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quantumaniac:

Why is Pluto not considered a Planet? 

We were all heartbroken in late 2006 when Pluto’s status as a planet was officially revoked. Since childhood we were all taught that there were nine planets, and even learned clever pneumonic devices about educated mothers and pizza to remember them - and all of that was changed forever. However, most people aren’t exactly sure what happened; what caused the big change? 

Although we’ve always really taken the definition for granted, astronomers, before 2006, didn’t have an official definition for what ‘was’ and ‘was not’ a planet. However, since its discovery in 1930, Pluto hasn’t really ever fit in. 

• It’s much smaller than any other planet - it’s even smaller than Earth’s moon. In fact, one of the planet’s own moons, Charon, is nearly half Pluto’s size!
• It’s small, dense and rocky, like the terrestrial planets (Mercury, Venus, Earth and Mars). However, its planetary neighbors are the Jovian planets (Jupiter, Saturn, Uranus and Neptune). On this premise, many astronomers believe that Pluto didn’t form with the rest of the solar system, and was trapped by Earth’s gravity sometime later - some even theorize that it could’ve been one of Neptune’s moons.
• Pluto’s orbit is weird. Most planets in the solar system orbit in a relatively flat, close-enough-to-perfect ellipse, but Pluto’s orbit is irregular, at an angle and even crosses over Neptune’s! 

On August 24, 2006, the International Astronomical Union (IAU), an organization of professional astronomers, gathered together and passed a few important statements about the definition of galactic objects. The IAU passed a few requirements for any and all prospective planets: 

1. It needs to orbit the sun - Pluto’s definitely a planet!
2. It needs to have a strong enough gravitational force to have formed into a spherical shape - Close enough! 
3. It needs to have “cleared the neighborhood” of its orbit - Damn. According to this, Pluto isn’t a planet since it interferes with Neptune’s orbit, and thus hasn’t completely “cleared the neighborhood.”

Additionally, another resolution defined a dwarf planet as: 

“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 not cleared the neighbourhood [sic] around its orbit, and (d) is not a satellite.”

So Pluto was revoked its status, and is now classified as a dwarf planet. Sorry buddy, but you’ll always be in our hearts.