Adler’s ‘What is a Planet?’ Explains Pluto Controversy

Adler Planetarium

Mention Pluto to someone, and if they’re not talking about a cartoon dog they’ll likely have strong feelings about the former planet no longer being a planet. “What is a Planet?” is the definitive exhibit on Pluto’s newest classification, the history of astronomical definitions, and the controversy behind the decision.

Whether you think Pluto should be considered a planet or not, the Adler Planetarium’s newest exhibit gives you so much information behind the decision that it will likely just reaffirm whatever position you had going into the exhibit. In 2006 the International Astronomical Union (IAU) put the definition of a planet up to a vote because it had never had an official definition. With their new definition Pluto was declassified as a planet and is now known as a dwarf planet. When you have a planet that most humans grew up with as a planet, was discovered by an American, and is the namesake of a Disney character, it is only natural for people to be up in arms.

A lot of the emotions brought up by the name change is highlighted in an excellent video that cuts together all the news coverage of the reclassification that includes network news, comedians, and, of course, Neil deGrasse Tyson. Moving on from that you’ll start to get into the details where you learn that while Pluto fit two factors on the new planet checklist—orbiting around the sun and being round—it doesn’t have the mass to clear the neighborhood around its orbit.

"What is a Planet" artifact

As you continue to move through “What is a Planet?” you’ll get to vote on whether you think Pluto should be considered a planet or stay as a dwarf planet and even hear from Adler astronomers about their differing opinions on the matter. However, the most interesting part of the exhibit is the collection of artifacts that help tell the story of planetary classification over the years. You’ll be drawn to the light box that shows that at one time we considered 23 celestial bodies to be planets, and next to it is a book that would help you at the time to memorize each one of those. This historical aspect will make you realize that while definitions are important they also change frequently.

“What is a Planet?” opens March 19 at the Adler Planetarium located at 1300 S. Lake Shore Dr. The exhibit is included with the price of admission. For more information visit

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