwarf is the luminous stellar core left behind after the star runs out of fuel and releases its gaseous outer envelope into space. Such will be our Sun’s fate some five billion years from now.
Dispatches from Planet 3 Page 3
