More problematic to his fellow scientists was Newton’s law of gravity itself. His mathematics implied that imperceptible ribbons of attraction somehow radiated over distances, both long and short, to keep moon to planet and boulder to Earth. For many, this feat appeared more resonant with the occult than science.
The German astronomer Johannes Kepler in the early 1600s had suggested that threads of magnetic force emanating from the Sun were responsible for pushing the planets around. A little later the French philosopher René Descartes visualized the planets carried around like leaves trapped within a swirling whirlpool by vortices of aether, the tenuous substance then thought to permeate the heavens.
Newton’s critics were now demanding a physical mechanism. What was replacing either magnetism or vortices? This led to Newton’s famous statement in the Principia: “I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses.” Newton did not want to stoop to speculating or conjuring up some kind of hidden cosmic machinery. It was enough for him that his laws allowed successful calculations to be made.
Total acceptance took a while, but as the years passed, the rest of the physics community did eventually come over to Newton’s side. And it was a comet, of all things, that provided the incentive.
Edmond Halley, Newton’s colleague at the Royal Society of London, had used his friend’s mathematical laws to make the first prediction of a comet’s return. After poring over historic records, Halley had compiled a list of twenty-four comets observed from 1337 to 1698 and computed their motions. Looking over this record, he came to recognize that a comet sighted in 1682 had much in common with comets previously observed in 1531 and 1607. For one, they shared the same orbital characteristics (all went around the Sun in the opposite direction to the planets). This made him suspect it was the same comet returning every seventy-five to seventy-six years. “The space between the Sun and the fixed stars is so immense,” wrote Halley, “that there is room enough for a comet to revolve, though the period of its revolution be vastly long.”
A photo of Halley’s comet taken at Yerkes
Observatory on May 5, 1910.
(Wikimedia Commons)
Based on his calculations, which took into account the additional tugs by Jupiter in the comet’s journey through the solar system, Halley made a prediction. “I dare venture to foretell,” he announced in his 1705 paper, “that it will return again in the year 1758.”
The comet appeared on schedule, just as Halley foretold. On Christmas Day in 1758, thirty-one years after Newton’s death and sixteen years after Halley’s, an amateur astronomer and gentleman farmer in Saxony named Johann Georg Palitzsch was the first to catch sight of the comet as a nebulous star in the nighttime sky. French observer Charles Mes-sier, already on the lookout for the comet, saw the same fuzzy object four weeks later from Paris. It was soon confirmed to be Halley’s returnee, and by March the comet was rounding the Sun.
The public was bedazzled, and the remaining critics of Newton’s controversial law of gravity were instantly silenced. Despite the lack of a mechanism, his law was at last triumphant among both scientists and the public. Who could argue with a theory that allowed for a spot-on prediction about the solar system’s behavior nearly a century in advance?
As a consequence, the universe came to be viewed as intrinsically knowable, ticking away like a well-oiled timepiece. And Halley’s name became forever linked to that special, periodic celestial visitor. Its next visit: 2061.
CHAPTER THREE
To Be . . . or Not to Be a Planet
How planets have been promoted and demoted
over the decades
THEY looked for five years toward the far edge of the solar system and found nothing. But in 1992 the tide at last turned for two American astronomers. Using a new digital camera mounted on the University of Hawaii’s 2.2-meter telescope atop Mauna Kea, David C. Jewitt and Jane X. Luu swiftly spotted their long-awaited quarry: a fuzzy spot of 23rd magnitude, four billion times fainter than the star Sirius. They had found the holy grail for planetary astronomers: an object orbiting the Sun beyond Neptune and Pluto. It was roughly 125 to 150 miles (200 to 240 kilometers) wide. They had wanted to name it “Smiley” (after the astronomer Charles Hugh Smiley), but given that an asteroid had already been named for Smiley, it’s today simply referred to by its catalog name, 1992 QB1. That’s astronomy code for being the twenty-seventh asteroidal object discovered in the second half of August in the year 1992.
Over the ensuing years, Jewitt, Luu, and others found many more objects like 1992 QB1 in the far reaches of the solar system, and they were christened with such captivating names as Quaoar, Sedna, Makemake, and Haumea. These newly discovered bodies were proof that the “Kuiper belt,” a thick ring of icy planetesimals beyond the solar system’s outer planets, indeed existed, as proposed in the mid-twentieth century by, among others, the Dutch-American astronomer Gerard P. Kuiper (although he originally thought the belt, remainders from our solar system’s birth, would have scattered away by now).
During that time of explosive discovery, Luu made a prophetic remark in Astronomy magazine about the new evidence: “The confirmation of the Kuiper belt changes our perception of the solar system. What we thought of as a planet is probably just the biggest member of a rather large population of objects.” She was thinking of tiny Pluto, only 1,400 miles (2,250 kilometers) wide. Smaller than our Moon, it had always been an oddball when compared with its gas-giant neighbors—Jupiter, Saturn, Uranus, and Neptune. The best ammunition to support this notion arrived when California Institute of Technology (Caltech) astronomer Michael E. Brown announced in 2005 that he and his colleagues had found an object in the belt heavier than Pluto. It has roughly a third more mass.
Brown’s newfound body was eventually dubbed Eris, after the Greek goddess who personifies strife and discord. It was a fitting name, because this groundbreaking work caused the International Astronomical Union to revamp the solar system’s membership. By 2006 Pluto was demoted in status to “dwarf planet,” no longer in the big time but simply one of the larger members of the Kuiper belt, like Eris and the others. For their pioneering roles in this transformation of the solar system, Jewitt, Luu, and Brown were awarded the 2012 Kavli Prize in astrophysics, a prestigious biennial honor that came with a cash award of $1 million.
Many a child (and adult) was horrified when the number of planets in our solar system dropped from nine to eight. It means the Italian-menu formula for remembering their order is out: instead of “My Very Educated Mother Just Served Us Nine Pizzas,” we have “My Very Educated Mother Just Served Us Nachos.” A banquet of pepperoni and cheese has been reduced to an appetizer.
Since the eighteenth century, we’ve been accustomed to astronomers adding planets to our solar system, not subtracting them: first Uranus in 1781, followed by Neptune in 1846. It seemed an unprecedented move for astronomers to take one away: the planet found by Lowell Observatory astronomer Clyde W. Tombaugh and greeted with such fanfare in 1930. Pluto, we hardly knew ye. But this is not the first time the solar system has undergone a substantial reconfiguring. Another planet once came and went in a similar manner—two centuries ago.
Ever since Johannes Kepler, in the early 1600s, was able to link a planet’s orbital period (the time it takes to round the Sun) to its orbital radius, astronomers sought an underlying pattern to the various distances of the planets from the Sun. In 1766 the Prussian scientist Johann Daniel Titius developed an elaborate mathematical scheme (based on earlier work by Oxford professor David Gregory) that appeared to account for the planets’ positions. Six years later, a self-educated astronomer soon to be a professor at the Berlin Academy of Sciences, Johann Elert Bode, drew attention to the pattern in a new edition of a popular book on astronomy that he had written, which led to the rule becoming known as “Bode’s law.” The one shortcoming of the law was that it did not account for an apparent gap between Mars and Jupiter, w
here the law predicted an intermediate planet should appear.
When the planet Uranus was discovered at the very distance from the Sun that continued the sequence beyond Saturn, the sway of Bode’s law (though not based on any physics) became near-mystical, immediately emphasizing the yawning gap between Mars and Jupiter. “Can one believe that the Creator of the Universe has left this position empty? Certainly not!” declared Bode. The success with Uranus encouraged astronomers throughout Europe to join forces to discover the planet everyone was sure was missing beyond Mars. The team jokingly referred to itself as the “celestial police,” dividing the sky into twenty-four zones so each could be thoroughly explored by one of the team.
Meanwhile, the discovery of an object orbiting in the “gap” was serendipitously made by one of the astronomers the “police” had intended to enlist—although he didn’t know it. Working from a new observatory he had founded in Palermo, Sicily, the monk Giuseppe Piazzi was assembling a star catalog, the most accurate in its day. On the evening of New Year’s Day in 1801, he routinely measured the position of a star in the constellation Taurus, the Bull. “The light was a little faint, and of the color of Jupiter,” he reported, “but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star.”
But, following his customary procedure, Piazzi measured the star again the next night and found to his surprise that it had shifted. Over subsequent nights, he kept track of its movements and saw that its path was not elongated, like a comet’s, but rather more circular. Privately, he wondered whether it might be the long-sought lost planet. “Since its movement is so slow and rather uniform,” he wrote a colleague, “it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public.”
By February, Piazzi was unable to continue his observations because the object was lost in the glare of the Sun, but he communicated his find to other astronomers. Although they could not observe the newfound body, the noted German mathematician Carl Friedrich Gauss was able to calculate its orbit from the limited data. That helped astronomers relocate Piazzi’s object once it was again visible, on December 31, near the very spot in the constellation Virgo, the Virgin, that Gauss had computed. More than that, its orbital radius closely matched that predicted by Bode’s law.
An image of Ceres taken by NASA’s Dawn spacecraft in 2017.
(NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
Piazzi named the object Cerere Ferdinandea (Italian for “Ceres of Ferdinand”), in honor of the patron goddess of Sicily and his own patron, King Ferdinand IV of Naples and Sicily. Bode excitedly wrote a paper in 1802 trumpeting the discovery (and not forgetting to crow about his own role in the endeavor): “Piazzi had, indeed, here discovered a very extraordinary object. It was most probably the eighth major planet of the solar system, which already thirty years before I had announced between Mars and Jupiter, but which until now had remained undiscovered.” Bode published a table updating his concept.
Ceres’s reign as a major planet, though, was a bit shorter than Pluto’s. William Herschel, using his large telescope in Great Britain, was quickly able to discern that Ceres was smaller than our Moon. And Heinrich Olbers, a German physician and accomplished amateur astronomer, soon found a similar object in the same region, which he christened Pallas. Over the next five years, two more, named Juno and Vesta, were found. Being hundreds rather than thousands of miles in diameter, these newfound objects appeared starlike (“asteroidical”) to Herschel in his telescope, so he suggested the name asteroid to describe this new class of objects. It took some time, though, for all astronomers to fully apply this term. As late as 1866, the Berlin Observatory’s annual yearbook continued to list the first four asteroids as major planets. Other observatories called them “minor planets” for a while.
In the nineteenth century it was believed the asteroids were the remains of a former full-sized planet that had somehow disintegrated in the distant past. Today it is known they are a field of debris—tens of millions of fragments of planetesimals that failed to coalesce into a major planet owing to the gravitational tugs of nearby Jupiter, and that then randomly smashed into one another like cosmic bumper cars. Ceres was a protoplanet that failed to grow up.
But no tears need to be shed for this celestial goddess. At the same time that Pluto got demoted in 2006, Ceres got re-promoted. As it is the largest object in the asteroid belt (containing a third of the belt’s entire mass) and rather round, with a diameter of about 590 miles (950 kilometers), the International Astronomical Union reclassified it as a dwarf planet, the sole one in the belt. It’s the queen of the asteroids, majestically orbiting the Sun once every 4.6 years. NASA’s Dawn spacecraft is currently exploring this dwarf planet, gathering data from an orbit several thousand miles above Ceres’s heavily cratered surface.
CHAPTER FOUR
The Watery Allure of Mars
The renegade astronomer who started the gossip about
water on Mars
WE take them for granted when walking along a shoreline or river bank. Looking down, we see once-jagged rocks now rounded and worn smooth by the flow of running water. Pebbles have never been big news here on terra firma—but they were an Earth-shattering, or rather a Mars-shattering, discovery when spotted on the red planet by the Mars rover Curiosity in 2012.
That finding and further evidence—erosion channels carved into the Martian landscape, large expanses of sedimentary deposits in former lakes (now dry craters)—all strengthen the case that liquid water once flowed freely over the surface of our planetary neighbor when the planet was warmer, and its atmosphere denser, more than three billion years ago. These recent revelations have made me wonder how Percival Lowell would have handled the news, if he were still living today. He’s the man who infamously dominated this whole conversation about water on Mars more than a century ago.
The oldest of five children, Lowell came from a well-established New England family. He was one of the Boston Brahmins, upper-crust Massachusetts townsmen who had made their fortunes creating the American textile industry. A few years after graduating from Harvard in 1876, Lowell traveled extensively, especially to the Far East, which led to his writing several well-received books on the region.
By the 1890s, though, restless and searching for individual expression, he renewed a childhood interest in astronomy. “After lying dormant for many years,” recalled his brother, “it blazed forth again as the dominant one in his life.” Independently wealthy, Lowell decided to establish his own private observatory atop a pine-forested mesa nestled against the small village of Flagstaff, Arizona (then still a territory of the United States). It was a daring venture for an amateur astronomer with no professional experience, especially since he found himself competing with the new and larger astronomical outposts then being built by universities and research institutions throughout the United States. In this rivalry, Lowell became the controversial outsider, insisting that his staff pursue the questions that interested him alone. His initial aim was to observe the particularly close approaches of Mars occurring in 1894 and 1896. Given his obsession with the red planet, the high perch on which his 24-inch (61-centimeter) refracting telescope rested more than a mile above sea level was soon dubbed Mars Hill.
Mars, with its vivid ruby luster, had been fascinating stargazers for millennia. This interest grew even more intense after the invention of the telescope. As magnifications increased over the decades, astronomers began to discern distinct markings on Mars’s surface. Bright patches around its poles, similar in appearance to our own planet’s Arctic and Antarctic regions, were seen to wax and wane with the Martian seasons. So earthlike were these phenomena that by 1784 the German-born British astronomer William Herschel was reporting that Mars “is not without a considerable atmosphere . . . so that its inhabitants probably enjoy a situation in many respects similar to ours.”
&
nbsp; Percival Lowell on the observer’s chair at
Lowell Observatory’s 24-inch telescope.
(Wikimedia Commons)
Scrutiny of Mars was particularly favorable in the fall of 1877, when Earth and Mars were at their closest, approaching in their orbits to within 35 million miles (56 million kilometers) of one another. The superb viewing conditions allowed the Italian astronomer Giovanni Schiaparelli to map numerous dark streaks crossing Mars’s reddish ochre regions. In his native language, he called these thin shadowy bands canali, or “channels,” which many deduced arose from natural geologic processes.
But Schiaparelli’s term was translated inaccurately, a gaffe that led to many fanciful conjectures. The most notorious, by far, was the assumption that the “canals” were irrigation works built by advanced beings, who were directing scarce resources over the surface of their planet for cultivation. The building of the Suez and Erie canals in the nineteenth century was still fresh in the public’s memory. “Considerable variations observed in the network of waterways,” wrote French astronomer Camille Flammarion in 1892, “testify that this planet is the seat of an energetic vitality. . . . There might at the same moment be thunderstorms, volcanoes, tempests, social upheavals and all kinds of struggle for life.” No one championed this extravagant vision more avidly than Percival Lowell.
With the opening of his observatory in 1894, Lowell immediately began to map Mars, adding 116 waterways to Schiaparelli’s original depiction. And within a year he published a book titled simply Mars, following up in coming years with Mars and Its Canals and Mars as the Abode of Life. The lines discerned on Mars, he declared, were assuredly artificial rivers conveying seasonal snowmelt from the planet’s polar caps. That they were even visible from Earth was likely due to the massive vegetation growing along the canal banks. Promoting his ideas in books and lectures like a blue-blooded carnival barker, he fancifully imagined his Martian civilization as dependent on its global irrigation system to remain extant. The very fact that the Martian features he saw were straight—like the canals, streets, and railways on Earth—increased the odds, he claimed, that they were produced by intelligent workers.
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