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What’s The Deal With Pluto? October 23, 2007

Posted by Weird Science Writer in Easy As Pie, Solar System, Space Exploration.
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When Lauren Tunnell heard that Pluto’s classification as a planet had been changed, she set out to discover just what was happening. She learned two things: scientists sure do like to argue and regardless of how we want to classify Pluto, it has some very cool and interesting features.

Pluto 2

Credit: National Aeronautics and Space Administration (NASA).

Once upon a time, students were taught that there were eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Then, in 1930, a ninth planet was discovered and it was named Pluto. For 76 years, there were nine planets in the solar system. That is, until 2006, when it was decided that Pluto isn’t really a planet, and there are only eight planets in the solar system after all. So, what happened to Pluto? And why has it been demoted from planet status?

New Information About Pluto

Pluto isn’t being ignored. In fact, scientists are learning more and more about this distant world. We now know Pluto’s approximate size, mass, temperature, and chemical composition. !n 1978, Pluto’s largest moon, Charon was discovered, and in 2005, we learned that Pluto has two additional moons, Nix and Hydra.

Other Possible Planets

However, we have also learned about a lot of other celestial bodies. Most notably, Eris was discovered in 2005. Eris’ orbit is far away from the sun (three times farther than Pluto!) but it is slightly larger than Pluto. People started to wonder if Eris should be considered the tenth planet.

If that isn’t confusing enough – then more potential planets started to turn up. It was discovered that Pluto doesn’t orbit alone, but is actually a part of the Kuiper belt – a large ring of rocky objects encircling the sun (like the asteroid belt but much bigger!). Although Pluto is the largest of these rocky objects that has been discovered so far, it really isn’t particularly special or distinctive from the other Kuiper belt objects. Nobody knows how many of these Kuiper belt entities exist, but at least five with diameters of 500 km or more have been identified. Does this bring our total number of planets to fifteen? What about the worlds we haven’t even discovered yet? Could this mean that there may be hundreds, or even thousands, of planets in the solar system?

What Makes a Planet?

All of this newfound information has forced scientists to reconsider what is meant by the word planet, and they started to consider the qualities that the eight major planets have in common. Check out these quick facts:

  • All eight of the major planets orbit the sun along the same plane. Kuiper belt objects, including Pluto, orbit on a different orbit that is diagonal to the orbit of the planets (Eris has an even more eccentric orbit than Pluto).
  • The planets are pretty big! Even Mercury, the smallest of the eight, is many times larger than Pluto.
  • The planets have their orbits all to themselves. None of the eight major planets have asteroids or other planet-like objects sharing their orbital path.



Credit: NASA. This image depicts Pluto in true color. Pluto is mostly brown, which according to NASA is thought to be from frozen methane deposits metamorphosed by faint sunlight.

So Yeah, What About Pluto?

Since Pluto fails to meet these criteria, it was decided that Pluto was, in fact, not a planet but something distinctly different. Pluto has now been reclassified as a dwarf planet, along with Eris and the asteroid belt object, Ceres. Over a dozen other celestial objects are also being considered for dwarf planet status (including Pluto’s largest moon, Charon).

It’s Not Quite Finished Yet

Of course, all of this is subject to change as we learn more and more about the solar system and require new words to describe the alien worlds we’ve discovered. One thing is for sure – there is still a lot we don’t know about the solar system but no doubt, scientists will continue to argue out the nuances of these discoveries as they are made!

Lauren Tunnell is an educator and freelance writer living in Houston, Texas.

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1. Hazel - October 30, 2007

This was a great, clear and informative post about one of my favorite celestial objects. I thoroughly enjoyed reading it, and the pics were terrific. I’m so fascinated by the current upheavals in our understandings of the worlds around us. It feels somehow very momentous like we’re on the cusp of a grand leap of understanding our place in this expansive mysterious universe of ours.

Anyway, this post was a lovely break in my day. Thanks.

2. Miss Weird Scientist - October 30, 2007

I’m glad you enjoyed the article! I think Lauren wrote a really cool and interesting piece. :) It still always amazes me how definitions and concepts in science change so rapidly and can stir up so much controversy.

3. Hazel - October 31, 2007

Me too,(amazed) with astronomy especially. Its such an amazing era at the moment with the advances of radio, telescope, satellite’s and propulsion systems. I think my next planet I want to study in depth is Mars – whats with all the missing probes? I love me a good mystery.

4. Laurel Kornfeld - November 18, 2007

This debate is definitely not finished yet. The requirement that an object have its orbit all to itself to be considered a planet is highly controversial. It was adopted by four percent of the IAU in an extremely controversial process and was immediately rejected by over 300 professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Most of the IAU members who voted on this were not even planetary scientists. And their determination that a “dwarf planet” is not a planet at all makes no linguistic sense.

Pluto and Eris are distinctive from the majority of Kuiper Belt Objects. Most of these are shapeless rocks, or asteroids. However, Pluto, Eris and a few other large KBOs have achieved hydrostatic equilibrium, meaning they have sufficient gravity to have pulled themselves into a round shape. Many astronomers believe that the only qualifications for an object to be considered a planet are that the object orbit a star and that it has achieved hydrostatic equilibrium. The real answer here is to establish a third category of planets (the first two being terrestrial and gas giant planets), the ice dwarfs, which would include objects like Pluto, Eris, and even Ceres. They would still fall under the larger umbrella of planets, just under a different subcategory.

Also, there is a significant difference between the orbital inclinations of Pluto and Eris. Pluto at most orbits at a 17 degree inclination to the other planets. Eris orbits at a 42 degree inclination. Interestingly, Mercury orbits at a 7 degree inclination, meaning Pluto’s orbit is more like that of Mercury than that of Eris.

5. Billy-joe bob - November 18, 2008

Pluto is the coolest planet IAU SUCKS for demoting it.We should have a rampage

6. kaleb henry - December 12, 2008

i think that you r awsome

7. Andrew F - October 8, 2009

Great information, clear and concise! Exactly what I was lookin for. I wonder if information in quizzes will be updated and indeed give a correct answer relating to the status that Pluto is no longer classed as a planet??? Doubtful.
It seems we are often educated with incorrect facts….

8. Mr.Science - June 7, 2010

I am very agreeing with you on that. Our school system should definetly try for a new text book. Mine still stats that Pluto is currently on of the planets in the solar system when clearly it was found that Pluto is a Dwarf Planet.

Talking about being “LATE” for school. hahahahahah

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