The relatively recent demotion of Pluto and discovery of Eris has a lot of people upset, for various reasons. I’ve heard people declare their emotional attachment to the poor dwarf planet, which I can understand, but I’ve also heard people scoff at the lack of precision in the entire field of astronomy.
They are absolutely right to. The field in astronomy is unique in many ways, but possibly one of the most important ones is that it is entirely possible that we will literally never see most of what we’re studying. Humanity as a whole will never touch a star, poke an exoplanet, or prod the cosmic background radiation; the vast majority of what we study about the universe is thirdhand inferences and cautious assumptions simply because everything is so inconcievably far away. To put it in perspective, if you take the distance between the Sun and the Earth to be an inch, Pluto would be three feet away. Alpha Centauri, the nearest star, would be four miles away, and the radius of the Milky Way would be ten times the radius of the Earth. The nearest galaxy would be ten times further away from the Earth than the Moon, and the universe, as far as we know, would span a diameter of roughly fifty times further away than – well – Pluto.
And this is on the scale of the distance between the Earth and the Sun being one inch.
So no, astronomers hadn’t defined the word “planet” until recently. Frankly, they had bigger things to worry about.
-Kristen Koopman, Sadielou.net, May ’09