What is a planet? Prior to August 24, 2006, the answer was simple, vague, but generally agreed-upon: large, round-ish, orbiting a star, perhaps. Most people could name nine examples without much controversy. But the ninth planet became suddenly controversial on that Thursday in August---the last day of the International Astronomical Union's 26th triennial General Assembly.
Many of the meeting's 2,700 attendees had already left Prague by then. The few hundred who remained sat in a large auditorium facing a screen bearing the IAU's proposed definition of a planet, which had a curious addition---that a planet, on top of being round and orbiting the sun, should also clear the neighborhood around its orbit. Pluto, which orbits in the cluttered Kuiper Belt, was on trial. The crowd voted, raising canary-colored sheets of paper for or against the definition---and with a margin of fewer than 10 votes, the Ayes won.
You know what happened next. Pluto became a joke, a martyr, an object of sympathy; its demotion to dwarf planet was meme-ified and turned into t-shirt slogans. That was popular culture. In scientific culture, its reclassification opened a rift in the continuum of specialists who study planets. On one end were the orbital mechanicians, who generally support the IAU's definition because fits their top-down view of how moving objects harmonize with the surrounding cosmic orchestra. At the other, geophysicists have spent the past decade harumphing against the illogic of a definition that ignores their bottom-up, size-agnostic view of accumulated space dust. Well, the geophysicists are done protesting. They've gone ahead and written their own definition of "planet."
The authors---members of the New Horizons mission that swung by Pluto in 2015---wrote a definition that is objectively simpler: A planet is a thing that's big enough to become round by the force of its own gravity. And it makes the solar system more complicated: Forget nine planets, the solar system now has 110. It also fixes some other hanging chads in the IAU's definition (like the fact that only things orbiting this sun could be called planets).
The group revealed their definition a few weeks ago. But it will be formally displayed in the poster hall at the 48th Lunar and Planetary Science Conference, taking place March 20 to 24 in Houston. Not that the partisans on either side of Pluto's identity politics are waiting until then to make some noise.
There are essentially two ways of studying a non-star object in space. "There are the dynamicists, who are interested in motions, orbits, and classes of objects based on mass," says Owen Gingerich, astronomer emeritus at Harvard University. "And then there are geophysicists who are very keen to look at planets as actual, physical bodies."
Gingerich was chairman of the planet committee at the 2006 Prague meeting, where a small but feisty group of dynamicists worked in the wording that a planet should be massive enough to clean all the debris out from its orbit. He didn't expect the debate to be so vociferous, and had in fact bought tickets to fly home from Prague two days before the final vote occurred. He says he's not enthusiastic about how the whole ordeal turned out. "Since then I have taken a view that the IAU didn't really have any right to define the word planet," he says.
Pluto's identity crisis began as an accounting issue. In 2005, CalTech astronomer Mike Brown discovered an object far out in the Kuiper belt that he later named Eris. But before that, it had to be catalogued by a bureaucratic subdivision of the IAU. Based on its mass, an astronomer named Brian Marsden assigned it a minor planet number (the IAU had invented the minor planet catalogue in the 1850s to keep track of all the asteroids between Mars and Jupiter). But there was one hitch: This new object was slightly more massive than Pluto. To keep things orderly, Marsden gave Pluto a minor planet number as well. Once word got out, the IAU was flooded with letters from ornery astronomers. The IAU resolved to fix the issue in Prague.
Mike Brown led much of the campaign against Pluto's planethood. His Twitter handle is @plutokiller, he wrote a book titled How I Killed Pluto and Why It Had It Coming1, and he accuses anyone who believes Pluto deserves to be a planet of nostalgia. When Pluto was discovered in 1930, a New York Times story said the world was "possibly the size of Jupiter." "If Pluto were discovered today, no one would even vaguely think to call it a planet," says Brown. Compared to Jupiter, Saturn, even humble Mercury---planets with real gravitational pull---Pluto is a mote of cosmic dust.
Not everyone is so obsessed with size. "I don't care about orbit, or the gravitational effect that a planet might have other objects," says Kirby Runyon, a post-doctoral geophysicst at Johns Hopkins University. "I only care about what it intrinsically is." Runyon wrote the abstract proclaiming the definition of planet from a geophysical perspective.
What's more, he says Mike Brown and the IAU can keep calling Pluto a dwarf planet. "I'm OK with there being two definitions of planet, one that deals in orbits and a separate one that lets planetary geologists do our jobs." According to Runyon, his definition is purely functional, because researchers like himself find more similarities than differences among the solar system's 110 objects---Mars, Earth, Titan, Pluto, Charon, the moon (yes, Earth's moon)---that fall into his definition.
And his definition has some powerful co-authors. Chiefly, Alan Stern, who became a minor celebrity as the principal investigator of the New Horizons mission that flew by Pluto in 2015. Stern is probably the world's biggest Pluto fan. Not surprisingly, he's prone to pick fights with the IAU, Mike Brown, and anyone else---including journalists---who he perceives as disparaging Pluto: "You're acting as if the IAU definition actually makes a difference in my field. There are a few people like Mike Brown, who uses dwarf planet because he has a book to sell, but they are the minority. I have a joke that IAU stands for Irrelevant Astronomical Union. It's amazing that the science press doesn't begin the way the political press does and call bullshit when they hear it."
He makes some valid points. For one, the IAU is not a police force, and scientists decide for themselves what to call things. The disappointing part for him is that the IAU had such a cultural impact, and turned Pluto into the butt of so many jokes. (The definition also had a minor effect on New Horizons---launched just eight months before the 2006 IAU vote---which was programmed to photograph Pluto according to coordinates derived from the world's north pole. North poles for dwarf planets are calculated differently than those for planets, requiring the team to update the probe's protocols.)
But his broader point---which is important---is that the astronomical community contains researchers who study the same objects in space at completely different scales, and it makes no sense for one subgroup to decide what another calls the things they study. "It would be like if the lunar and planetary scientists meeting in Houston next month were to decide on what is or isn't a black hole," he says.
Brown says that's all poppycock. "There's been nothing new in the past 10 years to justify this. If you look at the solar system without nostalgia forcing you to desperately want Pluto to be a planet, it would be impossible not to recognize the difference between the the eight big things and everything else," he says. And by the way, he adds, Stern and Runyon's definition also means the moon is a planet, so is Charon, and Jupiter has four planets orbiting it. "I'm not saying these things aren't interesting, but just because it's interesting doesn't mean there's any reason to put Pluto in the same category as the other eight planets," he says.
Neither Stern nor Brown were in Prague for the 2006 vote. Stern was helping his daughter move into her college dorm, and Brown was squirreled away on Washington's Orcas Island with his family. Neither saw a need to attend the proceedings personally. But since, both keep finding themselves the target of the same question: What is a planet? It all depends on your perspective.
1UPDATE 03/2/2017 1:10pm EST --- This passage originally misstated the subtitle of Brown's book as "...And Why It Deserved To Die."
