Of Cosmic Proportions

It's a merciful thing that Clyde Tombaugh wasn't around to see this day. Tombaugh, the ambitious Kansas farm boy who discovered the ninth planet, Pluto, in 1930, departed Earth in January 1997, when he died at the age of 90. And then Tombaugh departed it again last January, when his cremated ashes were blasted into space aboard the New Horizons space probe. But while New Horizons was streaking through the asteroid belt last week en route to a rendezvous with Pluto in 2015, a group of astronomers on Earth decreed that its destination now belonged to a new category of heavenly body, a "dwarf planet." Tombaugh knew something like that was afoot before he died, according to his 93-year-old widow, Patricia, who added that as a man of science, he would have understood the decision. For her part, though, she confessed some disappointment. "I feel like I sort of got demoted from my job being the wife of the discoverer of Pluto," she told the Arizona Daily Star. "Now I'm the wife of the discoverer of a dwarf planet."

It's not surprising that she should care. But what about the rest of us? Why is the world so captivated by the fate of this remote and almost invisible world that, if it landed on Earth, would barely stretch from Boston to Tulsa? This knob of rock and frozen nitrogen dwells so far out in the solar system--averaging about 3.6 billion miles, or almost 40 times the distance from Earth to the Sun--that in the 76 years since its discovery it didn't get to complete even a third of its orbit. And, of course, it was not really its "fate" that was at stake, but an arbitrary designation whose impact will largely fall on textbook publishers, planetarium gift shops and astrologers. The debate and vote by the International Astronomical Union at its meeting in Prague was a little reminiscent of the earnest discussions a few years back about whether the 21st century began on Jan. 1, 2000, or 2001.

But then why was Matthew Malkan, a UCLA astronomer, deluged with e-mails from people he hadn't heard from in years, wanting to talk about the IAU meeting (which he skipped, although he says now if he'd realized what a big deal it would turn out to be he might have attended). Why did Jan Weiss, visiting the Rose Center for Earth and Space at the American Museum of Natural History, feel a pang of sadness at the news about Pluto? "You grow up and there are nine planets, and now there aren't," she mused. "Imagine all those dorm rooms where they have to scrape Pluto off the ceiling."

But the solar system we thought we knew is changing faster than most of us can keep up with--part of a larger process of expanding our view of the universe. Who imagined, even a few decades ago, that we would someday see up close giant hurricane-like storms sweep across the surface of Saturn? The revisions began as long ago as the 1970s, when the Apollo mission brought back evidence that the moon had been formed out of a tremendous collision between Earth and another large object. Planets, once thought to form gradually out of coalescing dust and gas, are now viewed as the survivors of a violent process of collision and accretion, the winners in a Darwinian competition to build up enough gravity to control one's own orbit. A planet the size of Pluto has no place in the 21st-century solar system. But it still has a role to play in science, along with the hundreds of other nearby objects discovered in just the past decade. Understanding them offers us a window on how planets like our own are born and give rise to life.

The sad truth is, Pluto's claim to planethood has been shaky ever since its discovery, which came about more or less by mistake. It is by far the smallest of the canonical nine planets--about 2,300 kilometers (1,400 miles) in diameter, smaller even than Earth's moon--and with a unique orbit, tilted at an angle to the plane in which the other eight planets all travel (graphic). Too faint to be seen by the naked eye and barely crawling across the night sky, it remained unknown to the ancients, who recognized five heavenly planets (Mercury, Venus, Mars, Jupiter, Saturn) traversing the unchanging backdrop of the stars. By 1846, astronomers had discovered Uranus and Neptune, which together with Earth made eight relatively large bodies circling the Sun--some, like Earth, with smaller moons in orbit around them as well.

No one suspected there might be another until the early 1900s, when astronomers--notably Percy Lowell, who founded the observatory that bears his name in Flagstaff, Ariz.--detected a perturbation in the orbits of the outermost planets, suggesting they were being tugged by the gravity of something not yet seen. Lowell, who died in 1916, devoted the last years of his life to the fruitless search for this "Planet X." (He was better known up to that point for his regrettable speculations about the "canals" on Mars, which he thought had been built by Martians.) In 1929, the job of looking for Planet X was turned over to Tombaugh, an amateur astronomy buff who had built a homemade telescope using parts from an old Buick. Tombaugh hadn't even been to college--he later graduated from the University of Kansas and went on to a distinguished career as an astronomer--but he must have had exceptional eyesight and iron patience. His job was to take two photographs of the same part of the sky on different nights--using the observatory's new 13-inch telescope--and compare them, looking for objects that had moved. Each photographic plate contained hundreds of thousands of stars. Yet in less than a year, he picked out of some 15 million specks of light the one that would be called Pluto, in just the part of the sky that Lowell had predicted. This was hailed as a triumph of American science, only slightly tarnished when astronomers later realized that Pluto was actually much too small to have any effect on the orbits of other planets. The perturbations Lowell had noticed resulted from incorrect values for the masses of Uranus and Neptune. Pluto just happened to be in the right place at the right time to be found.