Pluto, artist rendition.
For many years, our solar system had 9 planets. However, in 2006 an unelected group of astronomers, over the objections of a majority of their peers, declassified the historical ninth planet, Pluto, into what they refer to as a "dwarf planet". Thus, for now, our solar system has 8 planets.
The question remains though: Is Pluto a planet? In 2006 the IAU classified a planet as having to have 3 criteria:
- The object must be in orbit around the Sun.
- The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
- It must have cleared the neighbourhood around its orbit.
The IAU declared Pluto failed to meet the third criteria, since its mass was only 0.07 times that of the mass of the other objects in its orbit. This has caused great debate and resistance within the astronomical community, with many deriding the decision as "crazy".
Public reaction has tended, for the most part, to fall onto the side of defending Pluto's lost position. Indeed, California, New Mexico, and Illinois have all passed state resolutions supporting Pluto as the 9th planet despite the IAU's ruling. Adding fuel to the fire is the fact that Pluto is the only planet discovered by an American.
All of this may change, however, in 2015. The New Horizons is a NASA robotic spacecraft currently hurtling towards Pluto. She is the fastest spacecraft ever launched, and the second fastest ship overall, just behind the legendary Voyager 1. Her mission is to gather data, take readings and photographs, and learn as much about Pluto as is possible. The ship will also encounter Pluto's moon Charon, as well as two small newly discovered moons named Nix and Hydra. New Horizons is also carrying some cultural mementos, including an American flag, a piece of Spaceship-One, a Florida State quarter, and, fittingly, some ashes of Pluto's discoverer, Clyde Tombaugh.
www.youtube.com/watch Student protests in New Mexico over Pluto's de-classification.
www.youtube.com/watch More reactions to Pluto's "termination".
So, until New Horizons arrives in 2015, the debate will go on. Planet or non-planet? Comment below, what do YOU think?
Comments
Pluto is not a planet
Pluto IS a planet!
Pluto IS a planet because unlike most objects in the Kuiper Belt, it has attained hydrostatic equilibrium, meaning it has enough self-gravity to have pulled itself into a round shape. As of now, there are three other KBOs that meet this criterion and therefore should be classified as planets—Haumea, Makemake, and Eris. Only one KBO has been found to be larger than Pluto, and that is Eris.
The IAU definition makes no linguistic sense, as it states that dwarf planets are not planets at all. That’s like saying a grizzly bear is not a bear. Second, it defines objects solely by where they are while ignoring what they are. If Earth were placed in Pluto’s orbit, by the IAU definition, it would not be a planet. That is because the further away an object is from its parent star, the more difficulty it will have in clearing its orbit.
Significantly, this definition was adopted by only four percent of the IAU, most of whom are not planetary scientists. No absentee voting was allowed. It was done so in a highly controversial process that violated the IAU’s own bylaws, and it was immediately opposed by a petition of 300 professional astronomers saying they will not use the new definition, which they described accurately as “sloppy.” Also significant is the fact that many planetary scientists are not IAU members and therefore had no say in this matter at all.
Many believe we should keep the term planet broad to encompass any non-self-luminous spheroidal object orbiting a star.
We can distinguish different types of planets with subcategories such as terrestrial planets, gas giants, ice giants, dwarf planets, super Earths, hot Jupiters, etc.
We should be broadening, not narrowing our concept of planet as more objects are being discovered in this and other solar systems.
In a 2000 paper, Dr. Alan Stern and Dr. Hal Levison distinguish two types of planets—the gravitationally dominant ones and the smaller ones that are not gravitationally dominant. However, they never say that objects in the latter category are not planets.
I attended the Great Planet Debate, which actually took place in August 2008, and there was a strong consensus there that a broader, more encompassing planet definition is needed.
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