Pluto is once again the solar system's biggest dwarf planet

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Pluto lost its status as a planet because of the discovery of Eris, another Kuiper Belt object that was slightly larger. But now Pluto has put Eris back in its place.

Astronomers were long uncomfortable with Pluto as a planet, because its small size and eccentric orbit didn't make it a good match for the four rocky inner planets and the four outer gas giants. Still, the issue was pretty much ignored until the discovery of another, slightly larger object out in the Kuiper Belt beyond Neptune forced the issue. This new object, named Eris, was a couple hundred miles in diameter larger than Pluto. If Pluto was a planet, then logic demanded that Eris was as well.

This wasn't a good solution for astronomers, as there was every chance other large Kuiper Belt objects would be discovered, and there needed to be some definition of a planet more elegant than simply giving it to anything that happens to be bigger than Pluto. Astronomers were finally forced to define the term "planet", with Pluto and Eris relegated to the new status of dwarf planet.


But the entire controversy was mostly the result of measurements that said Eris was bigger than Pluto. Now, it looks like that isn't the case. Recent observations of Eris by Chilean telescopes have provided the most accurate measurements yet for Eris's size, and it turns out Eris's radius is "almost certainly" smaller than 1,170 kilometers, and it's possible the final calculations will push that upper limit down another 50 or 60 kilometers.


Why does that matter? Well, Pluto's radius is 1,172 kilometers. Yes, by perhaps as little as two kilometers, Pluto is slightly bigger than Eris after all. That means it's the biggest dwarf planet in the solar system, and once again it's the ninth biggest object orbiting the Sun. To be fair, Eris still has it beat in terms of mass, which is about 125% that of Pluto. This means Eris is unusually but not impossibly dense.

So, is it time for Pluto to reapply for planet status? Honestly, two kilometers isn't much of a difference, and even if it was known a couple years ago that Pluto was slightly bigger than Eris, it probably still would have been demoted to dwarf planet. But I'm betting Pluto fans can take some satisfaction in the fact that Pluto really can be considered the ninth planet again with no other icy rocks around to get in the least until something even bigger than either Pluto or Eris is discovered out there, that is.


[Sky and Telescope; images are of Pluto and Eris and their respective moons.]