In 2006, the I.A.U. officially demoted Pluto from planet to dwarf planet. At this point, pretty much everyone knows that, and most people seem to have strong opinions on the matter (especially right now, as New Horizons sends back the first ever detailed images of Pluto). However, Pluto was not the first planet to be demoted. That particular dishonor goes to Ceres.
Discovered on New Year’s Day, 1801, Ceres was initially identified as the long sought missing planet between the orbits of Jupiter and Mars, making it the second planet discovered in modern times (Uranus was discovered about twenty years prior).
A few months later, however, astronomers discovered another object (named Pallas) sharing Ceres’s orbit. Within a few years, they found another (Juno) and another (Vesta). This started getting awkward. How could we have so many planets in the same (or nearly the same) orbit?
And so Ceres went from being the fifth planet to the first known asteroid.
What happened to Pluto is almost the exact same story. When Pluto was discovered, we had no way to know that it was one of a great many trans-Neptunian objects. Once we realized our mistake, Pluto had to be reclassified.
On the upside, the creation of a new category of celestial object allowed us to promote Ceres from asteroid to dwarf planet. So Pluto’s loss was Ceres’s gain.
But what do you think? Do you agree with the I.A.U.’s decision, or do you think Pluto and Ceres should have retained their planethood?
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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.
Planet Pailly 
I’m mostly neutral on planet definition politics. I do think people need to understand how different objects like Ceres and Pluto are from traditional planets like Earth or Mars. But they also need to be aware of how radically different Jupiter and the other gas giants are from the inner rocky planets.
Although when we start taking into account all of the “super Earth” and “hot Jupiter” exoplanets that are being found out there, drawing lines between all these different objects starts to look more and more like what it is, classifications which are inescapably arbitrary to some degree, and whose usefulness we need to revisit from time to time.
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I recently heard Neil de Grass Tyson say that maybe Earth shouldn’t be considered a planet because it has a lot more in common with Pluto than it does with Jupiter.
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Interestingly enough, in its original meaning, that of a wandering star, Earth was not a planet. It only became a planet when it became clear that it orbited the sun like all the others.
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Thanks! That would probably be perceived as much safer. It would definitely be cheaper. Once they’ve proven they can do it, they could go bigger.
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