When we place the known objects in the Solar System in order, four inner, rocky worlds and four, outer, giant worlds stand out. Yet it’s 2019, and astronomers (and planetary scientists) are more divided than ever over the definition of planet. (NASA’S THE SPACE PLACE)

This Is Everything That’s Wrong With Our Definition Of ‘Planet’

Not only can’t astronomers and planetary scientists agree, but the IAU made it worse for everyone.

If you were alive in 2006, you likely remember a momentous event in astronomy: the International Astronomical Union (IAU) took it upon themselves to redefine what it meant to be a planet. While eight of the nine classical planets in our Solar System were still in, from Mercury to Neptune, the smallest and most distant among them — Pluto — was out. Its demotion to the status of ‘dwarf planet’ was met with worldwide dismay, much to the chagrin of plutophiles everywhere.

What most people don’t realize is that until this resolution was made 13 years ago, there was no universally agreed-upon definition of a planet at all. In an interesting perspective over at Scientific American, Chris Impey discusses the history of how this fateful decision was made at the time. But in many ways, the definition created more problems than it solved. Here’s the story behind what it truly means to be a planet.

The largest galaxy in the Local Group, Andromeda, appears small and insignificant next to the Milky Way, but that’s because of its distance: some 2.5 million light years away. The Moon, the stars and planets, the Milky Way, and various nebulae are all distinctly identifiable in Earth’s night sky. (SCIENCETV ON YOUTUBE / SCREENSHOT)