Worldbuilding the Definition of Planet

What is a “planet”, anyway? A perhaps surprisingly contentious question, that extends far beyond the most recent flare-up of controversy, which centered around Pluto’s status in the planetary club, and despite the pretenses of the International Astronomical Union the jury’s still out on that one. If you ask me a planet is any gravitationally rounded object that’s not a star. That would include not only Pluto, but also the planetary-mass moons as well.

 Πλάνητες: where it all began

This is what’s sometimes called the geophysical or planetological approach, classifying objects by what they are, i.e. their intrinsic properties. The dynamical approach, on the other hand, classifies objects by how they move, i.e. their orbital dynamics. Historically the latter has been the most prevalent current in astronomy. From the beginning, the very word “planet” comes from the ancient Greek πλάνητες ἀστέρες, wandering stars, often contracted to just πλανῆται, wanderers, so named because these objects wandered in the sky relative to the background of fixed stars.

Even back then there was some ambiguity. In classical Greece the obviously disk-like Sun and Moon were often considered a different type of object from the five star-like πλανῆται, especially among astronomers, but lumping the two into the group to make seven πλανῆται seemed to be more common.

A reconceptualization came when Copernicus offered the heliocentric model of the solar system, which was eventually widely accepted, with the essential criterion in this view being that a planet was an object that orbited the sun. Notice how dynamics prevail in this definition as well as the ancient form. The truly revolutionary realization was that Earth itself was a planet.

Satellite Planets, past and present

In 1610 Galileo observed the four planetary-mass moons of Jupiter, which he referred to as “four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness”. Similarly, Christiaan Hyugens referred to Titan using, variously, the terms “planeta” (planet), “stella” (star), “luna” (moon), and “satellite” (attendant). The usage of “planet” to refer to satellites persisted for much longer than most people would probably think. In 1787 William Herschel referred to the two moons of Uranus he discovered as “secondary planets”. As late as 1868 the book “Smith’s Illustrated Astronomy” was still referring to satellites as “secondary planets”.

And get this: the usage has not died out even now. In the scientific literature of planetology references to the planetary-mass moons as planets persists; Titan’s haze, for instance, is referred to in papers as a “planet-wide haze” without any scientists missing a beat. The very field of “planetary science” includes all objects of planetary mass in its remit, no matter where they may be located. Alan Stern, most prominently, has expressly advocated for a definition of planet which encompasses the planetary-mass satellites, which he proposes to call “satellite planets”.

The Asteroid Belt: from in to out of the Planetary Club, and back again?

On the other hand, Ceres, the largest object in the Asteroid Belt, was discovered in 1801 and promptly declared a new planet, followed quickly by Pallas, Juno, and Vesta. At the time their small size and large number led them to be considered a different category, namely asteroids (meaning “star-like”), but they were still listed among the planets for decades. After the 1850s, more objects in the region were discovered, quickly increasing their numbers into the hundreds, so the collective term “minor planets” was coined. To this day the outfit that keeps track of asteroids is named the “Minor Planet Center”, their list the “Minor Planet Catalogue”. Hmph.

Seems the asteroids’ linkage to the planetary concept hasn’t completely died out either, though even the most liberal definitions like Stern’s would exclude them, with the exception of Ceres, which is gravitationally rounded and is indeed considered by the IAU to be a “dwarf planet”, the latest addition to the not-quite-planet club (and probably dumbest: dwarf stars are stars, dwarf galaxies are galaxies, but dwarf planets are not planets!?). In this case it was dreamed up to shuffle Pluto off the list of “real” planets after it was discovered that, like Ceres before it, it was not a singular object but rather the harbinger of a host of planetary-mass bodies in its region. Again, the dynamical mentality works its will.

Dynamic Mentality vs Geophysical Mentality

One reason the dynamical mindset has been so dominant is because for ages planets were viewed solely from Earth as stars wandering the sky, so distinguishing the category based on their orbital dynamics made sense. This continued to be true for a very long time, but now that we’re sending probes and, quite possibly, even people to these places it’s starting, little by little, to look more and more awkward to give an object a completely different classification based on where it’s located even if it’s geophysical properties are identical in either case. After all, a star is still a star no matter what its orbit is up to.

I suspect that the planetological approach will win out in the end. When you’re standing at an object or looking right at it close-up do you really care what its dynamics are or what kind of object it’s orbiting? Chances are, no. Planetary scientists have never drawn much distinction between, say, Ganymede and Mercury, and their view of things will attain ever-greater relevance in the future. Interestingly, the working definition of a planet seems to be considerably more liberal and planetological in the context of extrasolar planets.

Notice there’s never consideration of dynamical dominance or “clearing the neighbourhood”; if an object is round and not a star it’s put into the planet category. Indeed, sometimes it seems the working definition in this context is “any object that’s not a star”; PSR B1257+12 D was supposed to be only a fifth the size of Pluto, probably not even rounded by its own gravity, yet it was put into the planet category too! (yes, the claim of its discovery was later retracted, but that’s immaterial for the purposes of this analysis)

Defining a Planet by intrinsic Properties: even the Ancients did it!

The planetological approach isn’t even a new mentality; it’s always been with us. From time immemorial comets were never considered to be planets, despite being wandering objects in the night sky. The reason? Because having a fuzzy core and a long tail, not to mention that blue-white color, their intrinsic properties were obviously very different from the πλάνητες ἀστέρες. Even to the ancients geophysical properties trumped orbital dynamics, which in the comets’ case were fundamentally identical to the likes of the Moon, the Sun, and the other planets. Intrinsic properties were likely also the reason many ancient astronomers separated the Moon and the Sun into their own category.

In my view the primary reason the dynamic approach was so strong for so long was because for a long time the planets’ orbital dynamics were about the only property that was known about them. Notice that when information about their intrinsic properties was available, i.e. in the case of the comets, those properties won out.

The Kuiper Belt’s History, and alternate History, with the Planetary Club

So what does any of this have to do with worldbuilding? As in making a fictional universe? Well, one point here is that even now, after the IAU attempted to create an authoritative definition, there are many working definitions of and varying usages of the word “planet”. There’s nothing inevitable about the list we see today.

Which brings me to consider my own fictional universe. As I touched upon in my post “Etymologies of the Elements”, I think I’ll have Percival Lowell himself discover Pluto in 1916, bringing forward its discovery by a full 14 years. In real life Pluto was widely considered a planet at the time of its discovery, and due to it being the only known object in its region that classification stuck until the 2000s when other similarly-sized objects were discovered in the same area.

It’s a bit odd that Pluto was discovered so early and the other planets in the Kuiper Belt followed only so late, but Pluto just so happens to be by far the brightest Kuiper Belt object. Eris is slightly more massive, but throughout the 20th century it was located much further away than Pluto, and so is harder to spot; it’s fainter, and moves slower in the sky. The same disadvantage permitted the other planets like Quaoar, Sedna, Makemake, and Orcus to avoid discovery until the 2000s.

The slow movement was the kicker; for example, Orcus was being spotted by the telescope at Palomar as early as 1951, but wasn’t recognized for what it was until much later because it was moving slowly and nobody was looking particularly hard for planets in the far outer solar system at the time. Back then you had to laboriously compare photographic plates manually; because of the search for “Planet X”, an object perturbing the orbit of Neptune (that was later found to not exist because the original calculations for Neptune were in error), the Lowell Observatory was willing to commit the resources required to do so, but routine sweeps of the sky for such dim slow-moving objects were not done until the process could be automated with computers. That came in the 1990s. Better telescopes able to achieve higher resolution helped too.

The War for Pluto’s Planethood: even more epic in my Universe than in real life? Uh oh…

Which brings up a really interesting point in my fictional universe. The first space telescopes, of similar caliber to the Hubble, are launched in the 1940s, with whole fleets of them lofted up into the 1950s. Hubble can, of course, easily spot all of the currently known planets in the far outer solar system, even Sedna, which is much more distant than Pluto. Computer technology is a full half century ahead of real life, so the sort of sweeps that we were performing in the 2000s should be achievable by the 1950s or even 1940s.

Just a few decades after its discovery, Pluto’s brethren will be revealed, one of which will be even more massive! In addition, by the 1940s at the latest Charon, Pluto’s companion planet, should be discovered, making it trivially easy to calculate the true masses of both, revealing, as we found out in the 1970s, that “Planet X” was far less massive than originally thought. Would this jeopardize Pluto’s planetary status even more than in real life? In any case I’m sure a massive flare-up in controversy over what a planet is would occur in the mid 20th century.

The mid 20th century, on the other hand, also happens to be the same time human spaceflight is greatly expanding. To cosmonauts walking and living on the Moon, and ditto for the solar system’s other worlds in the not-quite-planets club, the view that the spherical and geologically differentiated body they were living on and looking at was a planet, regardless of where it was located, would seem natural and obvious, breathing new life into the concept that the planetary-mass natural satellites should count as planets. In addition, space probes would quickly reveal the fundamental similarities in intrinsic geophysical properties between “true” planets like Earth and Mercury on one hand and Ceres and Pluto on the other hand.

In my universe, as I suspect will eventually be the case in real life, the planetological view centered around intrinsic properties will prevail, but it will be a heated fight, with the revelations of the mid 20th century giving both sides of the debate ample ammunition. As touched upon in a few of my stories, it’s still a live question well into the 21st century, though like geographers and the word “continent” there doesn’t seem to be an attempt in my universe by anybody to craft an authoritative definition of “planet”. Rather, it evolves by common usage and convention among individual scholars, just as it always has, and, hopefully, always will be. Now that’s a nice thought, isn’t it?


Although it’s a nebulous concept, planethood has always been something of a natural category, from the ancients’ πλάνητες ἀστέρες to the moderns’ planetary-mass objects. As such, it will always have a prominent place in the hearts and minds of men. So, science-fiction writers and worldbuilders, aspiring and seasoned alike: what does the concept of “planet” look like in your universe? What’s your take on the subject? Applying a little imagination to just this one seemingly insignificant detail can add quite a bit of flavor to any fictional world, be it in the past, the present, or the future.

3 Replies to “Worldbuilding the Definition of Planet”

  1. The discovery of Pluto was, in a sense, the result of a mistake. An error in calculating the perturbation of Neptune’s orbit led Lowell to look for a much more massive object than it turned out to be. Without that error, he might not have suspected there was another body out there to look for, and Pluto might not have been discovered till later.

    1. It was pretty weird that Lowell miscalculated where Planet X was and right there just so happened to be Pluto. Though like I said, Pluto is much easier to spot than the other KBOs, so it very likely would have been the first one discovered in any case, but I don’t think in most timelines there would have been a 62 year (!) gap between Pluto and the first other KBO’s discovery, nor would there have been a 75 year gap between Pluto’s and Eris’s dates of discovery.

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