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Sorry, Pluto: You’re really not a planet

Pluto from the New Horizons project, 71 million miles away

The first color image of Pluto sent back by the New Horizons probe. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

In 2006, Pluto was officially demoted from “planet” to “dwarf planet.” But this debate flared up again earlier this year, when astronomer David A. Weintraub wrote a column predicting that Pluto would be returned to “planet” status sometime in 2015. And it will likely remain in the news for the next few months, as the New Horizons probe becomes the first spacecraft to visit Pluto in July.

Before that happens, we need to get one thing clear: there are very good reasons why Pluto isn’t a planet.

“There are eight bodies in the solar system that dominate the others,” says Mike Brown, the Caltech astronomer who in 2005 discovered Eris — a dwarf planet more massive than Pluto — and wrote How I Killed Pluto and Why It Had It Coming. “Those eight are planets, and the other thousands are not.”

His point isn’t just that the eight planets are orders of magnitude bigger than Pluto (though they are). Equally important is the fact that in terms of size and location, Pluto fits neatly in a group with dwarf planets like Eris, Haumea, Makemake, and likely thousands of undiscovered rocks that orbit in the Kuiper belt, outside Neptune. If we really want to call Pluto a planet, our list of planets is going to be expanding dramatically in future years.

For more of this article by Joseph Stromberg, check out