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Yes Virginia, Pluto Really is a Planet

Posted by Guy Pirro 09/12/2018 12:33AM

Yes Virginia, Pluto Really is a Planet

What is a planet? For generations the answer was easy -- A big ball of rock or gas that orbited the Sun... And there were nine of them in our Solar System. But then astronomers started finding more Pluto-sized objects orbiting beyond Neptune. Then they found Jupiter-sized objects circling distant stars -- First by the handful and then by the hundreds. Suddenly the answer wasn't so easy. Were all these newly found things planets? The International Astronomical Union (IAU), the organization that is in charge of naming newly discovered worlds, tackled the question at their 2006 meeting. They tried to come up with a definition of a planet that everyone could agree on. But the astronomers couldn't agree, so they voted and picked a definition that they thought would work. The results have been mixed. In the end, the IAU did accomplish one thing -- They figured out a way to turn something simple into something complex. Now, a recent study by the University of Central Florida concludes that the reason Pluto lost its status as a planet is not valid and that a proper definition of a planet should be based on its intrinsic properties rather than ones that can change, such as the dynamics of a planet’s orbit -- the rationale that was used by the IAU to strip Pluto of its planetary status.


  • bikeboy [Bob MacArthur]
  • 09/12/2018 06:21PM
Pluto should absolutely should be a planet along with Eris, Ceres and others. I don't recall science being done by a hand count like what the IAU did. Enough mass for interior differentiation and a spherical shape are good requirements. Clearing its path not really valid for most of the objects in our solar system. Also orbits a star is also becoming an issue. What about rouge planets?


Thanks for your comment. I agree with your statement that Science should not be done by a hand count. But if it must be done through a vote, it should at least have some semblance of fairness and inclusiveness.

For an eye-opening summary of the goings-on at the IAU meeting in Prague on that fateful day, read the BBC report that was written on the following day. It concluded that the Pluto vote was "hijacked."

"Only 424 astronomers who remained in Prague for the last day of the meeting took part (in the vote)."

Alan Stern is quoted as saying "I was not allowed to vote because I was not in a room in Prague on Thursday the 24th. Of 10,000 astronomers, 4% were in that room - you can't even claim consensus. If everyone had to travel to Washington DC every time we wanted to vote for President, we would have very different results because no one would vote. In today's world that is idiotic. I have nothing but ridicule for this decision."

Ernest Rutherford once said "All science is either physics or stamp collecting."

The IAU's attempt to categorize planets clearly fell into the later camp.. And it was a poor attempt at that.

I believe the IAU could have done better.

Guy Pirro

I believe the IAU totally missed the mark when demoting Pluto in 2006. When they did it, I was irate and questioned their rationale. In my opinion, Pluto was ALWAYS a planet and NEVER anything but a planet. Now retired teacher, every once in a while I would have teens ask me in class about astro topics and the like, and when Pluto would come up I would tell them not to listen to the morons who claim otherwise. There are currently nine planets and Pluto IS one of them.

  • dgruss23 [David Russell]
  • 09/17/2018 12:08AM
A better taxonomy would recognize that all spherical sub-stellar bodies formed in a proto-planetary disk are "planets" and then you can categorize types of planets based upon composition, mass ranges, and dynamics. Dynamical classes would be:

Principal planets - same as IAU planets
Belt planets - same as IAU dwarf planets
Satellite planets - spherical moons.
Rogue planets - free floating planets ejected from their circumstellar orbit after formation.
  • bikeboy [Bob MacArthur]
  • 09/17/2018 03:57AM
Interesting groups. It makes sense.

Or what about

Rocky/Terrestrial (Mercury, Venus, Earth, Mars, Ceres)
Gas (Jupiter, Saturn, Uranus, Neptune)
Ice (Pluto, Eris, Makemake, Sedna, etc)

  • dgruss23 [David Russell]
  • 09/17/2018 11:53PM


Well, you do want to keep the classification systems distinct. Dynamics is dynamics and composition is composition. The Solar System's planets are more diverse than normally recognized. A good thing to understand is that the planets are generally classified based upon bulk composition. There are three composition components that make up planets: Rock (silicates and iron), Ice (C,H,N,O compounds such as H2O, CO2, NH3, CH4...), and Gas (H and He).

Now if you broadly classify by which of the three composition components makes up more than 50% of the mass of a planet, then the terrestrial planets are 100% rock with Mercury being a "metallic" planet that is >50% iron. Jupiter and Saturn are the gas giants. Uranus and Neptune are Ice giants as they are ~63% ices by mass with ~12% H/He gas and ~ 25% rock.

Pluto is actually a rock planet with ~70% of the mass silicates and iron with the remaining 30% ices. The ice component is differentiated to the outer layers of the planet, but the bulk of the mass comes from rock.

There are also other composition classes among exoplanets. I have argued for a composition scheme that is coded rather than named. It is discussed here: