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.

Pluto

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* Presumably the Trojan and Greek camps around Jupiter don’t count.

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