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80 years of Pluto

Lowell Observatory file, circa 1950
Astronomer Clyde Tombaugh demonstrates how he used a device known as a blink
comparator to discover Pluto at the Lowell Observatory in Arizona on Feb. 18, 1930.


Astronomers had their doubts about Pluto from the very start. At first, even its discoverer wasn't sure where the little world fit in the planetary parade. But 80 years after it was found, Pluto has demonstrated that it's a survivor.

The dwarf planet first came to Earth's attention on the afternoon of Feb. 18, 1930, when former Midwest farmboy Clyde Tombaugh saw a curious speck on a pair of photographic plates in a contraption called a blink comparator, set up at the Lowell Observatory in Flagstaff, Ariz.

The 24-year-old Tombaugh had been painstakingly checking plates for months, looking for the telltale signature of a "Planet X" beyond the orbit of Neptune. When he saw a speck on one plate that seemed to jump to a slightly different position on another plate, made just a few days later, he suspected that he had found his quarry.

Only problem was, the darn thing looked much smaller than what Tombaugh was expecting to see. As described in astronomer David Levy's biography of Tombaugh, the speck was so small that the discoverer wondered whether he had merely spotted the moon of a yet-unseen planet.

That's the way it's gone for Pluto in the eight decades since. Even when the world was getting used to the idea that a "ninth planet" had been found, some astronomers suspected that there were more mini-worlds yet to be found out there - and that Pluto might not deserve its planetary title. Beginning in 1992, smaller objects were indeed discovered on the solar system's icy rim, known as the Kuiper Belt. And in 2005, Caltech astronomer Mike Brown finally found something bigger than Pluto, another world that came to be known as Eris.

A 'fraudulent' find?
"The moment that I saw Eris ... I was pretty convinced that this was the 10th planet, and that's what it would be forever," Brown recalled. After all, if Pluto was a planet, its bigger cousin Eris had to be a planet, too.

But Brown also admitted that referring to Eris as a planet "felt a little fraudulent, even at the time.'" And if that's how he felt about Eris, what did that say about Pluto?

Such thoughts set the stage for the International Astronomical Union's disputed definition of planethood in 2006, which classified Pluto, Eris and their ilk as dwarf planets ... but not planets per se. The rationale was that the word "planet" should be reserved for worlds that have cleared out everything else that orbits at the same distance from the sun.

To some astronomers, that definition doesn't make sense - and I lay out the detailed arguments in my book, "The Case for Pluto." But to Brown, whose Twitter nickname is @plutokiller, the definition was surprisingly sensible.

"The fact that astronomers actually got their act together and made sense of the solar system instead of just going on sentiment ... to this day, I'm still shocked. But I'm happy they did it," he said.

Now Brown is working on his own book about Pluto and the planet-hunting life, titled "How I Killed Pluto and Why It Had It Coming." Despite the pugnacious title, even Plutokiller sounds as if he's developed a soft spot for his prey.

"It's without question one of the most interesting Kuiper Belt objects to study," Brown said. "I feel bad when I say mean things about Pluto. I still feel like I have to defend the rationalness of the decision to demote Pluto."

Astrophysicist Neil deGrasse Tyson demoted Pluto in 2000, even before the IAU did, when he left it out of a planetary lineup at New York's Hayden Planetarium (in his capacity as the planetarium's director). Today, he's taking pains to point out that he's never been a "Pluto-hater."

"I felt in the year 2000, as I do today, not hard-line against Pluto - as I've been stereotyped - but having what I think is a rather simple point of view: that we need a lexicon commensurate with the richness of knowledge that we now have about what orbits the sun," he said. "And what we've been trying to do is shoehorn new discoveries and new ideas into an outdated classification scheme.

"I was never as hard as news programs tried to get me to say. I exist nowhere saying that there are eight planets in the solar system. I've never said that, ever," Tyson said. "What I said is that if you group objects by similar properties, then the solar system breaks into a whole other kind of understanding."

Meeting the family
Tyson goes on a coast-to-coast odyssey to lay out that understanding in "The Pluto Files," a "Nova" documentary premiering on PBS television stations on March 2. During the hour-long program, we see him sizing up planetary stand-ins with fellow astronomers on Harvard's football field, discussing Pluto the planet (and the dog) with Walt Disney's great-nephew, getting a close shave in the Illinois town where Tombaugh was born, and meeting up with Tombaugh's family in New Mexico.

The program even shows Tyson making his peace with Clyde Tombaugh's daughter, Annette Tombaugh-Sitze, as he points out Pluto's place in the Hayden Planetarium.

"Things are beginning to change in people's attitude toward Pluto," Tombaugh-Sitze told me this week. "People are softening."

Clyde Tombaugh's 97-year-old widow, Patsy Tombaugh, also thinks the debate over Pluto has "calmed down" in the past couple of years, although she wouldn't be surprised if the controversy continued to simmer for the next 80 years. "It looks like we're going to have to keep on discussing this," she told me.

Something wonderful
One of Pluto's biggest advocates is Alan Stern, a planetary scientist at the Southwest Research Institute who serves as the principal investigator for NASA's New Horizons mission to Pluto and the Kuiper Belt. Stern told me today he sensed that the "pendulum is dramatically swinging back."

"We hear more and more people referring to Pluto as a planet. ... Even Neil Tyson has become sort of ambivalent," he said.

In Stern's view, the IAU "really, objectively screwed up" by approving a definition that, for instance, would classify Earth as a non-planet if it were orbiting at Pluto's distance from the sun. "It's pure absurdity. People see through it, once they think through it," he said.

M. Buie / SwRI / NASA / ESA
Click for video: Hubble Space Telescope images, taken in 2002-2003, were
combined to produce these maps of Pluto. Click on the image to watch Pluto spin.

Every decade brings a sharper picture of Pluto, and just last week, scientists unveiled a map based on Hubble Space Telescope imagery that revealed dramatic change on the dwarf planet. But those revelations won't hold a candle to what New Horizons could see when it passes by Pluto in 2015.

There might be ice volcanoes, or puzzling amounts of methane, or thin streaks of clouds. The possibilities are so wide-ranging that Stern shies away from suggesting what we'll see. "Every time we've been to a new planet, we found out we were naive and wrong," he said.

"We're going to get surprised, we're going to get shocked. That's the fun of it." Stern said. "When we get to Pluto, when you ask me what we'll find, I'll give you an answer that's guaranteed to be right. The answer is this, in two words: something wonderful."

Update for 10 p.m. ET: How will posterity judge Pluto and Clyde Tombaugh 80 years from now? Here are the perspectives from our panel of experts:

  • Neil deGrasse Tyson: "My hope ... and I would not have said this before making this film ... My hope is that Clyde Tombaugh gets learned about and remembered as not simply the discoverer of Pluto, but as a kind of hero. ... He was a farmboy, not highly educated but self-taught, made homemade telescopes, got hired by this observatory, and had the perseverence and the discipline to make an important discovery. Whether or not you think of it as the ninth planet, Pluto is important as the first of a new class of objects in the solar system. In fact, I think that's a more important distinction to have than simply being known as the most diminutive planet."
  • Alan Stern on Clyde Tombaugh: "He not only discovered Pluto the planet, but a whole new class of planets, the ice dwarfs. It's the brightest light in the Kuiper Belt, in what we later came to realize is the most populous region of the solar system. Of course, it took a long time to see that. Columbus didn't know he was in North America, but Clyde lived long enough, into the 1990s, to realize the magnitude of what he saw in the 1930s."
  • Mike Brown on Pluto's status: "I can't imagine anything major happening that would lead to yet another reclassification of dwarf planets, with one exception. The one exception would be if somebody finds, in the very outer parts of the solar system, something that bridges that gap. Right now there's such a dramatic gap between the smallest planet, Mercury, and the biggest non-planet, Eris, that it's really, really easy to draw a line there. ... My suspicion is that this is going to stick for the next five years, and that it will stick for the next 80 years."
  • Mike Brown on Clyde Tombaugh: "The fact that he found the first Kuiper Belt object 60 years before anyone else found the next one is just a testament to what an amazing job he singlehandedly did. ... That will always be part of the legend. Eventually, I suspect, he really becomes known as the discoverer of the Kuiper Belt. ... I predict that in 100 years, if you read books about the history of astronomy, there will be a big section about Clyde Tombaugh - and there will not be a section about Mike Brown. And I think that's OK."

Correction for 8:30 p.m. ET Feb. 18: I originally wrote that Clyde Tombaugh compared photographic plates for more than a year, but he actually did that task for a mere eight months. The Planetary Society's biographical page points out that he took pictures of the night sky starting in early 1929, but the photographic plates piled up for a few more months before he was given the job of comparing them as well. Tombaugh began doing that task in June 1929.


I'll be discussing the 80th anniversary of Pluto's discovery at 1 p.m. ET Thursday on "Looking Up With Mal and Dave," airing on WKXL 1450 radio in Concord, N.H. Even if you're not in New Hampshire, you can listen online.

Astronomer Ken Croswell is observing the anniversary by pleading Pluto's case in The Wall Street Journal. (You might have to click into the article from a Google News search page to read the whole thing.)

... And wouldn't you know it? Just weeks after I laid out "The Case for Pluto" at Powell's Books in Portland, Ore., the city of Gresham has declared that Thursday will be celebrated as Pluto Day. The party begins at 5:30 p.m. at the Plaza del Sol in Rockwood.

Join the Cosmic Log team by signing up as my Facebook friend or following @b0yle on Twitter. (Every new follower brings me closer to my goal of exceeding @NeilTyson's Twitter tally.) For the full story on Pluto and planethood, pick up a copy of my book, "The Case for Pluto."