Pluto is no longer a planet.
The IAU has been toying with this for a while but they finally did it, establishing three major guidelines in order to define what a planet actually is. The third guideline is what did Pluto in.
According to the IAU, for an object to be regarded as a planet:
- It must be in orbit around the Sun
- It must be large enough that it takes on a nearly round shape
- It has cleared its orbit of other objects
Since Pluto’s orbit carries it across Neptune’s, we can hardly argue that it’s cleared its orbital path of other objects.* Thus, officially, there are eight planets in our solar system, just as there were before 1930 when Tombaugh first proved Pluto’s existence.
What Pluto might be, in case you’re wondering, is a captured comet. There are nuclei of comets surrounding the Sun in a very distant grouping or shell called the Oort Cloud. Distant gravitational events can jostle these nuclei on a sunward spiral, causing them to fall toward the Sun’s gravity well and, eventually, grow a tail due to a combination of solar heating and solar “wind”. Pluto and its moonlet, Charon, may have been comet nuclei that managed to fall into a more or less stable orbit rather than go Halley on us.
Also, comets might have caused more than one mass extinction here — as well as brought the water that life needs so desperately. So while Pluto isn’t regarded as a planet any longer, its history and the controversy surrounding it have surely got it more attention than Mercury is likely to ever garner.
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* Presumably the Trojan and Greek camps around Jupiter don’t count.
