Why Isn't Mercury Considered a Dwarf Planet Like Pluto?

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Stocktrek/Getty Images

Why isn't Mercury classified as a dwarf planet? It's not much larger than Pluto. originally appeared on Quora - the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Corey Powell, Book author and science editor at Aeon, on Quora:

Several others have nicely explained the criteria established by the International Astronomical Union (IAU) to define what qualifies as a “planet.” You can read about that decision, along with the official ruling, here: Pluto and the Solar System.

Many people, including many planetary scientists, have taken exception with the IAU’s third criterion, that a true planet must have “cleared the neighborhood around its orbit.” Pluto’s resulting demotion to “dwarf planet” also upset a lot of astronomy fans. I was dubious about that part of the IAU definition myself, but over time I’ve come to agree that it makes sense.

Rather than the abstract idea of “cleared the neighborhood,” think about it in terms of populations. Mercury is the only object of its kind in its part of the solar system. The same holds true for all of the other major planets: Each is a population of one. If you put another Earth in the same orbit as the real Earth, the whole system would become completely unstable.

That’s not true for the asteroids, though. Ceres is the largest asteroid, but there are a dozen other ones in broadly similar orbits that are at least 1/4th its size. When astronomers discovered Ceres, they originally considered it a planet. Then they realized that there are many more asteroids all around it, and reclassified it as a big asteroid instead.

The same is true for Pluto. When Clyde Tombaugh discovered Pluto, it was in a class of one. Now we know that there are dozens of similar but slightly smaller objects orbiting in the same zone of the outer solar system. Pluto is part of a whole population known, broadly, as Kuiper Belt Objects. Like Ceres, then, it doesn’t fit the traditional definition of a planet.

Calling Pluto a dwarf planet doesn’t make it any less interesting or significant, incidentally. In some ways Pluto is now even more significant, because it is the archetype of a whole population. Studying it reveals a lot about how the outer solar system formed and evolved over the eons.

True, Mercury is also considerably larger and more massive than Pluto. What really matters, though, is that Mercury is one of a kind and Pluto is not. Put it this way: if there were thousands of objects circling the sun closer to Venus, and Mercury was just the largest of the bunch (including others nearly the same size), would we consider it a proper planet? Probably not.

So it’s nothing personal against Pluto. Calling it a dwarf planet is just a way to try to establish some logical consistency to a word that arose long before anyone understood the true structure of the solar system.

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