Remind Me Again, Why Isn't Pluto a Planet?

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Last week, news broke that a team of astronomers using the Hubble Space Telescope had discovered a new moon in orbit around Pluto.

They were actually looking for signs of a ring system when they stumbled across another tiny object — now imaginatively called "P4" — bringing Pluto's moon count to four.

This is quite a nice little news story in its own right, and covered by my Discovery News colleague Irene Klotz (see the July 20 article: "Hubble Discovers New Pluto Moon"). However, what I hadn't anticipated was a deluge of messages asking me if this means Pluto is now a planet again.

Well, the answer is a resounding no. It is still, and always will be, a dwarf planet.

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But what rules have we applied to Pluto to forever condemn it to the back seats of the solar system rankings?

Before we look at the well established rules of planetary terminology, it's worth remembering why we even need such a definition.

Like many of you, I was taught as a school kid that there were nine planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto along with thousands and thousands of tiny objects called asteroids. For a long time, Pluto was believed to larger than Mercury but after the discovery of Charon (Pluto's largest moon) in 1978, the foundations of its planetary status started to wobble.

By studying Charon, astronomers could accurately determine the mass of Pluto and surprisingly found it to be much smaller than Mercury and even our own moon.

During the late 20th century, more objects started to be discovered at comparable distances to Pluto's orbit and beyond; one of which, Eris, was even thought to be larger than the ninth planet in our solar system. Eris was discovered by Mike Brown and his Palomar Observatory-based team in 2005.

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These discoveries led the International Astronomical Union (IAU) to setup a committee in 2005 to consider an official definition of a planet. There were a number of different definitions that were considered, but in 2006 a final, all-encompassing set of criteria was identified that, once and for all, knocked Pluto off its planetary pedestal.