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What Counts as a Planet?

pluto, with neptune in the background
NASA images/Shutterstock

What’s a planet? At first glance, the answer may seem obvious, but the more you think about it, the more complicated it gets.

Planets, of course, are the large, sphere-ish objects hanging around in space, like the one we live on. However, not all round space objects are planets. There are suns, moons, and things like Pluto that seem to baffle even experts.

So, where do we draw the line between planet and not-planet? Let’s find out!

Where Does the Word “Planet” Come From?

The word “planet” comes from an ancient Greek word, planēt, which meant “wanderer.” This certainly seems appropriate for the planets of space.

The first known use of “planet” was in the 14th century, a century that also gave us words like “abyss.” At first, the term referred to just seven celestial objects: Mercury, Venus, Mars, Jupiter, Saturn, the moon, and the sun. That’s because early observers saw that these objects seemed to move in the sky separately from the stars.

By the 1600s, though, people started to use “planet” to mean only the celestial objects that orbited the sun. Gradually, we discovered the rest of our solar system’s planets, for a total of nine. This pretty much gave us the modern definition of planet—although the 1992 discovery of exoplanets, or planets outside our solar system, changed things a bit.

Then, in 2006, the International Astronomical Union (IAU) made headlines with a stricter definition of planets that left Pluto out. Pluto was no longer considered a planet, but a dwarf planet instead.

The move was controversial (at least among a public who grew up with nine planets in the solar system), but the IAU is ultimately in charge of planetary science definitions. So, the concept of dwarf planets changed the definition of planets once again.

Of course, changes to the definition aren’t over: definitions and words evolve with time. In the future, the meaning of “planet” may change yet again, as it has many times before. However, many other discoveries have also complicated this definition historically— let’s take a closer look.

Defining Planets Through History

silhouette of astronomer and telescope against night sky

Space study is a relatively new field in human history, and like most things, we got a lot of it wrong at first. Looking at celestial objects through the world’s first telescopes (and before that, by merely looking at the sky) was one thing. Knowing what we were looking at was quite another.

In ancient Greece, people thought the Earth was the center around which the known planets rotated. In the centuries that followed, of course, we learned that Earth itself was a planet orbiting the sun and gradually discovered the other planets in our solar system.

However, many things complicated our definition of planets along the way, even before the relatively modern additions of exoplanets and dwarf planets. Asteroids were one.

Most asteroids hover in the asteroid belt between Mars and Jupiter. First discovered in 1801, asteroids are sometimes called “planetoids” or “minor planets,” which hints at their complicated relationship with planets. At first, people thought they were a type of small planet, although we now know they’re quite different.

But although asteroids generally aren’t planets, some objects challenge that notion. For example, there’s Ceres, the asteroid belt’s largest object, which is considered both an asteroid and a dwarf planet. Ceres was actually the first asteroid ever discovered, back in 1801, but was only classified as a dwarf planet in 2006.

In the late 20th century, we also started discovering many large, planet-ish objects in the Kuiper Belt (where Pluto resides), a belt of celestial objects outside Neptune’s orbit.

We’re still learning about the Kuiper Belt and everything it contains. Someday, the dwarf planets and other objects found there could change the definition of planet yet again. Plus, there’s also Sedna, a strange planet-like object that appears to orbit all by itself, way outside the Kuiper Belt. Its orbit takes 10,000 years! Learning more about it could tell us more about planets in general.

To make matters even weirder, the strange orbits of some Kuiper Belt objects have led to the theory of “Planet X” or “Planet Nine.” This hypothetical planet could orbit beyond Pluto, with enough gravity to cause the strange orbits seen in the Kuiper Belt, but we’re not yet sure if it exists or not.

Why Is This Definition So Difficult?

As you can see, the definition of planet is complicated because there are many different types of spherical objects in space, and our abilities to observe those objects are limited.

Many significant advances in space knowledge only started in the second half of the 20th century, when technology allowed us to send research equipment and even people into space. However, we knew of planets for centuries before that and struggled to define them with the equipment at hand. Thanks to technological advances, our definitions changed many times, as did our planetary knowledge.

Here, we’ve touched on a few of the odd discoveries that complicated the definition of planet, like Ceres and Planet Nine. However, these are only some of the many weird planet-like space objects we know of—and more will surely be discovered in the future.

For now, according to the IAU, a planet is something that:

  • Orbits the sun
  • Is round, or mostly round, thanks to gravity
  • And clears the neighborhood around its orbit

By this definition, dwarf planets can’t be planets: they aren’t large enough to “clear the neighborhood.” However, they are mostly round and orbit the sun, like planets.

To make things extra confusing, the IAU doesn’t really define what they mean by “clear the neighborhood.” This definition also doesn’t attempt to describe exoplanets outside our solar system. Many experts have criticized the IAU definition of planets.

No matter what, defining planets is ultimately a somewhat arbitrary thing. For example, where do you draw the line between “not round” and “mostly round”? Do exoplanets, which can be very different from our solar system’s planets, need a separate definition? What about “rogue planets,” which don’t even orbit stars?

Scientists may eventually answer these questions. However, many scientists are far more interested in researching other worlds, no matter what you call them, than in trying to pin down the definition of a planet.

As with all words and scientific concepts, we do our best with what we know so far—but the definition of “planet” is always subject to change.

Want to learn more? While we’ve touched on the essentials, you can also check out The Planetary Society’s fascinating deep dive into defining planets.

Elyse Hauser Elyse Hauser
Elyse Hauser is a Seattle-based writer and editor with a Master's in Writing Studies from Saint Joseph's University. Her work has appeared in publications like Racked, Vine Leaves Literary Journal, and Rum Punch Press. She was awarded a 2017 Writing Between the Vines residency.  Read Full Bio »