So, Really, Isn't Pluto A Planet?

Updated on May 10, 2017
Greekgeek profile image

Daughter of a rocket engineer, granddaughter of a planetarium director, I've been a huge fan of astronomy and space exploration all my life.

Pluto and Charon vs. Earth

Pluto and Charon compared to Earth. Side by side, they'd fit inside the United States.
Pluto and Charon compared to Earth. Side by side, they'd fit inside the United States. | Source

It's Geologically Active, for Goodness' Sake!

The funny thing is, another planet went through the same process as Pluto in the 1800s. Ever heard of Ceres? When it was first discovered in 1801, it was declared a planet. Almost 50 years later, it was demoted to asteroid amidst angry protests, after astronomers discovered the Asteroid Belt full of other little rocky worlds, some of them not much smaller.

Sounds familiar, eh? Now Ceres is classified as a dwarf planet.

It's been so long that the Ceres' "Asteroid or Planet?" controversy has passed from living memory. Pluto has ruled the solar system as "cutest planet" since 1930. Sadly, few people are paying attention to the Dawn spacecraft exploring Ceres. It may be fascinating, but it's no longer a household name.

Pluto followed the same same story as Ceres, except that no one's going to forget it just because it's been reclassified. My grandparents remember Pluto's discovery! It was discovered by American Clyde Tombaugh on February 18, 1930, within the lifetime of the oldest members of the New Horizons Team.

Some of the Kuiper Belt Objects discovered in the past ten years. (More have been found since this chart was created, and another moon, Vanth, has been found around Orcus.)
Some of the Kuiper Belt Objects discovered in the past ten years. (More have been found since this chart was created, and another moon, Vanth, has been found around Orcus.) | Source

Meet the Other Dwarf Planets

The existence of a zone of small, icy worlds in the outer solar system was suggested in the 1940s, but the first one after Pluto was only spotted in 1992. Since then, astronomers have found over 70,000 Kuiper Belt Objects. Most are small blobs you'd probably think of as asteroid, although they're more ice than rock. However, we've started finding Pluto-like ice worlds as well. Some have moons. Others are larger than Charon. Eris is practically Pluto's twin: a few miles smaller, but quite a bit heavier!

After the discovery of Eris, astronomers began to argue: Should we keep adding more and more planet names to the solar system roster, creating a nightmare for schoolchildren to memorize? If not, where should we draw the line?

Faced with a growing list of "Plutoids," the International Astronomers Union reclassified the largest ones as "dwarf planets" in August 2006. NASA changed its FAQs to reflect this (NASA's just a U.S. government agency; it doesn't have authority over the whole world, let alone the solar system). The new rule was: if a body has sufficient mass to pull itself into a round shape, but isn't big enough to clear its orbit of debris, it's a dwarf planet.

The iconic "Heart" photo of Pluto, taken July 13, 2015, the day before the New Horizons flyby.
The iconic "Heart" photo of Pluto, taken July 13, 2015, the day before the New Horizons flyby. | Source

Planet or Dwarf Planet?

So, what should we call Pluto?

See results

Come on, Is it a Planet or Isn't it?

However, the debate continues among astronomers. New Horizons' scientists have called both Pluto and Charon "planets" in interviews, and I've never heard them say "dwarf planet."

It's an emotionally-charged issue. People love Pluto. They're outraged, complaining that Pluto's been "demoted." For its fans, Pluto is a matter of popular mythology, not just science! Now that we've seen Pluto close-up, it seems impossible not to call it a planet.

The thing is, for all we know, some of the other dwarf planets may be just as wonderful. And many of them have moons.

If I had to answer the question, I'd say this. "Dwarf planet" is a useful term, just like "gas giant." The ice dwarves, the gas giants, and the Earth-and-its-pals worlds are three different kinds of planet. (However, the divisions are not that absolute: other solar systems have gas giants uncomfortably close to their suns. It's complicated.)

For me, the controversy is simply a distraction from talking about Pluto. However, I like the "dwarf planet" term for one reason. It means that there's so many other Plutos out there that we need a name for them! That's exciting news. It means the exploration of the solar system isn't finished. There's new worlds waiting to be discovered and explored. By studying Pluto, we'll learn about the ones too far away for us to reach...

... for now.

Note: This page was originally part of my "New Horizons Has Reached Pluto!" article, where you'll find the latest photos and exciting discoveries from the Pluto Flyby.

Say It Ain't So, Neil!!!

Despite all the pontificating about Pluto being the "last planet," our survey of the solar system did not end in 1930. I am suspicious of any 20th century astronomer claiming that 21st astronomers' discoveries don't count.

Five Fantastic Minor Planets (All but Ceres Are Artist Concept Art)

Neil DeGrasse Tyson

The Pluto Files: The Rise and Fall of America's Favorite Planet
The Pluto Files: The Rise and Fall of America's Favorite Planet

Neil DeGrasse Tyson weighed on on the Pluto controversy a few years ago with this great book. (In fact, it has a lot about the history of Pluto's discovery and a fascinating review of the Kuiper Belt discoveries, although he'll need to write an appendix after the New Horizons mission is over!)

 

Petition to Return Pluto to "Planet" Status

Do you care passionately about Pluto being a planet? The New Horizons team does, too! In fact, their Twitter channel invites us to sign this Change.org petition.

Our FAVORITE (Dwarf?) Planet

Pluto and Charon, composite portrait from the July 14, 2015 Pluto flyby.
Pluto and Charon, composite portrait from the July 14, 2015 Pluto flyby. | Source

Questions & Answers

    © 2015 Ellen

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      • profile image

        Raven 2 years ago

        I can't call it a planet 'cause it's not really a planet and I can't call it a dwarf planet. I mean it's not fair! Lets just say Pluto is unique.

      • Kristen Howe profile image

        Kristen Howe 2 years ago from Northeast Ohio

        Great hub about Pluto and other dwarf planets in our solar system. Lizzy's right. It should be a planet and nothing else.

      • DzyMsLizzy profile image

        Liz Elias 2 years ago from Oakley, CA

        Yes, that's true; a dwarf planet is still a planet. But, there was a point of time during which the scientific powers that be had decreed that Pluto was not a planet at all. ;-)

      • Greekgeek profile image
        Author

        Ellen 2 years ago from California

        Many of the other dwarf planets have moons, too. They probably have more moons than we can see.

        Are all those Pluto-like worlds planets? What do we call the new ones we keep discovering? It doesn't make sense to say that Pluto is, and they're not, just because we didn't have telescopes powerful enough to find them until now.

        I honestly don't know. But I'm not convinced that Pluto has been "declassified" from planetary status. A dwarf planet is a planet, just like a dwarf peach tree is still a peach tree.

      • DzyMsLizzy profile image

        Liz Elias 2 years ago from Oakley, CA

        It has a moon. It's a planet. End of discussion.

        Funny--while it was "declassified" from planetary status for a few years, my step-daughter got hold of a t-shirt that read, "In MY day, there were NINE planets."

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