Why Is Pluto Not a Planet?

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Pluto on July, 14 2015. Image taken by New Horizons.

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NASA/Johns Hopkins University Applied Physics Laboratory/Sou

What is a Planet, why Pluto was reclassified, and how Pluto got its name

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When Pluto was “demoted” from a planet to a dwarf planet back in 2006, it was a sore subject for many. Then, in 2015, a spaceship gave us our first-ever close-up of Pluto, and everything we knew changed! Read more about why Pluto is no longer a planet, how it got its name, and learn some cool Pluto facts.

In 2006, the International Astronomical Union (the world body that names the contents of the universe) demoted Pluto to a Dwarf Planet because it really does not match the other eight planets. 

  • It’s far smaller, with only 4% the mass of even tiny Mercury.
  • And it has a very unplanetlike orbit from every angle. 
  • But the clincher was finding more Plutos out there. Eris is bigger than Pluto, while Makemake, Quaoar, Sedna, and a few others are almost as large. If Pluto’s a planet, then those others must be planets, too.

It became clear that there’s a Kuiper Belt—a band of icy objects at the edge of our solar system—with thousands of small, unplanetlike bodies, and Pluto’s one of them. A whole different ball game from the “original eight” planets. So, if you’re one of those who’d like to see Pluto called a major planet again, be aware that you’re opening the door to lots more “major planets” that will be tiny ice-balls with odd names, all of which will be smaller than our moon.

That said, it is still considered to be one of the largest bodies in the Kuiper Belt. Because Pluto is the biggest object in this region, some call it “King of the Kuiper Belt.” 

Size is relative, perhaps. Pluto—smaller than Earth’s Moon—is about 1,400 miles (2,380 km) wide.

What Defines a Planet?

Rules have now been created. According to the International Astronomical Union (IAU), the object must:

  1. Orbit the Sun
  2. Be round (so big enough for gravity to squash it into a round ball)
  3. Have cleared other objects from its orbit.

Pluto did not meet the third rule. It orbits among other floating icy bodies in the Kuiper Belt.  

While the other planets travel around the Sun in almost perfect circles, Pluto takes an oval-shaped path, and the Sun is nowhere near its center. Pluto’s path is also much more tilted than the nice, orderly plane in which most other planets orbit.

Image credit: NASA.

What Is a Dwarf Planet? 

A “dwarf planet” has a definition too. It must meet the criteria of a planet above as well as a fourth rule: it must not be a satellite.

There are hundreds of dwarf planets. A couple of the more well-known dwarf planets are Ceres and Eris.

Some folks ask if Pluto is an asteroid. Actually, it’s called a “plutoid” or an “ice dwarf” because it has an orbit outside Neptune’s (plus, it’s extremely cold).

However, Ceres is so small (especially compared to Pluto) that it is classified as both a dwarf planet and an asteroid. It’s one of the smallest dwarf planets but also one of the largest asteroids, making up approximately a fourth of the asteroid belt’s mass.

Visiting Pluto for the First Time

On January 19, 2006, a NASA probe named New Horizons was launched; it arrived in Pluto’s neighborhood in July 2015. Since then, we’ve received our first close-up images of Pluto—and we’re also studying the edge of our solar system and how it formed.

The dwarf planet is much more geologically complex than we had imagined. It’s not just a ball of ice.

  • Pluto has a very prominent heart-shaped feature on the surface! It’s a glacier that’s the size of Texas and Oklahoma.
  • Pluto has blue skies, spinning moons, and mountains as high as the Rockies.
  • There are snow-capped peaks on Pluto, and it snows—except the “snow” is red!
Pluto’s photo by New Horizons on July 13, 2015—plus, a creative interpretation.

What’s in a Name?

Anyway, what’s in a name? Until about a century ago, rabbits were classified as rodents. Then their order was abruptly changed so that now they’re lagomorphs. That’s mostly because they have four incisor teeth instead of two. But hey, they still hop around. So Pluto is Pluto regardless of which mental box we try to make it fit.

The debate about Pluto’s planetary status goes on, even among astronomers. Some argue that Pluto’s official planetary status should be restored. They suggest that the requirement that a planet must “clear” other objects from orbit doesn’t make sense—and that there are plenty of examples. 

Other astronomers today say that the name is not important. One made the comparison of naming similar to calling any island a continent. 

How Was Pluto Discovered?

Pluto was discovered in 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh. The existence of this planet was actually proposed years before by American Percival Lowell, who had theorized that slight disturbances in the orbits of Uranus and Neptune were caused by the gravitational pull of another planet. Lowell even went so far as to make calculations to estimate where to find what is now Pluto. Although he didn’t succeed in finding this object by his death, the search continued until Tombaugh discovered the tiny planet using a new astronomic technique of photographic plates combined with a blink microscope.


How Pluto Got Its Name

  • Pluto’s name is not from Disney character of Pluto (Mickey Mouse’s dog)! Venetia Burney from England, just 11 years old at the time, suggested the name Pluto in 1930. Her grandfather, a librarian, knew many astronomers and sent it to a friend who sent in the name to the Lowell Observatory. She is the only girl (so far) who has named a world!
  • She suggested the name Pluto partially because it followed the other planets’ naming based on classical mythology. Pluto” is the name of the the Roman god of the Underworld (equivalent to the Greek Hades).
  • The name also honors Percival Lowell, as the first two letters of the name Pluto, “PL,” are the initials of this famous American astronomer who tirelessly hunted for it and at whose observatory (in Arizona ) it was finally found.
  • As for the Disney cartoon dog, Pluto, it was actually named after the planet, not the other way around. The cartoon dog was originally named Rover. In 1931, a year after Pluto’s discovery, the Walt Disney folks decided to exploit the newly found world’s publicity, and changed the character’s name to that of the planet.
  • Pluto’s five known moons—Charon, Nix, Hydra, Kerberos, and Styx—also have names associated with the underworld. Charon is the name of the river Styx boatman who ferries souls in the underworld; Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology; and Styx is named for the mythological river that separates the world of the living from the realm of the dead.


More Cool Facts About Pluto

  • Pluto orbits the Sun about 3.6 billion miles (5.8 billion km) away on average, about 40 times as far as Earth.
  • We could never live on Pluto. Because it’s so far away from the Sun, its temperature is about 400 degrees below zero Fahrenheit! 
  • If you weigh 100 pounds on Earth, you would weigh only 7 pounds on Pluto.
  • If you lived on Pluto, you’d have to live 248 Earth years to celebrate your first birthday in Pluto-years. 
  • A day on Pluto lasts 153 hours, or about 6 Earth days.
  • Pluto has blue skies and is very hazy, thanks to a thin atmosphere of nitrogen, methane, and carbon monoxide. 
  • The largest of Pluto’s five Moons, Charon, is so big that Pluto and Charon orbit each other like a double planet.

Even though Pluto has been reclassified to a dwarf like so many other Disney characters, Pluto is now a whole new world with all the photos and information from New Horizons—the first-ever spacecraft to visit that strange tiny world.

Tell us what you think of Pluto—and what you think of it being a “dwarf planet.”

Love learning about space? Visit planet Mars, Earth’s neighbor.

About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann

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