Asteroids | The Technicality that Lead to Pluto’s Justified Demotion

Noah von Hatten
6 min readFeb 16, 2024
Image Credit: NASA 2015, New Horizons Mission

The discovery of the asteroid belt proved to us that it was wholly necessary for Pluto to lose its status as a planet.

Pluto resides at the farthest edge of our solar system, orbiting our sun at a relative snail’s pace (one year on Pluto lasts 248 earth years). For 76 years, we shared our solar system with eight other worlds. Depending on your age, you may have grown up learning to memorise a planetary line-up that included Pluto. However, less than eight decades after US astronomer Clyde Tombaugh first sighted it in 1930, Pluto lost its status as a planet.

For hundreds of years astronomers applied a very simple definition for planets. A planet, according to our oldest understanding of the concept, was any celestial object that was neither a star or our own moon.

This definition, though simplistic, held up for a long time. The majority of the planets discovered within our solar system throughout the centuries were correctly classified according to it. However, this base classification would eventually prove too simple, and too flawed to be relevant on its own.

As the centuries dragged on, improved optics and technologies allowed to us to peer deeper into the sky. Astronomers began to uncover an increasing number of objects orbiting our sun.

According to what is now called the Titus-Bode Law, which states that each planet is twice as far away from the sun as the previous one, many scientists reasoned that there should be another planet between Mars and Jupiter. Finding this supposed planet was a topic of much fascination throughout the late 18th and early 19th centuries.

In 1801, Italian astronomer Giuseppe Piazzi discovered a small object located roughly half-way between Mars and Jupiter, exactly where the supposed planet was supposed to be. In 1802, German Heinrich Olbers discovered another similarly small object following the same orbit. Another planet it would seem. These discoveries would be repeated twice more. By 1807, four new “planets” had been discovered.

However, these were strange planets. They were very small, and all existed within the same approximate area of space between Mars and Jupiter. By the mid 1800’s astronomers had worked out that there were dozens and dozens of similarly small objects floating in that area of space. Today we know them to be asteroids.

Image Credit: NASA, The Brightest Points are Asteroids

Asteroids are not planets, we know that intuitively now, however, astronomers initially had no reason to believe they weren’t, since the term was used to refer to anything that wasn’t a star or a moon.

Even though they passed the planet test of the day, it became apparent that this new discovery of characteristically different objects would need to be differentiated from the other objects found in the solar system. The alternative would be to recognise the hundreds of newly discovered objects as hundreds of new additions to the line-up of planets.

The discovery of the asteroid belt called into question what was a planet, and what wasn’t. It forced scientists to consider how we define something as a planet and what characteristics an object would need to have to fit this designation. The terms asteroid and asteroid belt came into use by the 1850s to refer to these newly discovered objects and distinguish them from the planets.

The discovery of the asteroid belt also laid the ground for Pluto’s eventual demotion, a century before humanity caught its first glimpse of our most distant neighbour.

When Pluto was discovered in 1930, astronomers agreed that what had been found was indeed a planet, and Tombaugh was congratulated. However, issues with the description of Pluto as a planet started to present themselves pretty quickly.

First, Pluto was initially believed to be quite large, approximately the size of Neptune in fact. This was later revised again and again until we arrived at its final size, now believed to have a diameter 1/6 that of earth’s. The discovery of Pluto’s moon Charon in 1978 — which is almost half Pluto’s own size, causing Pluto to wobble in its orbit — put further holes in Pluto’s case. Pluto’s orbit also intersects with Neptune, which is quite unusual, being the only planet which had an orbit that crossed that of its neighbouring planet.

Image Credit: NASA, Pluto and Charon

Put simply, Pluto acts differently than the other planets in the solar system, which was a serious cause for questions within the scientific community. However, Pluto’s planet-hood remained intact until events in the early 21st century lead to a deeper understanding of the region of space that it occupies.

In 2005, another object in the same region of space as Pluto, with nearly the same size, was uncovered and named Eris. Other, smaller objects were also discovered in the same region. History seemed to be repeating itself. Just as the asteroid belt’s discovery lead to discoveries of an increasing number of objects in the same region, the expansion of our understanding of the regions around Pluto lead to the discovery of even more objects. These objects together came to be known as the Kuiper Belt.

This forced the scientific community to define what made a planet a planet, lest chaos ensued. Possibly hundreds of new objects were expected to be uncovered, which would dramatically inflate the number of “planets” in our system. That would be a lot of little balls to model for school science projects. But, if these new objects weren’t planets, what the heck were they?

We simply know intuitively that the planets such as Earth, Mars, Jupiter, Saturn, etc. are planets. But what makes them so? A hard definition was needed.

So, when the International Union of Astronomers met in 2006 the following definitions were created:

  1. In order to be considered a planet the object in question must have sufficient mass that its own gravity formed it into a sphere-like shape.
  2. It must orbit a star, but not be a star itself, or a satellite of another planet.

However, according to this new definition many of the objects found in the Kuiper Belt could be considered planets, including Eris. Thus, a third requirement was added.

The object must have cleared it’s orbit of other objects either by incorporating or pushing away other material in its path. It other words, it must have a large enough presence to dominate its orbit.

Eris, and the other Kuiper Belt objects, wouldn’t be considered planets. Planets could now be categorically defined, providing a little bit more order to our continued exploration of the farthest reaches of our solar system. Unfortunately, the new definition sealed Pluto’s fate. Since it lives within the Kuiper Belt, and hasn’t managed to push everything else around it away, it can’t be considered a Planet.

The term Dwarf Planet was invented to classify smaller planet-like objects that met some, but not all of the requirements of a proper planet.

Image Credit: NASA, Closest Flyover during the New Horizons Mission

Pluto’s demotion may not have been popular. And it could be argued that the definition of a planet agreed upon by the IUA in 2006 is just as flawed as earlier understandings of the term. Other planets have also failed to completely clear their orbits. Rogue planets also exist, which are worlds that have broken free of the orbit of a star. Regardless, Pluto’s demotion was important.

If we go around simply referring to everything new we uncover in the night sky as a planet, we soon lose the ability to define what we discover. The term planet also completely loses any sense of usefulness. Without definitions the expansion of understanding is limited.

Pluto’s demotion could be construed as necessary collateral damage. New objects required new definitions, and those new definitions didn’t leave room for Pluto to keep being a planet. Perhaps someday another new discovery will force us to re-examine our definitions yet again. Science after all isn’t fact, it’s a process. The process of asking a question, and trying to answer it. Will Pluto someday become a planet again? Who knows. But maybe it doesn’t need to to help us learn more about the tiny corner of the universe we call home.

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Noah von Hatten

Writer, photographer, star gazer, watch-collector, and history enthusiast from Germany living in the shadow of the Canadian Rockies.