It’s Good to Know: Not Your Mother’s Planets Anymore

Everyone knows that the nine planets, in order of their distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Right?

Wrong!

Maybe you got caught napping, as I did. Seems they’ve added two more in the past few years - Ceres (between Mars and Jupiter) and Eris (now the farthest, beyond Pluto).

Ceres and Eris - as well as Pluto - are "dwarf planets."

Adding two dwarf planets into the mix - not to mention "demoting" Pluto to dwarf status - has resulted in quite a controversy. Some experts now place the number of planets at eight, nine, or 11. It all depends on whether they consider one or more of the dwarfs to be "planets."

 What’s the difference?

According to the International Astronomical Union, a planet has three characteristics. It orbits around a star. It has sufficient mass so that its self-gravity can "overcome rigid body forces" and preserve a nearly round shape. And it is neither a star nor a satellite of a planet.

A dwarf planet is a celestial body that fits the definition of a planet, but isn’t one of the eight "dominant bodies" in our solar system. Or you could use the definition offered by Mike Brown (professor of planetary astronomy at Cal Tech): A dwarf planet is something that looks like a planet but isn’t quite a planet.

With such a fuzzy definition, it’s no wonder that planet classification is hotly debated in the astronomy world.

[Ed Note: Charlie Byrne is ETR’s Associate Publisher. Sign up for e-mail delivery of his blog and get edgy and useful ideas on copywriting, marketing, and other category-defying topics.]