I've just encountered yet another article in the October 2010 issue of Astronomy magazine regarding the ongoing debate of whether Pluto is or is not a planet. Some states (most notably Illinois where Clyde Tombaugh, Pluto's discoverer back in 1930 was born) have even passed laws that state when Pluto is directly overhead in the skies of these states, that Pluto regains its planetary status. How ridiculous.
For the record, I like Pluto. I think like most people who have even a passing interest in astronomy, Pluto is a world near and dear to many a heart. Maybe it's its underdog status: this tiny speck of ice and rock occurring after a string of gigantic gas giants. It's practically ghetto in the bourgeoisie establishment of the outer Solar System. I look forward to the New Horizons probe encountering Pluto in the still far off year of 2015. It'll be a brief encounter, but it will finally show us what this world looks like and if the probe is fortunate, it may survive long enough to encounter other objects in what is now known as the Kuiper Belt. It is the fact that this belt of icy objects akin to the more familiar Asteroid Belt exists that I have come to accept Pluto's reduced status. A little history:
Back in 1801, a new object was discovered by Giuseppe Piazzi between Mars and Jupiter. It was named Ceres and for almost a half-century, it was considered the eighth planet (Neptune would not be discovered until 1846). In fact; Pallas (1802), Juno (1804), and Vesta (1807) were also considered planets. That means, for a time, the Solar System had eleven planets. However, as more and more objects were discovered in that region of the Solar System, it became clear that Ceres was not a planet; but rather, the first of a new class of objects that we now know as asteroids. All these new planets were summarily demoted and the Solar System was now back to its familiar eight worlds (Neptune had since been discovered). I wonder if there was an outcry then to save those "planets" or if the reclassification in light of an increasing number of discoveries rendered the argument moot.
A key difference between the Asteroid Belt and the Kuiper Belt is the amount of sunlight. Sunlight is much denser per square inch in the Asteroid Belt than in the far reaches of the Solar System. This makes the detection of even small, dim objects easier than at the greater distances where Pluto can be found. Therefore, when Pluto was discovered, it seemed like it was alone -- the last object that would be found out there. The region was simply too dim (as well as enormous in area compared to the Asteroid Belt) to find even Pluto-sized objects let alone the many likely smaller objects out there. Long exposures times and limited time at the large telescopes to take such images makes such a search prohibitive. It wasn't until 1992 that new objects began turning up in that region. Technology had made searches for new objects easier and once the search began, objects began turning up relatively quickly. They have been given a mix of familiar-sounding and exotic names such as Eris, Sedna, Makemake, and Quaoar (asteroids bear a similar mix of names as the discoverer gets to name the object - subject to approval by the IAU). Simply put, Pluto was no longer alone but merely one of many. It is currently the largest Kuiper Belt Object discovered but that title may one day be stripped.
Like Ceres, Pluto is merely the first of a new class of objects in the Solar System. Unlike Ceres though, these objects have been named after the former planet and are called Plutoids. So while I was at first a bit miffed at Pluto's demotion, through an understanding of an analogous history, I am at peace with the world no longer being the ninth planet of our Solar System...and so should you.
As a side note, a spacecraft called Dawn (NASA really comes up with some shitty names for spacecraft these days) will visit Vesta in 2011 and Ceres in 2015 so we'll be able to see them up close as well for the first time in the near future.
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