Wandering stars: A brief history of defining ‘planet’

From wandering star to Earth-orbiter, the word ‘planet’ has changed its meaning many times. Pluto’s exile is only the most recent.

 Illustration by Nichla Smith

Illustration by Nichla Smith

If words have power, then the word ‘planet’ must be one of the most powerful in the space community. Whether the subject is the planets in our solar system, dwarf planets like Pluto or Ceres, or exoplanets orbiting other stars, the public is automatically interested. According to planetary scientist and New Horizons’ principal investigator, Dr Alan Stern, this is because “people don’t live on black holes, or active galactic nuclei, or asteroids: they live on a planet. It’s important to them because that’s where they live.”

I've always found the possibility of other worlds to be both inspirational and humbling. To think that there are planets upon planets waiting out there to be discovered, reminds me of the beautiful mystery of the universe that can both fuel and overwhelm the beautiful mystery of the human brain.
 Laura, 23, writer and hostess

Humans have observed planets for at least four thousand years. We know that ancient Babylonian astronomers recorded the cycles of Venus, Mercury, Mars, Jupiter, and Saturn, ascribing meanings to their positions in the sky. Occasionally, the sun and moon were also included in this list of planets visible to the naked eye. Each planet — including the sun and moon — was associated with a god: Mercury was associated with the god of wisdom, Venus with the goddess of love and war, and the Sun with the god of justice, and so on.

This fascination with planets was not unique to the Babylonians. For nearly all of recorded human history, people have ascribed special importance to planets. As Dr Jean-Luc Margot, an astronomer and professor at UCLA, put it, “For thousands of years, people have noticed that these planets are different. That may be why it’s built in to our culture that these objects are special. The sort of objects that, initially, we could see with our naked eye in the night sky, that behaved differently from the stars.”

 
  The   Venus Tablet of Ammisaduqa contains a record of the rise and set times of Venus on the horizon in the seventeenth century BC.    Wikimedia Commons    (CC BY-SA 3.0)

The Venus Tablet of Ammisaduqa contains a record of the rise and set times of Venus on the horizon in the seventeenth century BC. Wikimedia Commons (CC BY-SA 3.0)

 
I like planets because they're a changing part of the night sky. I like trying to identify them. I love conjunctions… It's like, ‘here are two objects which do not have much to do with each other at all actually, but you can see them together when usually you can't!’
– Anna, 31, software engineer

The first known use of the word ‘planet’ in English, according to the Oxford English Dictionary, was in the year 1300 in an account of the lives of Christian saints. In other languages, though, the word dates back much further. The word ‘planet’ comes from the ancient Greek ἀστήρ πλανήτης (astēr planētēs), meaning ‘wandering star’. The ancient Greeks, like the Babylonians, associated the planets with gods, often corresponding with the Babylonian pairings. Other ancient societies, like those in China and India, also connected planets with personal attributes and predictions of future events, in what would later be referred to as astrology.

In the fourth century BC, astronomy became more quantitative, focusing on the scientific and mathematical side of observing the universe beyond Earth. This specialisation split astronomy from astrology and religion. Astrology dealt with the symbolic associations between the planets and everyday life as it still does, largely unchanged, to this day. Astronomers, however, studied the planets using observations and mathematics, determining how far each was from Earth and predicting their paths across the sky.

Planets represent the possibility of a new home for the human race in some distant future — a possibility that seems all the more important as evidence mounts that we might be destroying our planet's environment, or at the least, we know it will not last forever.
– Mark, 50, Associate Professor of History

In the early 16th century, the word ‘planet’ suddenly became much more specialised. Instead of denoting anything that moved with respect to the stars, ‘planet’ meant only objects that orbited the Earth. The same planets the Babylonians knew of, their moons, and Earth itself were included. While the Sun was still widely thought to orbit the Earth, it was no longer considered a planet, as the evidence that it was actually a star continued to grow. By the late 17th century, when the public started to accept astronomers’ realisation that planets orbited the sun, the name had stuck. The moon, since it did not orbit the sun, was struck from the list of planets.

The 19th century brought a crackdown on planets. With just eleven objects categorised as planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus, as well as Vesta, Juno, Ceres, and Pallas), the working definition became dependent on size. Those objects too small to make the cut were downgraded to ‘asteroids’. One might think that this would be a fine line (especially given modern debates over Pluto), but it was not. The largest object to be reclassified, Ceres, was less than one three-hundredth the mass of the smallest planet: Mercury. Also, all of the objects that were reclassified shared similar orbits in what is now called the asteroid belt, between Mars and Jupiter. Shortly after this mass reclassification, in 1846, Neptune was observed for the first time.

 
   In the 19th century, objects that were too small lost their planet status and reclassified as asteroids.   Artist impression of asteroid belt.     NASA/JPL-Caltech/Wikimedia Commons    (public domain)

In the 19th century, objects that were too small lost their planet status and reclassified as asteroids. Artist impression of asteroid belt. NASA/JPL-Caltech/Wikimedia Commons (public domain)

 

The last object to be officially added to the list of planets was Pluto. A planet beyond Neptune had been theorised for some time before Clyde Tombaugh actually saw Pluto in 1930, as slight wobbles in the orbits of the outer planets suggested a body larger than Earth lay beyond them. In 1915, estimates put Pluto’s mass at about seven times that of Earth. Now we know that Pluto’s mass is only about one five-hundredth that of Earth. It’s easy to see, then, why Pluto was initiated as a planet without question, and also why that confidence in its planethood waned with time.

A planet is big and round and it can be lots of different colours.
– Olive, 5

In 2006, though, the word ‘planet’ underwent its first major transformation since the reclassification of the asteroid. The International Astronomical Union (IAU), a group of astronomers charged with standardising the names and designations of astronomical objects, decided it was time for a change, stating: “The word ‘planet’ originally described ‘wanderers’ that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.” They issued a formal definition of the word ‘planet’ with a new set of criteria. According to the IAU, a planet must: orbit the Sun, have enough mass to remain approximately round due to its own gravity (the hydrostatic equilibrium criterion), and have cleared its orbit of other objects (the dynamical dominance criterion).

 
   The New World Order: papier m  â  ch  é     school diagrams just got a little harder.     Farry/Wikimedia Commons    (public domain)

The New World Order: papier mâché school diagrams just got a little harder. Farry/Wikimedia Commons (public domain)

 

In the same resolution, the IAU defined a dwarf planet as an object that directly orbits the sun (and thus isn’t a moon), has enough mass, but has not cleared its orbit. Essentially, a dwarf planet is a planet that still smashes into lots of things as it orbits the sun. Because it’s not big enough to obliterate or absorb them, it hits them like a tetherball every time it goes around.

It seems like all of the things that we use to classify a planet a planet causes the classification of 'planet' to align with the properties of the rock we call home... We humans use what we know as a ruler, so when it comes to planets, I'm quite fond of them, because that's where we live — on a planet.
– Tim, 38, student and mechanical designer

These new IAU definitions have been controversial. Dr Margot described the new definition of ‘planet’ as being “problematic in a few ways. One is that it applies only to our solar system: that’s a problem. And two, it is not quantitative: that’s a problem. And there is, in fact, a third problem, which is that it relies on some idea of roundness, which is not observable, not easily quantifiable, and it’s inherently problematic as a classification tool.” Dr Margot, along with many other scientists, prefers the criterion of dynamical dominance: if an object orbits a star and has cleared its orbit, it is a planet. “The nice thing about dynamical dominance is that we can show fairly easily that any object that is dynamically dominant is going to be round, just because it has enough mass,” he insisted. That makes it simply another measurement of size and mass that sets a higher bar than the hydrostatic equilibrium criterion.

Dr Stern takes exception to the IAU’s definition of ‘planet.’ He prefers the geophysical planet definition, in which any object with enough mass to become round (the hydrostatic equilibrium criterion), but not enough to cause nuclear fusion (which would make it a star), is a planet. The result, he said, is that “you end up with a very large number of planets, in our solar system and in others, as a result. But we’ve just got to get used to the data that there are a lot of planets and that most of them are this really tiny type like Pluto.”

 
  Pluto’s demotion from the planet club had some Pluto lovers up in arms. Why are planets so important to us?   Javacolleen/Flickr  (CC BY-NC-ND 2.0)

Pluto’s demotion from the planet club had some Pluto lovers up in arms. Why are planets so important to us? Javacolleen/Flickr (CC BY-NC-ND 2.0)

 

Pluto’s exclusion from the 'planet club' has caused some degree of public outrage. There have been petitions, “Save Pluto” movements, and even outright refusal to believe that Pluto is not a planet. Dr Margot said that “50 years from now, I fully expect that people will not care that Pluto is no longer a planet, just like we no longer care that Ceres and Vesta and Pallas are no longer planets.” 

I like planets because they're all so different, so each one expands our knowledge of how varied the universe is.
– Jason, 65, retired

Along with many other scientists, Dr Sarah Hörst, assistant professor of Planetary Science at John Hopkins University, “[doesn’t] particularly care one way or another if people consider ‘dwarf’ an adjective or if people are adamant that dwarf planets aren't planets.” Dr Stern, on the other hand, disagrees. When he coined the term 'dwarf planet' in 1991, it was meant as a description analogous to those we use for stars: “Did you know the sun is a dwarf star? That doesn’t make it not a star, does it? It just means that it’s a small star compared to giant stars, and Pluto is a small planet compared to giant planets.”

Regardless of the IAU’s categorisation of Pluto, its definition certainly needs some work: no one will argue that extrasolar planets or exoplanets, which orbit other stars, are not planets. However, excluded by the IAU’s definition, they remain officially uncategorised by the scientific community. The classification scheme for exoplanets remains fairly casual, relying on comparative classifications like 'hot Jupiter' or 'super-Earth.' Even though we can’t see most of these planets yet, classification would simplify and clarify the language astronomers use to describe them. “It wouldn’t serve us very well if we had a classification that required us to wait thousands of years until we can get to the planet, see what it looks like, and then decide whether it’s a planet or not,” Dr Margot explained.

 
  Our traditional meaning of the word planet hasn’t always meant what we think it has. But changing knowledge ingrained in primary school is never easy.   Eden, Janine and Jim/Flickr  (CC BY 2.0)

Our traditional meaning of the word planet hasn’t always meant what we think it has. But changing knowledge ingrained in primary school is never easy. Eden, Janine and Jim/Flickr (CC BY 2.0)

 
They're interesting because we haven't ever had manned missions to any planets. We only have gotten pictures from space probes, and we could get more information about them if we had people looking at them. And we don't, so we don't get as much information. Basically, if we didn't have planets, we couldn't live. ‘Cause we live on a planet. ‘Cause usually when you think of space you don't usually think of Earth. But Earth is a planet.
– Archie, 7, aspiring astronomer

Dr Hörst put it most simply: “The definition might need to evolve; history certainly tells us that our understanding of the solar system and the universe is incomplete at best.” The language that we use is constantly evolving: the word 'galaxy' comes from the Greek for 'milk,' and the only remaining reminder of that is the name of our galaxy, the Milky Way. Perhaps in another thousand years, the original meaning of ‘planet’ will be nearly forgotten, too. We will almost certainly discover new astronomical bodies, and with those new discoveries will come new names. 

Regardless of whether they are planets, dwarf planets, exoplanets, or simply 'worlds,' as many scientists have taken to using as an umbrella term, humans will always be fascinated by them. On that, the astronomers all agree. As Dr Hörst said, “People love planets! People love the unknown, the unexplored, the weird, the beautiful, the different. Planets are all of those things. But so are asteroids and Kuiper Belt Objects and moons.” Classifying these objects is important to how we discuss them, and the word ‘planet’ has a rich and dynamic history. But when we look at those objects  — planets or not — from down here on Earth, what matters most is the awe, fascination, and scientific inquiry that they inspire. 

Edited by Bryonie Scott and Tessa Evans, and supported by Sam Brentnall.