When we place the known objects in the Solar System in order, four inner, rocky worlds and four, outer, giant worlds stand out. Traditionally, these were defined as planets. Will the way we define these words change? (NASA’s The Space Place)

You Won’t Like The Consequences Of Making Pluto A Planet Again

In 2006, astronomers demoted Pluto out of its planetary status. Geophysicists want to bring it back. Here’s what would happen if we did.

Ethan Siegel
8 min readMay 15, 2018

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Since its discovery in 1930, Pluto was heralded as the ninth planet in our Solar System. Pluto was the first world ever discovered beyond Neptune, and for nearly half a century, was the only world known beyond our last gas giant. Generations of schoolchildren learned mnemonic devices about their very educated mother just serving them nine pickles, with Pluto, the very last, lonely one out there, becoming the favorite of so many.

After 76 years, however, astronomers seemingly demoted Pluto to dwarf planet status, placing it alongside the large asteroid Ceres and other worlds out there in the Kuiper belt, reducing our Solar System’s planetary count to a mere eight. Last year, a team of scientists put forth a new definition of planet that would bring Pluto back into the fold, and this definition has been endorsed by Alan Stern and David Grinspoon, authors of a new book on the New Horizons mission and “the planet” Pluto.

Here’s what it would mean if we listened to them:

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Ethan Siegel
Starts With A Bang!

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.