There is a clear divide in our household. On one side we have the more-cerebral, less-emotion camp; and on the other side we have the more-emotion, less-cerebral camp. This comes to a head every time we talk about the designation of Pluto as a dwarf planet.
One of the qualities I appreciate most about engineering, and science in general, is the emphasis on precision of language. Scientists use very specific terminology to describe things, events, measurements, etc. This focus helps resolve conflicts, and removes a lot of ambiguity, especially when you start working with complex systems like Particle Physics, Engineering, and Astronomy. In August of 2006 the IAU (International Astronomical Union) created an official definition for the term "planet". This definition would be the beginning of the end of Pluto's spot among the planets orbiting the Sun.
"Why? Why would we do such a horrible thing?" my wife asks. "Can't we just grandfather Pluto in?" My mind immediately begins pondering the consequences of "grandfathering in" old scientific findings for nostalgic purposes. This is a foundational aspect of science: new information forces us to reconsider old truths. As long as humans have been looking up we've known there were lights in the night sky that moved differently from all the others. The English word "planet" literally comes from the Ancient Greek word "planasthai" which means "to wander". Some cultures thought these were the souls of the deceased, others thought they were deities. It wasn't until technology gave humans the ability to physically see planets that we were able to appreciate them for what they really are. This takes us back to the question the IAU was trying to answer with their definition, what are planets?
The definition of a planet is fairly dry and space-nerdy: the object must orbit the Sun, the object must be massive enough to be rounded by its own gravity, the object must have cleared the neighborhood around its orbit. Unfortunately, Pluto only meets two of these three criteria. Since the early-1990's over 2,000 objects have been observed in the Kuiper belt (that's generally Pluto's neighborhood) and contemporary mathematical models show that there are probably orders of magnitude more tiny objects floating around out there. It is for this reason that in September of 2006 Pluto officially lost its designation as the 9th planet in our solar system.
Neither my wife nor I are professional astronomers, so our personal opinions will never carry any weight in the scientific community. However, I do know one thing for certain: regardless of Pluto's official designation, it will always hold a special place in our hearts and minds.
A despondent Pluto laments to a gruff Mars, "Earth says I'm not a real planet." Mars responds, "Yeah, that's cuz you're not."
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