Editor's note: this post has been updated to reflect a correction.
Many people were mystified by the action astronomers took a dozen years ago to demote Pluto to the status of a dwarf planet. Public reaction was swift and mostly negative. School wall charts and mobiles were forced to change. Mnemonics for planets in the Solar System had to be shortened. A beloved Disney character was slighted. The good will astronomers generally feel from the public was tested.
I was there. I’d prefer not to say how I voted, but I was in the room where the deed was done.
This issue came to a head in the 1990’s as astronomers learned more about the Kuiper Belt, a set of rocky objects extending from the orbit of Neptune to three times Neptune’s distance from the Sun. Eris was discovered there in 2005, similar in mass and size to Pluto. It’s likely that ongoing surveys will discover other Kuiper Belt objects as massive as Pluto in the future.
There’s a precedent for demoting Pluto. When Ceres was discovered in 1801 by Giovanni Piazzi it was hailed as a planet. But as other large objects were found between the orbits of Mars and Jupiter, they all—Ceres, plus Pallas, Juno, and Vesta—were reclassified from planets to asteroids. With its highly eccentric, inclined orbit, Pluto had always been an oddball. It’s less massive than any of the terrestrial planets, and seven moons in the Solar System.
The International Astronomical Union (IAU) held one of its triennial assemblies in Prague in August, 2006. I made the trip to hear talks on supermassive black holes and cosmology but my colleagues who worked on planets told me there might be fireworks at a session on Solar System definitions. I wasn’t disappointed.
The session was held near the end of the two-week meeting and many attendees had already left. Debate was spirited, the tone occasionally tinged with acrimony. A resolution defining planets in a list of eight that excluded Pluto passed by a large majority, although the vote was not counted.*
Less than 5 percent of the membership of the IAU voted and the room was full of astronomers like me, with no particular expertise on planetary science (but on the other hand, we had no skin on the game and no axe to grind). The new IAU rules said that a planet must meet three criteria: it had to orbit the Sun, it had to be massive enough for gravity to make it round, and it had to have “cleared out” its orbital neighborhood. Pluto stumbled at the third criterion.
After the vote and the demotion, discussion among astronomers continued. Johns Hopkins University hosted an event called “The Great Planet Debate.” Planetary scientist Mike Brown wrote a valedictory memoir titled Why I Killed Pluto and How It Had It Coming. Exoplanets were left in limbo by the definition. It seemed arbitrary and unsatisfying to use a definition that excluded thousands of “worlds” being discovered around other stars. Also, the New Horizons mission got a close look at Pluto in 2016, and revealed it to have a layered atmosphere, at least five moons, an underground ocean, an evidence of geological activity. In many ways, it’s as dynamic and complex as Mars.
Current debate centers on the third criterion: that a planet must have cleared out material in its orbit. In other words, there should be no object of comparable size at that distance. The rule is hard to apply consistently since it depends on details of the formation process. Planetary scientist Phil Metzger reviewed the research literature and found only one instance in the past two hundred years where orbit-clearing had been used to classify a planet. He said of the IAU rule: “It’s a sloppy definition.”
Astronomer Ethan Seigel added that the IAU definition gives too much weight to the location of the planet. For example, Mercury wouldn’t be defined a planet at the position of Jupiter, yet a world much smaller than Mercury would be a planet if it orbited a red dwarf star. Definitions that are arbitrary and capricious tend to annoy scientists. (On the other hand Jean-Luc Margot of the University of California, Los Angeles, has argued that Mercury would in fact clear its orbit that distance and would thus qualify, according to the IAU definition.)**
A simpler strategy would be to adopt the second criterion: a planet is any object whose gravity is strong enough to make it round. Choosing this definition would let Pluto be readmitted to the planet fold, but open the door to a dozen moons and outer Solar System bodies. But a division between small, lumpy rocks and round, geologically interesting worlds makes sense. I could vote for that.
*Editor’s Note (7/17/19): This sentence was edited after posting. It originally said that 237 voted in favor of demoting Pluto and 157 voted against doing so, with a mere 40 votes deciding the outcome.
**Editor’s Note (7/17/19): This paragraph was edited after posting to include a reference to Margot’s positon on Mercury.