America was rocked when the New York Times reported, on January 22, 2001, that Pluto was no longer a planet. A huge uproar followed as we all grew up knowing there were nine planets, and yet suddenly there are only eight. A solid truth about the universe was thrown into question.

Such is the nature of science, and such is the problem of labels. As Neil Degrasse Tyson, Director of the Hayden Planetarium, explains, the word planet became stretched to irrelevancy. According to Tyson, there are many objects in our solar system, and many ways to describe them. The word planet originally meant wanderer, as the ancients observed the planets moving in different patterns than the rest of the heavens. Clearly these celestial objects that could move independently of the stars must be powerful indeed, certainly magical, and maybe even divine. But with scientific observation we now realize planets do not wander, they orbit in a predictable way, and the number of objects in orderly orbit around the sun is vast. So how do we describe these objects?

Planets and dwarf planets of the solar system

Planets and dwarf planets of the solar system (NASA)

Ancient people counted seven planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus. Neptune was discovered in 1846 by noting something off in the orbit of Uranus, and then calculating where a next planet must be. Tiny Pluto was added in 1930, making nine. The problem is that Pluto is not the ninth largest object circling our sun. Eris is larger and very far out there, and Pluto is number ten. At only 0.2% the mass of Earth, with a width that would span New York to Denver, Pluto really is tiny, and its role as a planet unclear.

Tyson makes the case for describing the solar system as it actually is. We can list the gas giants Jupiter, Saturn, Uranus, and Neptune, or the rocky bodies Mercury, Venus, Earth, Mars, and Pluto. We can list the dwarf planets Eris, Pluto, Ceres, Haumea and Makemake. Or we can list the solar system’s objects by size starting at the largest and cutting off at some arbitrary point when we’ve reached the top 10, or 12, or 20. The simple label of “planet “does not actually describe anything useful, other than celestial objects visible to the naked eye that move differently from the rest of the heavens, which is descriptive of our experience but not of the objects themselves.

If such a material concept as planets can be called into question, any label about humans and human behavior must be suspect. As with planets, we may find some labels useful shorthand, while needing to get more descriptive if we really want to describe our differences accurately, or really understand each other at any depth. Some people call this political correctness when we try to actually get our labels right, but sometimes it is essential, because the old labels may no longer apply to our enhanced understanding. Pluto isn’t changing, but our understanding of our universe is.