The story of Pluto: the hard luck ex-planet that gets no respect.


Eighty-four years ago today, on February 18, 1930, the celestial body known as Pluto was discovered. In contrast to the dramatic discoveries of other things made by humans in the past–Antarctica, the moons of Jupiter, penicillin–the discovery of Pluto was pretty dull. Clyde Tombaugh, a 23-year-old astronomer fresh on the job at Lowell Observatory in Flagstaff, Arizona, was staring at some photographic plates of the stars side-by-side when he noticed a tiny speck was out of place in one of the plates. All the other stars were in the same place. Hence, something out there was moving. From mathematical predictions made by the founder of his employer institution, Perceval Lowell, Tombaugh and his colleagues already suspected there was a planet beyond Neptune. Now they had the proof.

When the discovery of the new planet hit the front pages, the big question was what they were going to name it. Venetia Burney, the 11-year-old daughter of an Oxford University professor, wrote to Lowell Observatory suggesting the name Pluto, lord of the underworld in classical mythology. The directors of the observatory loved the idea. Since 1846 the solar system officially only had eight planets, after William Herschel discovered Neptune. Now the solar family grew to nine. Walt Disney named Mickey Mouse’s dog after the new planet.

1930, however, was probably Pluto’s heyday. Its fortunes began to decline almost immediately. In 1931 the new planet was thought to be about the size of the Earth, but new astronomical work done in the late 1940s determined that was too generous and Pluto was in fact about the size of Mars. In the 1970s the official estimates dropped again–now researchers thought Pluto was 1/100 the size of Earth. In 1978, improved optics revealed there was something else out there: Pluto had a moon, quickly named Charon, but instead of orbiting Pluto the two seemed to orbit each other like drunken co-dependent lovers. The discovery of the moon made it possible to be more precise about how big Pluto was. Final estimate: a measly 0.0218 the size of Earth. By the turn of this century it was clear that Pluto, never very big to begin with, was a really dinky place.

surface of pluto

The surface of Pluto is very forbidding. In addition to being incredibly cold, its atmosphere consists mostly of methane–the key ingredient in flatulence.

Why should this matter? Well, the size of Pluto affects how much gravitational pull it has, and hence how it can affect other celestial bodies. Perceval Lowell had predicted his “Planet X,” which in 1930 everybody thought Pluto was, based on gravitational anomalies in the orbit of Neptune, which were believed to have been caused by another planet. (We’ve seen before on this blog how conclusions like that can turn out to be erroneous). But at roughly the size of a large Winnebago, Pluto was far too small to have that kind of pull. (Okay, it’s larger than a Winnebago–I just made that up. But still). The answer? It turned out that astronomers had been miscalculating the mass of Neptune all along. There never was any “Planet X.”

The next blow to Pluto’s reputation came in 1979. At the time of its discovery it was the most distant planet, so far that it takes 248 years to go around the Sun once. Its orbit is on a totally different plane than all the other planets, however, and in 1979 its screwy orbit carried it, for a brief time, slightly closer to the sun than Neptune. This remained true until 1999, when again Pluto became more distant than Neptune. By this time, however, especially aided by the Hubble Space Telescope, astronomers were discovering all sorts of stuff out there, mostly icy rocks hurtling around at the far reaches of the solar system. Pluto didn’t really seem that much different than them.

Finally, in 2006, Pluto suffered the ultimate indignity. An astronomer named Mike Brown discovered another icy rock out beyond Neptune that was (wait for it…) even bigger than Pluto. The thing he discovered is called Eris, and it caused an immediate problem: if Eris is bigger than Pluto, and Pluto is “officially” the ninth planet of the solar system, does that make Eris the tenth planet? Worse still, if there are other chunks of ice and rock still out there that turn out to be that same size or bigger–which is likely–are we going to call those things planets too? How many planets are we going to end up with? How many is too many? The International Astronomical Union, which in a 2003 document tried to define what a “planet” actually was, didn’t want to go there. It declared that neither Pluto nor Eris nor any of the other icy rocks that Mike Brown has discovered–including two, I am not making this up, named after characters from Xena: The Warrior Princess–are really planets. They’re now “TNOs,” Trans-Neptunian Objects. Pluto’s reign as the ninth planet of the solar system (1930-1979, and 1999-2006), or even as the eighth planet of the solar system (1979-1999), was over.

clyde tombaugh

Clyde Tombaugh was a very interesting fellow. In addition to discovering Pluto and teaching at New Mexico State, he was quite interested in UFOs and claimed to have seen one himself.

Clyde Tombaugh, who was very young when he discovered Pluto and died very old, did not live to see “his” planet demoted. He passed away in 1997. Venetia Burney, however, did. She died in 2009, saying that she didn’t really care that much but tended to think Pluto should still be a planet.

All is not lost, however. In 2006, the year of Pluto’s demotion, NASA launched a probe called New Horizons, which is scheduled to reach Pluto next year. Clyde Tombaugh’s ashes are on board. Assuming New Horizons completes the trip, he will truly have gone where no man has gone before.

The artist conception of Pluto’s surface was created by the European Southern Observatory (ESO) and is used under Creative Commons 3.0 (Attribution) license.


  1. There are a few erroneous statements here. First of all, you tell only one side of the story about Pluto when the reality is that its status remains a matter of ongoing debate. Pluto is NOT an “ex-planet,” and the fact that it and Charon orbit one another does not mean they aren’t planets. What it does mean is they constitute a binary planet system, much like the many binary-star systems we see. BOTH Pluto and Charon are small planets.

    Second, Eris is not larger than Pluto. It was initially thought to be so, but in November 2010, a team of astronomers led by Dr. Bruno Sicardy obtained a more accurate measurement of Eris’s size when Eris occulted a star, and determined that Pluto is actually marginally smaller than Pluto although it is 25 percent more massive. The latter means it is more rocky than Pluto, which, far from being an iceball, is estimated to be 70 percent rock.

    Most importantly, only four percent of the International Astronomical Union voted on the controversial demotion of Pluto, and most are not planetary scientists but other types of astronomers. Their decision was formally opposed in a petition signed by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Ironically, Stern is the person who first coined the term “dwarf planet,” but he intended it to designate a third class of planets in addition to terrestrials and jovians–small planets large enough to be squeezed into a round shape by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all.

    Many like-minded astronomers reject the notion that an object must perturb its neighbors or “clear the neighborhood of its orbit” to be considered a planet. Instead, they prefer the equally scientific geophysical planet definition, which centers not on where an object is but on what it is. According to this definition, a planet is any non-self-luminous spheroidal body orbiting a star or free floating in space (the latter to account for rogue planets). The spherical part is important because once an object reaches a certain threshold in mass and size, gravity takes over and squeezes it into a spherical or nearly spherical shape. These objects are much more akin to the larger planets in that they have geology, weather, and layering into core, mantle, and crust, just like Earth does. THIS is what differentiates objects like Eris and Pluto from the overwhelming majority of tiny objects in the Kuiper Belt. Blurring the distinction between tiny, shapeless rocks or iceballs and complex worlds is simply bad science.

    In 2009, a group of astronomers who advocate a geophysical planet definition asked the IAU leadership to re-open the discussion of planet definition at that year’s General Assembly, and the IAU leadership refused–and has continued to refuse every request to reopen the discussion.

    Interestingly, Tombaugh knew of efforts to demote Pluto, which, though revived in the 1990s, date all the way back to its discovery, and he adamantly objected to them.

    So far, now object bigger than Pluto has been found in the Kuiper Belt. Even if one is eventually found, there is absolutely no scientific merit to the claim that our solar system cannot have “too many planets” because kids won’t be able to memorize them. No one objects to there being billions of stars and billions of galaxies. No one says Jupiter can have only four moons, not 67, because no one can memorize 67 names. Memorization is not important for learning. We don’t ask kids to memorize the names of all rivers or mountains on Earth. What is important is that kids understand the different types of planets and their characteristics.

    What is genuinely sad is the way the media has simply gone along with the IAU decision, treating it as some sort of gospel truth, rather than reporting the reality, which is that it is just one view in an ongoing debate. Science is not determined by decrees from “on high,” which is why no one, including you, has to accept the notion that Pluto and Eris aren’t planets because of a vote by 424 people in 2006.

    The real paradigm shift, as Dr. Stern often says, is not from a solar system of 9 to 8 planets, but of one from 9 to 50, 100, or more planets. As Stern says, “The solar system made a lot more planets than we learned about in grade school. The solar system is teeming with planets.”

    For more on the pro-Pluto-as-a-planet side, I encourage you to read the book “The Case for Pluto” by Alan Boyle and my Pluto Blog at .

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