So, Really, Isn't Pluto A Planet?
Pluto and Charon vs. Earth
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.
Eris and Its Moon
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.
Planet or Dwarf Planet?
So, what should we call Pluto?
Say It Ain't So, Neil!!!
So 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.
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
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!)
The "Dwarf Planet" Controversy: Recommended Links
- Dr. Neil deGrasse Tyson on killing Pluto 'All I did was drive the getaway car'
Dr. Neil deGrasse Tyson argues that he's been unfairly blamed for the death of Pluto. Neil, I love ya, but you claimed part of the credit for it YOURSELF when you wrote "The Pluto Files."
- Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern
New Horizons' Principal Investigator Alan Stern explains why he thinks the International Astronomers Union's decision to reclassify Pluto as a Dwarf planet was wrongheaded and unscientific.
- Looking Back on Killing Pluto: Q & A With Astronomer Mike Brown
Caltech planetary astronomer Mike Brown, whose team discovered Eris, explains why the dwarf planet term is necessary. (You KNEW it would turn out to be a political feud, right?)
- Mike Brown's Table of "Dwarf Planets"
A table of Kuiper Belt Objects discovered so far. He's being too lenient about what he calls a "dwarf planet," but the "nearly certainly" one surely are, and I think some of the "highly likely" ones may be. (Charon's 1212 km, Ceres is 950km.)
Petition to Return Pluto to "Planet" Status
Our FAVORITE (Dwarf?) Planet
© 2015 Ellen