Today is the two year anniversary of Pluto’s demotion from planet status. I had invited several people to contribute to this event. Chris Schierer has remarked that Pluto is really really small and is very very hard to see. Kim Bosco tells us how Pluto was found and why Kansas is key! Jim Cronen opines about classification and lastly, Steph Sisson has composed a planetary haiku for the event:
Like your dark namesake,
return to deepest shadow.
Retrograde motion
I also have an opinion and some comments on this event
You may recall that in 2006, the International Planetary Union decided on a definition of “planet” that did not include Pluto.
The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
The clinching criteria was the (c) item. Pluto doesn’t have a large enough mass to have “cleared the neighbourhood”. Unfortunately, as several people have pointed out, if Mercury (a planet) were transported to the Kuiper Belt, it wouldn’t be a planet anymore. The definition is fundamentally arbitrary and therefore hasn’t much scientific usefulness.
This has implications for the ongoing search for “planets” in other solar systems. To date, most of the discovered extra-solar (“not in our solar system”) planets have been large ones. Jupiter-sized or so. These fit anyone’s definition of planet. Several planets close to the size of Earth have also been located. Soon, as techniques and technology improve, even smaller bodies will be found. We’re going to have to pigeon-hole these orbiting piles of rock and ice into our preconceived definitions of what is a planet, and what is not. People will think differently of a planet then they will a moon, or an asteroid, or a Kuiper belt object, or whatever. Don’t forget that the moons Titan (orbiting Saturn) and Ganymede (orbiting Jupiter) are both larger than Mercury, although less massive. Don’t tell me that your school teachers didn’t concentrate more on Mercury than they did Titan or Ganymede because I’ll know you’re lying.
These preconceived notions, while not detrimental in and of themselves, can lead to dangerous blindspots when performing that grandest of tasks: science. While I have faith that eventually these blindspots are exposed and examined, wouldn’t it be better to not force astronomers, astrophysicists and other scientists to pre-ordain orbiting bodies into the categories of “planet” or “not-planet”. A more all-inclusive term is needed for the various piles of rock, ice and gas that circle their central furnaces. “Planet” should probably not be used anymore in science, with an exception:
For the purposes of Grade School science instruction, I think that we should call the most recent 9 planets (including Pluto, for historical reasons) the “official” planets of our solar system and then discard the term in later science instruction, explaining that “planet” is only a convenient label for several large bodies that circle our sun and that it is not a scientific term.
Pluto isn’t a planet anymore, alas. It still has a large place in the hearts of those of us who care.