Why is Pluto a Dwarf Planet?

Many people don't like hearing that scientists reclassified Pluto from a planet to a dwarf planet. "What do you mean, Pluto isn't a planet? It was when I was in school!" is a common response I hear. Or, "Well, Pluto will always be a planet to me." Some people even staged protests when the announcement was made back in 2006. 

Pluto Protesters. Image Credit: NMSU/Darren Phillips

I understand why people react this way. They grew up knowing that there were 9 planets in the Solar System. It seems like a basic fact that should not change. They are emotionally attached to Pluto being a planet. In fact, most arguments that Pluto should be called a planet are based on emotions, not facts. 

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Pluto was the first object discovered in our Solar System beyond Neptune. It was an exciting discovery, so of course it was called a planet, even though it never quite fit in with the other eight. When it was first discovered, it was thought to be the size of Earth. However, it is much smaller (2/3 the diameter of Earth's Moon!), and its orbit is tilted relative all the other planets. 

The planethood of Pluto was seriously called into question when astronomers discovered that it was part of the Kuiper Belt, a belt of icy asteroids that orbit the Sun out beyond Neptune. In the 2000's, objects were discovered in the Kuiper Belt that were spherical and Pluto-like. The astronomer who discovered many of these objects is Dr. Michael Brown. He discovered Sedna, Haumea, Quaoar, Makemake, Eris, and 12 other Pluto-sized objects in the Kuiper Belt. Most important in the question of Pluto's planetary status - Eris is larger than Pluto. 

Image Credit: Wikipedia Commons

Astronomers knew they would have to either add all these round Kuiper Belt objects - and more are being discovered all the time - to the Solar System as planets, or come up with a new definition. Thus, the category Dwarf Planet was created. There are five official dwarf planets! Pluto, Eris, Haumea, Makemake, and Ceres. And this year, two will be explored for the first time ever. Dawn will fall into orbit around Ceres in just about a week, and New Horizons will fly by Pluto in July. 

New image of Ceres taken by Dawn on February 12. These images are the best we have of any dwarf planet so far. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Instead of being sad that Pluto isn't a planet any more, wouldn't it be better to learn more about all the dwarf planets? And to discover what other fascinating worlds might be lurking in the Kuiper Belt, just waiting for an intrepid astronomer to find them? 

If you are interested in learning more about dwarf planets and how they are discovered, you are in luck! This Thursday, March 5, the Dome Planetarium at the Peoria Riverfront Museum will be hosting a FREE simulcast of a lecture by Dr. Michael Brown, the astronomer who discovered three of the five dwarf planets. He will be describing the discovery process, properties of the dwarf planets, and the possibility that there is an even larger planet waiting to be discovered. Dr. Brown will speak at the Adler Planetarium, and we will live stream the lecture and stunning visuals here in Peoria, using our Uniview software. The event is free, but seating is limited, so reserve your spot by calling our front desk at 309-686-7000. Click here for more information!