One of the stranger arguments against counting Pluto as a planet is that if we were to include any object of its size and mass that doesn’t orbit a bigger planet, then our solar system would just have too many planets in it for our brains to handle.
The idea got a lot of play back when the International Astronomical Union was debating whether Pluto should be considered a planet. But evidently it’s become so widespread that even critics of the IAU’s decision are buying it:
In a sense, the IAU chose to cauterize a wound, a weakness in the definitions, that if left unchanged, would have led to who knows how many planets in our Solar System.
Mostly, that article goes to argue for a new set of definitions for celestial bodies that isn’t just haphazardly tailored to produce a specific set of planets in our solar system. But it still doesn’t explain how the discovery of a new planet would constitute a wound. Evidently, without official intervention, we should fear that a bunch of fuzzy pictures of small planets might start spilling onto high school astronomy charts, and freak out our impressionable youths who need to be able to count everything using their fingers.
Yet this is apparently exactly what the author fears. The piece doubles down on the idea later on, raising the specter of “the number of planets rising year after year and never settled and with no means to make reasonable comparisons between planetary systems throughout our galaxy and even the Universe.” Imagine if someone used that reasoning to argue that our galaxy has too many stars.
I think what’s really going on here is that a lot of preconceived ideas about our solar system, and planet formation in general, have been challenged over the last couple decades. Every advancement in our ability to detect planets outside our solar system has challenged our preconception about how a planet should form and behave.
For instance, the first extrasolar planets we found were in orbit of pulsars, which wasn’t supposed to be possible, and then we find a bunch of worlds bigger than Jupiter that orbit closer than Mercury does to our sun. By now, when we’re almost able to detect planets relatively similar to Earth, we’ve found plenty of evidence for planetary systems that look nothing like our own.
At the same time, our own solar system was getting dramatically more complicated. We could tolerate Pluto when it looked like a singular oddity, but when you find a whole swarm of other Kuiper belt objects, then people start asking if Pluto is really special enough to be called a planet. (The same thing happened to Ceres, which was considered a planet before we learned the asteroid belt was a thing.)
Which brings me to another idea. We tend to think every planet needs to be special. Right now, every planet in our solar system has its own special defining traits: Mercury is the hot rock; Venus has the crazy atmosphere; Earth is, well, Earth; Mars is a big red desert; Jupiter has the giant storms and all the moons; Saturn has the rings; Uranus is flipped on its side; and Neptune is…really blue.
Pluto used to be special, too. It was the little ball of ice with a big moon and a slanted orbit. The fact that it was another terrestrial world was actually nice: it bookended the solar system, so the smallest and second-smallest planets were on opposite ends of the lineup.
But then we found Haumea, and Makemake, and Eris, and suddenly Pluto didn’t look so unique anymore. And when it stopped being unique, then it had to stop being a planet.
In fairness, I would like Pluto to be considered for some rather unscientific reasons. For one, Pluto fits my own arbitrary sense of what a planet ought to be. For that matter, so do the rest of the dwarf planets. But more fundamentally, I want our solar system to have more planets in it. I don’t think having a neat list of eight does justice to how huge and varied our solar system is.
I always wanted us to find a tenth planet, partly because I liked the idea that even in our solar system, there was still more out there to be found. From a scientific standpoint, that’s true regardless of whether Pluto is a planet or not, but astronomy has always been about discoveries that capture the human imagination, and the discussion over the last decade has been less inspiring than exclusionary.
There’s probably no coincidence that the lead scientist on the first mission ever to visit Pluto doesn’t like that the world was redefined out of planethood, or that one of the scientists whose discoveries led to Pluto’s reclassification can joke about “killing” it. “Planet” has always been less of a scientific term than a status symbol, and the “dwarf planet” idea is a way to keep the rabble out.
Maybe if Ceres had stayed a planet, we’d have a better impression in popular culture about what the asteroid belt actually looked like—not a crazy mass of donut-shaped rocks for the Millennium Falcon to dodge around, but a bunch of lonely rocks that couldn’t draw enough mass together to become another Mars. Maybe Eris could have been used to show how many new planets there could be in our own solar system, waiting to be found.
Or maybe not. But either way, adopting arbitrary limits about what should count as a planet, specifically to end up with fewer planets, is a sad and silly solution to an imaginary problem.