NASA Administrator Jim Bridenstine declared Pluto a planet again in a video this week. I’m assuming Bridenstine was half-joking, since there is someone laughing in the background, and I think I see a smirk on his face. But now is as good a time as any to discuss the complicated issue of Pluto’s planethood.
Pluto is a planet. Pluto is also not a planet. The word “planet” is in fact an outdated concept that fails to capture the universe’s complexity.
A planet initially referred to anything that wandered across the sky, including the comets, the Sun, and the Moon. As scientists looked closer at the objects in the sky, they realized that Earth orbits the Sun, and that the other objects in the solar system could be further sorted. Planet earned a “you know one if you see one” kind of definition—Mercury, Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are pretty obviously planets. The Moon wouldn’t be a planet, because it’s a moon. We’ve since assigned importance to these named planets and identify strongly with them.
When Clyde Tombaugh spotted Pluto in 1930, he saw a spot wandering across the sky. By definition, he was looking at a planet.
But generally, the more you interrogate a category, the less well-defined the category becomes. In 1992, scientists David Jewitt and Jane Luu discovered an object in the then-theorized Kuiper Belt beyond Neptune, called Albion. Later followed Eris, Makemake, Haumea, and researchers soon realized there were a bunch of Pluto-like objects in the outer solar system. Eris was especially problematic, since it is more massive than Pluto. The question became—do we make these large Kuiper Belt Objects planets, or do we demote Pluto?
In 2006, the International Astronomical Union picked option two by defining an unsatisfying set of criteria for a planet. Planets must be round due to their own gravity, they must orbit the Sun, and they must “clear their neighborhood,” a pretty unscientific phrase that basically just means that they’re the dominant gravitational body in their orbit. Pluto and Eris earned the designation of “dwarf planet,” since they didn’t “clear their neighborhood.” It kind of makes sense. Look at Pluto and Kuiper Belt Object Charon together, and you’ll see that Charon doesn’t orbit Pluto; the two orbit a point between them.
Since then, it’s been hard for people to accept the loss of Pluto’s planethood—which makes sense, because we love our planets. Plus, NASA’s very expensive New Horizons mission to Pluto had just launched in 2006, and visiting a dwarf planet when you thought you were visiting a planet is akin to scientists declaring some other mountain the tallest partway through your Mount Everest ascent. Two years ago, scientists including New Horizon’s principle investigator and Pluto evangelist Alan Stern proposed to basically make every round object smaller than a star a planet. That definition also doesn’t work, by the way, because there are plenty of objects that straddle the line between star and planet.
This debate is not objective. Today, the word is most important to astrologers, nostalgic humans, and universities and agencies like NASA whose funding relies on the word’s definition. Neither Pluto nor the rest of the objects in the solar system nor the laws of physics have heard of Jim Bridenstine or NASA or Alan Stern or any humans, by the way.
Pluto, without a doubt, fits the ancient definition of planets. But as we learn more about planets, we’ve realized that it simply doesn’t belong in the same category as the other eight things we call “planet.” The problem isn’t with Pluto. The problem is that we assign far too much weight to the dated term, and language hasn’t evolved as fast as our understanding of the universe.
You will continue to see scientists, crackpots, and news articles debate Pluto’s planethood. But in actuality, it’s one of many large, round, and interesting icy masses beyond Neptune... and that’s it. Reality doesn’t satisfactorily fit into our semantic compartments.