Ever since Pluto lost it’s title as the “9th planet” there has been some controversy as to exactly what it is. When first discovered, Pluto was the only known object of any significant size in the solar system, aside from the other eight planets, so it seemed logical to group it with the planets and call everything else an asteroid or commit, since they were all distinctly smaller.

Now, however, the line has gotten more blurred as more “dwarf planets” have been discovered out in the Kuiper belt. These objects are large enough to be round and some are larger than Pluto, but they are distinct from the other planets, which are either terrestrial or gas giants. If these were to all be counted as planets this would result in the solar system having dozens, possibly hundreds of “planets.” This begs the question of exactly what to define as a planet, and all the definitions anyone has been able to come up with seem to exclude Pluto. The official definition from the International Astronomical Union is “a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.”

The only definition anyone seemed to be able to put forward that would make Pluto a planet was to define planets as objects with sufficient gravity to produce a round shape. This has two major issues, the first being how one determines what is round enough to be a planet, since any solid object will have some distortions. The other problem is that it would result in a real real lot of planets in the solar system, putting the “big eight” in the same category as many asteroids (and they seem to be pretty distinct from most other objects). Ceres, an object in the asteroid belt, and the largest known asteroid would easily qualify for the title. Currently, they are only “dwarf planets,” which are NOT planets, even if they have the word planet in them. (Things can be something with an adjective attached and not be the original item. Believe me, I once tried to talk a cop into a “rolling stop” being a type of stop. It didn’t work)

Now, the IAU has a new title for Pluto, and it seems to be custom made: Pluto is a “Plutoid.” Apparently a plutoid is a category of objects which are outside the orbit of Neptune AND large enough that their own mass maintains a round shape. Therefore, Ceres and any other asteroids within the inner solar system don’t qualify. Another way of putting it is to say that if something is in the Kuiper Belt and qualifies as a “dwarf planet.” Potentially there could also be objects which qualify out in the Oort cloud or even the scattered disc – there may be thousands or even millions, but such objects are far beyond the capabilities of current detection methods. Yet another issue is that the exact boundaries of the Kuiper belt, Oort cloud and scattered disc are not very precisely defined.

This is all well and good, I guess, even if it seems a bit redundant and unnecessary, but it seems this will really only apply to our local solar system. In other solar systems, the orbiting bodies may be arranged totally differently, blurring the line and making the definition less suitable. Of course, at present we don’t have the capacity to detect such small bodies orbiting distant stars.

In the end, none of this really changes anything in the universe. Pluto is the same as it ever was, and only artificial definitions set out by humans are changed, so it’s all semantics. Nature does not always draw hard boundaries between things to make it easy to define and categorize. Hopefully this will at least shut up some of the Pluto fanboys (and fangirls), at least until hundreds more “plutoids” are discovered and thus make the little ball of ice suddenly seem less special again.

---

This entry was posted on Thursday, June 12th, 2008 at 4:43 pm and is filed under Good Science, Space, media. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

---

View blog reactions