My Very Educated Mother Just Served Us Nine Pizzas

For those who love mnemonics to remember basic listing of general knowledge thingamajigs, the above represents the arrangement of the nine planets of our Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto).

However, astronomy books and this mnemonic will have to undergo some changes soon. Pluto, as voted upon by the International Astronomical Union (IAU), is no longer considered a planet.

A planet is now distinctly defined 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, and (c) has cleared the neighbourhood around its orbit."

I believe Pluto has a little problem with item (b) and (c). It's quite small to attain self-gravity and attract the small objects in its orbit and clear it off its path.

Pluto has been demoted to "dwarf planet" category instead, alongside 2003 UB313 (temporary name), which is a celestial body found orbiting outside Pluto's and is roughly the same size as the latter. In which case many argued how come UB313 (codenamed Xena) is not being classified as a planet when Pluto is?

I wonder what became of another orbiting celestial body called Planet X which we learned about in my late high school years? It was also found orbiting outside Pluto but its path is vertical as opposed to the known planet's horizontal orbit (when a simplified drawing is made).

Now, the more compelling question is, how are astrologers affected in their readings for the alignment of planets and the zodiac signs? Think of the dire consequences to fortune-telling! :-)

Happy weekend, everyone!

Read more at the IAU website
Photo credit: Physics at Capilano College