When people started using the word “planet,” they were referring to those seven objects. Even Earth was not originally called a planet – but the Sun and Moon were.

Since people use the word “planet” today to refer to many objects beyond the original seven, it’s no surprise we argue about some of them.

Although I am trained as an astronomer and I studied more distant objects like stars and galaxies, I have an interest in the objects in our Solar System because I teach several classes on planetary science.

Asteroids, the first demoted planets

The word “planet” is used to describe Uranus and Neptune, which were discovered in 1781 and 1846 respectively, because they move in the same way that the other “wandering stars” move. Like Saturn and Jupiter, if you look at them through a telescope, they appear bigger than stars, so they were recognized to be more like planets than stars.

Not long after the discovery of Uranus, astronomers discovered additional wandering objects – these were named Ceres, Pallas, Juno and Vesta. At the time they were considered planets, too. Through a telescope they look like pinpoints of light and not disks. With a small telescope, even distant Neptune appears fuzzier than a star. Even though these other, new objects were called planets at first, astronomers thought they needed a different name since they appear more star-like than planet-like.

William Herschel (who discovered Uranus) is often said to have named them “asteroids” which means “star-like,” but recently, Clifford Cunningham claimed that the person who coined that name was Charles Burney Jr., a preeminent Greek scholar.

Today, just like the word “planet,” we use the word “asteroid” differently. Now it refers to objects that are rocky in composition, mostly found between Mars and Jupiter, mostly irregularly shaped, smaller than planets, but bigger than meteoroids. Most people assume there is a strict definition for what makes an object an asteroid. But there isn’t, just like there never was for the word “planet.”

In the 1800s the large asteroids were called planets. Students at the time likely learned that the planets were Mercury, Venus, Earth, Mars, Ceres, Vesta, Pallas, Juno, Jupiter, Saturn, Uranus and, eventually, Neptune. Most books today write that asteroids are different than planets, but there is a debate among astronomers about whether the term “asteroid” was originally used to mean a small type of planet, rather than a different type of object altogether.

How are moons different than planets?

These days, scientists consider properties of these celestial objects to figure out whether an object is a planet or not. For example, you might say that shape is important; planets should be mostly spherical, while asteroids can be lumpy. As astronomers try to fix these definitions to make them more precise, we then create new problems. If we use roundness as an important distinction for objects, what should we call moons? Should moons be considered planets if they are round and asteroids if they are not round? Or are they somehow different from planets and asteroids altogether?

I would argue we should again look to how the word “moon” came to refer to objects that orbit planets.