Clyde Tombaugh might have missed it had he been a little less attentive as he stared through an eyepiece while switching back and forth between photographic images of the night sky.
A recurring speck on two successive images that also contained perhaps another 300,000 dots -- pinprick-sized images of stars and other space objects -- was all the evidence there was that Tombaugh was onto something extraordinary.
But it was enough. Tombaugh knew that he was getting the first view of the solar system’s ninth planet. He had discovered Pluto.
It has been 75 years since Tombaugh first spotted the tiny icy planet on Feb. 18, 1930. Since then, Pluto’s very classification as a planet has been questioned. Astronomers agree, though, that the discovery at Flagstaff’s Lowell Observatory remains a remarkable one.
“Whatever Pluto is, it is an extremely intriguing little world,” said Bob Millis, director of the Lowell Observatory.
The observatory’s search for the solar system’s ninth planet, a “Planet X” that had been hypothesized but never seen, was begun by Percival Lowell.
Lowell founded the observatory in 1894 in the mountains of this northern Arizona community to search for intelligent life on Mars.
Shortly before his death in 1916, Lowell published calculations estimating where a ninth planet might be. He and other astronomers believed that a planet existed beyond Neptune because of a perceived wobble in Uranus’ orbit that suggested it was being affected by another celestial body, said Kevin Schindler, senior supervisor of public programs at Lowell.
The wobble was an illusion; Pluto was far too small to disrupt Uranus’ orbit.
But because of the “serendipity of science,” Lowell’s calculations proved accurate, Schindler said. Tombaugh found a planet where Lowell said it should be.
The telescope used to find Pluto was built specifically to search for Planet X. The base was constructed with local rocks. A piece of railroad track supported the giant camera used to photograph the night sky.
The camera took an hour to capture a fist-sized section of the sky on 14-by-17-inch glass plates.
Each section of sky was shot twice several nights apart, allowing Tombaugh to look for movement. Planets, because they are relatively close to Earth, would move from plate to plate, while stars would appear to be in the same place, Schindler said.
Because the plates contained 200,000 to 300,000 objects each, Tombaugh looked at them through a machine that allowed him to flash back and forth systematically, peering at tiny sections.
It took Tombaugh less than a year to find the ninth planet, which was later named for the Greek god of the underworld. The discovery put the observatory on the map and gave Tombaugh instant acclaim.
Shortly after Tombaugh’s death in 1997, however, a debate arose over Pluto’s place in the solar system. It started in the astronomical community but spilled into the public limelight after someone noticed that the Hayden Planetarium at New York’s American Museum of Natural History wasn’t categorizing Pluto with other planets.
Planetarium Director Neil deGrasse Tyson said he has “folders of hate mail from third-graders,” and the outraged notes keep coming five years after the display opened.
Hayden didn’t kick Pluto out of the solar system, as some have accused it of doing, Tyson said.
Instead, the planetarium grouped Pluto with its apparent cousins in the Kuiper Belt -- small icy bodies beyond Neptune -- making it “the king of the comets. I think it’s happier there than being the puniest planet,” he said.
The planetarium abandoned the planet label for Pluto because recent research suggests that it may actually be part of an entirely new group of space objects.
Until the early 1990s, astronomers had only seen Pluto and its moon, Charon.
Although scientists had hypothesized that other objects were out there, the first views of Kuiper Belt objects weren’t captured until 1992. Since then, about 1,000 have been sighted, and 100,000 might be out there, Millis said. Pluto seems to behave like these objects.
Pluto is also very small; its diameter is only about one-fifth of Earth’s. And it has a much more elliptical and tilted orbit than the other eight planets, said Hal Weaver, project scientist on the New Horizons mission, which hopes to launch a probe to Pluto next year.
“You start to see where Pluto fits in better with Kuiper objects,” he said.
Some astronomers have suggested that the International Astronomical Union, a group of astronomers, should demote Pluto. So far, it hasn’t.
Part of the problem is the lack of an official definition for a planet. Setting standards like size limits or orbital patterns potentially invites other objects to take the planet label, while throwing Pluto out.
Pluto “is just not like anything else we know about,” Schindler said. “If you don’t call it [a planet], what else do you call it?”
Like other planets, Pluto is very spherical. Asteroids and comets tend to be misshapen, more football- or cigar-shaped, said Weaver, a senior scientist at Johns Hopkins University’s Applied Physics Laboratory.
Pluto also has an atmosphere and seasons.
Astronomers hope to learn more about Pluto with the probe, but it will take nearly a decade to reach the icy dwarf, Weaver said.
Astronomers have said that regardless of what Pluto is called, it is a significant find because it is the first of a new class of objects beyond Neptune.
Weaver said Pluto could be important for what it tells us about the origin of the solar system. Its unexplored ice deposits may help explain what the solar system was like when the sun formed.
But the debate over Pluto’s status won’t likely be resolved without a better definition of planets or better understanding of the Kuiper Belt, he said.
“Some people thought [the debate] was a slight of Clyde Tombaugh. You were trying to take away his planet,” Weaver said. “I’m sure that it was nothing personal.”