HowDoWhy

Our solar system has an interesting and diverse cast of characters. We even have a black sheep in our solar family in the form of Pluto and at the center of all the recent controversy about its status has been one surprisingly difficult question to answer - what is a planet?

You’d think that pretty much any response that includes “big round thing orbiting the Sun” ought to hit the mark but it turns out answering the question is more complicated than that and, while the International Astronomical Union (IAU) may have finally settled on a set of definitions that has resulted in one very notable change in our solar inventory, the truth of the matter is that there are still plenty folks out there who disagree with the terms and conditions. The 2006 gathering of the IAU defined a planet as follows:

• A planet is a celestial body that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
• Is in orbit around a star, and is neither a star nor a satellite of a planet.
• It has cleared the neighborhood of other objects in the path of its orbit.

This definition was adopted rather than the originally proposed draft which would have added at least three new planets to our solar system - Ceres, Sedna and Eris. All three are comparable in size to Pluto and in a similar orbit. The long debate essentially settled the question by assigning them the classification of Small Solar System Bodies (SSSB). Basically, any object (other than a moon orbiting a planet) that meets only #1 is now an SSSB. If it meets two of the definitions above, it would be considered a “dwarf planet.”

While it’s great that we’ve finally settled on the definition for a planet, there was a casualty in all of this. We lost Pluto. I’ll write more about what happened to poor Pluto in an upcoming post this week.