Lowell Observatory Historian Kevin Schindler followed Sedona-based author Diane Phelps Budden’s talk on Clyde Tombaugh’s discovery of Pluto during the Sedona Heritage Museum’s “Tiny Pluto Has a Big Heart” presentation on March 27 with a discussion of the dwarf planet’s place in astronomical research and pop culture.
Schindler dealt with the history of observation of the planet since its 1930 discovery from Flagstaff, the 2015 New Horizons mission to image Pluto and its designation as Arizona’s state planet in March 2024.
“In 1781, astronomer William Herschel discovered what we know today as Uranus, and that was the first planet discovered that you needed a telescope to see,” Schindler said. “When astronomers looked at it, they realized maybe there’s another planet out there because Uranus … seemed to be wobbling and they figured that wobbling must be due to another planet out there whose gravity is pulling on it. For years, astronomers looked for another planet. And in 1846, Neptune was discovered right where they thought there would be a planet, [but] that didn’t seem to account for it all … and that’s what started the search for the ninth planet.”
Schindler added that he believed that if Tombaugh had not discovered Pluto in 1930 from Lowell Observatory, the planet might not have been found until the 1980s because of the methodology Tombaugh used, searching through time-lapse photographs showing up to 300,000 stars in a single image with a blink comparator, looking for changes in the position of any of the imaged objects.
“Other scientists didn’t have the patience, but [Tombaugh] was hired to do that,” Schindler said. “This is the only planet discovered in the United States, and we’ll get to this a little bit later, about its reclassification, and why, some people are still a little crinkly about that. For years, all we knew about Pluto was it was a pretty little dot out there because we didn’t have telescopes big enough.”
That changed in June 1978 at the U.S. Naval Observatory about six miles away from Lowell when James Christy and Robert Harrington discovered Charon, the first and largest of Pluto’s five known moons.
Christy had observed that images of Pluto included a blob that seemed to move around the dwarf planet. The elongation direction cycled every 6.39 days, which is Pluto’s rotation period. Christy was able to find additional instances of Pluto appearing elongated in pre-discovery images, which confirmed the moon’s presence. The name was a reference both to the Greek mythological ferryman to the underworld and a nod to Christy’s wife Charlene.
When a moon orbits a planet, it helps astronomers determine the planet’s mass, as the motion of the two bodies in relation to one another is dependent upon their masses, and Charon is also notable for being the largest known satellite relative to its parent body in the solar system. Schindler noted that the discovery of Charon allowed astronomers to rule out early size estimates for Pluto comparable to the mass of Jupiter.
“In 2006, astronomers were finding other bodies in the solar system that seemed to be about the size of Pluto, maybe even bigger,” Schindler said, “And so some astronomers started saying, ‘We can’t call Pluto a planet anymore, if little Pluto is a planet, then all these other things have to be planets’ … Which is completely idiotic to me. There are millions of species of beetles. We don’t call the beetle because it’s convenient. It’s because … that’s what they are.”
“The IAU downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet,” the Library of Congress’ website states. “Essentially Pluto meets all the criteria except one — it ‘has not cleared its neighboring region of other objects.’”
“Pluto has a moon. Pluto has an atmosphere, and I would say it sounds really planet-like to me,” Schindler said. He suggested that the most relatable definition of a planet comes from “Star Trek”: If it looks like a planet when you look out a spaceship window, it should be a planet.
“The science community decided one of our top priorities is to explore Pluto close up to really see what it looks like,” Schindler said. “So over years of starts and fails, the spacecraft that became known as New Horizons came about. It was funded, funding was pulled back. Senators had to get involved to get the funding back. And they went back and forth. But finally, it launched in 2006 and this was the fastest-moving spacecraft on record, 36,000 miles per hour. That’s 10 miles per second. That means it would take about 10 seconds to get from here to Phoenix … this [was] going 20 times faster than a bullet.” Schindler compared the significance of New Horizons for students to the effects of the Voyager missions in the 1970s, which allowed young people to experience the remote exploration of a new planet.
