The history of Pluto goes way back before it became a tiny twinkle in Clyde Tombaugh’s blink comparator. Kelly Beatty, a senior editor at Sky & Telescope magazine, told the story of Pluto in his keynote address Saturday at the annual banquet of the Seattle Astronomical Society.
In a way, according to Beatty, the hunt for Pluto dates back to the late 1700s. The Titius–Bode law (since repealed) of the distances to the planets from the Sun worked well, with one exception: according to the law, there should be a planet between Mars and Jupiter. Thus a group of astronomers calling themselves the “celestial police,” led by Franz Xaver von Zach, set out to find this elusive object. They did it; on New Year’s Day, 1801, Giuseppe Piazzi discovered Ceres. It wasn’t long before Juno, Vesta, and Pallas we found. At first all four were labeled planets, but now they’re known as the four largest asteroids—and possibly the first celestial objects to be demoted in status.
Dumb, fool luckLater, because of irregularities in the motion of Uranus, astronomers predicted another planet out beyond its orbit. But even after the discovery of Neptune in 1846, anomalies remained. Percival Lowell and William Pickering predicted there was yet another planet beyond Neptune. The hunt was on for Planet X, and Pluto was finally discovered by Clyde Tombaugh in 1930. We soon learned that Pluto was pretty small, which lent some irony to its discovery.
“Pluto didn’t have any effect on Uranus and Neptune at all,” Beatty said. “It turns out that the mathematics were incorrect, the positional accuracy of those early observations was bad. There was no basis to the prediction whatsoever, and by dumb, fool luck Clyde found the planet Pluto that he had been seeking within about six degrees of the predicted position. Freakingly by accident.”
We didn’t know a whole lot about Pluto for a long time. The best photos we could get were fuzzy Hubble Space Telescope shots. Astronomers found methane ice on Pluto in 1976, and its moon Charon was discovered in 1978. Pluto’s atmosphere was discovered in 1988 when it occulted a star.
Pluto on thin ice
“In 1998 the bottom fell out of the pro-Pluto movement,” Beatty said. The beginning of the end was the discovery of another distant object in what we now call the Kuiper Belt. Astronomers figured that there had to be more out there than just Pluto, and we now know of more than 1,800 of them. Many of these objects are locked in a 3:2 orbital resonance with Neptune, just like Pluto.
“Not only is Pluto not alone, it’s not even unique in its orbit,” Beatty said. “Things did not look good for Pluto and its planet status.”
The International Astronomical Union (IAU) started hearing chatter that Pluto should not be a planet, and in 1999 it passed a resolution declaring that it still was. Brian Marsden, who headed the minor planet center of IAU, really wanted to classify Pluto as an asteroid, according to Beatty.
Then, in 2005, Eris was discovered. At the time it appeared to be bigger than Pluto, though we now know it is slightly smaller. It was bureaucracy that finally knocked Pluto off the planet list. Different committees at the IAU name planets and asteroids, so to decide to which committee to refer the new discovery for naming, they had to decide what it was. This led to the new definition of planet, under which neither Eris nor Pluto fall. The IAU declared Pluto to be a dwarf planet in 2006.
Beatty is not fond of the IAU definition of planet: an object that orbits the Sun, has enough mass to be round, and has “cleared the neighborhood of its orbit.”
“It’s a really stupid definition,” Beatty said, mostly because it’s hard to know the mass of faraway objects, and so the definition is difficult to apply. Plus he finds it puzzling that a dwarf planet is not a planet.
“We have dwarf stars which are considered stars.” he pointed out. “We have dwarf galaxies that are considered galaxies. A chihuahua is still a dog.”
Planet or not, the New Horizons flyby of Pluto in 2015 gave us a ton of new information about it and its moons. Beatty shared numerous photos of and findings about Pluto from the mission. It’s mostly made of rock, and might have liquid water below its surface. The surface features are mostly hard-frozen water ice, with a little frozen nitrogen, methane, and carbon dioxide mixed in.
Most fascinating is evidence of geology happening right now in the form of flowing nitrogen ice.
“Pluto’s surface, against all odds, out in the frozen corner of the solar system,” Beatty marveled, “has flowing glaciers on it.”
The last of the Pluto data from New Horizons arrived on Earth back in October, but the mission isn’t over. The probe is headed out for a look at the Kuiper Belt object 2014 MU69, at which it will arrive on January 1, 2019.
Beatty said this great new data about Pluto was worth the wait.
“We finally know what this planet/dwarf planet/interesting world looks like,” he said. “It was a 30-year effort from the time the Pluto missions were first conceived until we finally got out there. Some of the people involved, like Alan Stern, were there every year of the way, and boy, what a rich reward they have for their efforts.”