Every few months, an article appears in some news source or another announcing the discovery of a new species. Sometimes the specimen discovered is something never before seen, but seemingly far more often, the announcement is something a bit more subtle. Members of one species were found to actually be something new. The operative word here is “found” and the reason why is the reason I’m writing this post.

Species don’t exist in the sense you likely think they do. We’re taught from learning basic animals in our earliest years of schooling to memorizing facts in high school or college biology courses that the organisms of the world are divided into categories, each of which is subdivided into further categories, further and further, until you end up at the species level.¹ The fundamental dogma here is that things of the same species are alike and things of different species are different.

What defines a species, you ask? Well it depends on who you’re talking to and when. In 1942 Ernst Mayr introduced the “biological species concept” centered around the idea that organisms of the same species can mate and produce viable young, and organisms of different species cannot. But of course, coywolves are hybrids of the coyote (Canis latrans) and wolf (Canis lupus), and various crucian hybrids exist between types of carp (Cyprinus carpio for the common carp) and goldfish (Carassius auratus). Before Mayr the common consensus of species concept was morphological — species share common physical traits. In recent decades the common consensus has shifted toward the genetic — species share genetic traits. But how many traits? How close is close enough to be a species; how far is far enough not to be? This is a problem going back to the earliest conception of species.

Planets work the same way. I learned in school that Pluto was a planet. My younger siblings learned that it was not. Many were disappointed when on August 24, 2006, Pluto was demoted from “planet” to “dwarf planet”. Did Pluto change between its discovery and original designation of the “planet” label in 1930 and its demotion in 2006? Not in any meaningful way; change on the planetary scale (or perhaps the dwarf-planetary scale) occurs in timescales of million of years. So why was it demoted?

To astronomers, a planet is something special. A planet isn’t just any celestial body orbiting a star — we’ve got plenty of those we label asteroids or comets — planets are the ones that feel substantially different from the others. They’re not part of a group; they’re unique. As the advancement of telescopes enabled the discovery of more and more objects with orbits and characteristics similar to those of Pluto, we stopped seeing Pluto as special.

Why does Pluto get to be a planet when Quaoar, Makemake, Haumea, and finally Eris don’t? If those four do, what about Sedna? What about the rest of the celestial bodies orbiting in the Kuiper belt? Where do we draw the line? Neptune is special, it’s far larger than any celestial objects in the area. It’s also a gas giant, composed chiefly of gas held together by gravity while the others are far icier. Uranus is special for similar reasons. Jupiter is so massive its gravity may be a major reason Earth hasn’t been hit by deadly asteroids much in the past few billion years. Saturn even has rings! So Pluto isn’t special enough to be a planet.

But this feels so arbitrary! You’ll remember I mentioned asteroids and comets as designations for celestial bodies that aren’t special enough to be planets. But Pluto, too close to the public’s hearts to be demoted all the way to asteroid, was given the new designation of “dwarf planet,” a category created just for Pluto. And that’s after the discovery of 2060 Chiron and subsequent uncertainty about whether it’s more asteroid or comet led to the introduction of the new designation of “centaur” in 1977 for Chiron-like bodies. Why Centaur? Because in Greek mythology Chiron was the “wisest and justest of all the centaurs”².

The gap between the way most people view terms like “species” and “planet” is a misinterpretation between essentialism and nominalism. It’s the direction the arrow of causality points. An essentialist understanding of species would dictate “there’s a natural thing we call a species, and we’re just figuring out which one each organism is.” Meanwhile, a nominalist might say “we want to understand the world, and making convenient categories is an easy way to do so.” While most might think we live in a world defined as in the first case, very few natural categories exist; almost all are manmade.

Look around you. How many categories for things that weren’t created with their categories predefined feel essentialist to you? A Mazda Miata is a Miata because it was created from a master specification detailing how Miatas are made. A starfish is a starfish because someone long ago spotted one and proclaimed “Look! It’s not a star-shaped rock, it’s more like a fish!” and there are plenty of animals that are by definition starfish that you might think don’t deserve the distinction³.

There’s one final piece of this puzzle, which takes us all the way to the incentive system in academia. Categories are deeply interwoven with research and scientists, and research and science tends to be done in academic settings. The types of categories with the greatest essentialist-nominalist conflict tend to be those in the scientific literature, and incentives in many academic fields push strongly in the direction of creating new categories.

Academia doesn’t run on dollars — if you think you’re underpaid, ask a doctoral student how hard they work and how much money they make — it runs on reputation. Careers in research make or break on the quantity and quality of one’s discoveries. Quantity is quantity, and quality is determined by how prestigious the publication where the work is published and how many heads it turns. “Discovering” a new species or a new planet is a great foundation for a promising career in the sciences; discovering many is even better. Making a change to the basic facts of the universe every child learns alongside their ABCs lands your name in the history books.

So how in academia do you change an established fact in the literature? How do you demote Pluto as a planet? You make an argument. You provide evidence. You get your argument published somewhere prestigious. You talk to researchers. You push and push until your position is accepted, because if you think getting your viewpoint accepted by the masses is good for your career, what do you think happens if your arguments are broadly rejected? Researchers are people, the academic institutions are run by people, and changing the very facts underlying our world comes down to making a convincing case.

Most of the categories we view as natural are created. Often the problem is the instinct to categorize itself. Almost every property of a celestial body is continuous, not discrete. Is it icy? A little. Is it large? Very. Without discrete distinctions to form the boundaries of our categories, the types of categorization we favor are often inconsistent with the reality of the situations we seek to categorize. If you choose to divide celestial bodies into categories like planet, asteroid, comet, or indeed centaur, the categories will by necessity be arbitrary. The Creator didn’t differentiate between a planet and a dwarf planet, and with at least hundreds of billions of each in our Milky Way Galaxy alone, the distinction will always be a line drawn in sand.

Species are the same. Sometimes two members of two different species can be more similar physically or even genetically to each other than to other members of their own. Genetics (more accurately, genotype) is functionally continuous given how many hops there are between one species and another. Transitioning genetically from, say, a banana to a human would require somewhere on the order of billions of mutations. Can there ever be a non-arbitrary answer to where along that line you say it’s no longer a banana and starts being a human? Or does it become something else entirely in the interim, and where does that happen?⁴

So we either accept that our categories are fundamentally flawed, not because we haven’t learned enough about the things they categorize, but because of fundamental limitations of the ability of those things to be categorized at all. Or we continue to be awed when we see announcements like “researchers discover that the white-eyed individuals of Borneo, in fact, represent an entirely new species”. The difference between the cream-vented bulbul (the original one, Pycnonotus simplex) and the cream-eyed bulbul (the new one, Pycnonotus pseudosimplex)? One has red eyes. Definitely a new species.