The two sides of the emo­tive de­ci­sion to de­mote Pluto from full plan­e­tary sta­tus

Sky at Night Magazine - - PLUTO’S HISTORY -

The of­fi­cial act that took plan­e­tary clas­si­fi­ca­tion from Pluto was IAU Res­o­lu­tion 5A, ‘Def­i­ni­tion of a Planet in the So­lar Sys­tem’, on 24 Au­gust 2006. This doc­u­ment states that for an ob­ject to be con­sid­ered a planet must sat­isfy three con­di­tions. It states:

“A planet is a ce­les­tial body that (a) is in or­bit around the Sun, (b) has suf­fi­cient mass for its self-grav­ity to over­come rigid body forces so that it as­sumes a hy­dro­static equilibriu­m (nearly round) shape, (c) has cleared the neigh­bour­hood around its or­bit.”

Since Pluto is not mas­sive enough to clear its or­bit of de­bris, it fails to meet the third con­di­tion. There’s also the prob­lem that if Pluto were to be classed as a planet then so too would many other ob­jects in the Kuiper Belt, and the planet count in the So­lar Sys­tem would rise dra­mat­i­cally. There are some who have ar­gued to keep Pluto as a planet for sen­ti­men­tal rea­sons: for much of its his­tory, it has been thought of as a planet and there­fore should con­tinue to be so. Some as­tronomers point out that there are in­con­sis­ten­cies in the word­ing of Res­o­lu­tion 5A – while it is true that Pluto hasn’t cleared its or­bit, nei­ther has Earth or Jupiter. Earth or­bits with 10,000 near-Earth as­ter­oids, while 100,000 Tro­jan as­ter­oids lie within Jupiter’s or­bit. One could ar­gue that both Jupiter and Earth also fail the IAU def­i­ni­tion. Those who hold this view say that a bet­ter def­i­ni­tion of what a planet is might be to say that any ob­ject or­bit­ing the Sun that has a sur­face area greater than 1,000km can be called a planet. If this def­i­ni­tion were to be adopted both Pluto and Eris would be classed as plan­ets.

The IAU voted to de­mote Pluto in Au­gust 2006

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