Planets, or not planets?
Published on Aug 16, 2006 at 10:01 pm.
1 Comment.
Filed under planets.
So, what is a planet? Sadly, astronomers have never had a real definition. In ancient times, a planet was a “star” that appeared to wander from constellation to constellation. That was the last coherent definition of a planet that we had. Copernicus showed that the planets and Earth all orbit the Sun. Soon, it became apparent that, in that case, the Earth was a planet. Giordano Bruno then extemded that idea to be that perhaps the planets were Earths. Galileo’s telescope showed that the planets, indeed, were spherical objects. Soon it became apparent that the four inner planets were much larger than the two outer ones (only six were known at the time). Eventually, a seventh was found: Uranus. In 1801, Ceres was found. At first, Ceres was declared a planet. However, no amount of magnification showed any disk, so astronomers realized that it was way too small to be a planet. It was demoted to “asteroid”, a new category devised to include Ceres and many other similar bodies. More asteroids were found. Then, in 1930, Clyde Tombaugh discovered Pluto. Early estimates of Pluto’s size were wildly too big. Pluto was declared a planet. But over the years, we learned Pluto’s real size. It is tiny. You could fit about 3.5 Plutos inside of our Moon. It is made of ice and rock, not rock like the four inner planets, nor gas like the four big planets. It doesn’t fit. For years, astronomers have argued that it was a mistake to put it on the list of planets. Then, not long ago, the object called 2003 UB313 was discovered. The problem is that 2003 UB313 is bigger than Pluto. Is it a planet? What about several objects just a shade smaller than Pluto? Are they planets? Just what is a planet. The International Astronomical Union has been working on a definition of “planet” for several years. A committee was formed to decide on a definition. It failed to do so, so a new committee was formed. This one has come up with a definition that has been proposed to the IAU for a vote.
There’ve been rumors on what would make for a good planet definition. You can draw an arbitrary line and say “bigger than this” is a planet. But, what about something a tiny bit smaller? If bigger than Pluto is a planet, what about something just a few miles smaller? Or a few miles smaller than that? Or a few miles smaller even than that? Is it fair to say that one thing is a planet, but something just a tiny bit different isn’t?
Finall, though, the proposal has been made public. Under the new definition, a planet is a body that meets several criteria:
If a body is big enough that its gravity is able to pull it roughly into a sphere, and it is larger than 800 km across, and it orbits a star instead of another planet, then it is a planet. Recognizing that a binary planet is possible, such a designation would be given if two center of mass of the two bodies lies outside of either one.
A distinction is made between the regular terrestrial planets, the gas giants, and 
the smaller bodies that would fit this definition. Pluto fits the definition, as does 2003 UB313. So does Ceres. But, Ceres is rocky, and Pluto and 2003 UB313 are icy. So, Ceres is a “dwarf planet,” and Pluto, 2003 UB313, and other such icy bodies are “plutons.” Since Pluto’s moon Charon is so large, the center of mass between Pluto and Charon is outside Pluto. That means that Pluto and Charon are both planets. In fact, they are a double planet system. That makes 12 planets.
So, everyone congradulates themselves. But wait! The International Astronomical Union has to actually vote on this definition before it becomes official. Then, it will clear things up, right? No. It just changes the arguments.
Several of the asteroids are almost spherical, as are a number of the objects out past Pluto. How close to spherical do they have to be? In fact, even the major planets, notably Saturn, are not really spherical — they are oblate spheroids. Saturn is so oblate that (if you’d just ignore the rings) you can tell with the naked eye that it is wider across the equator than from pole to pole. Does that mean it isn’t a planet? What about something just a bit more oblate? Where do you draw the line? Even Ceres isn’t a perfect sphere. What about Vesta, which is not quite a spherical as Ceres, but is not too far off of a sphere itself? Also, icy things are easier for gravity to pull into a sphere than are rocky things. Is it fair to call an icy thing in the outer Solar System a planet when something slightly larger and much more massive in the asteroid belt isn’t, just because the asteroid is rocky and harder to pull into a sphere? In fact, there are about a dozen objects that almost fit 
the definition, and are objects of interest. They need to be studied, and a later committee will vote on their planetary status. Some of these are pictured in this second schematic.
So, rather than clearing things up, I don’t see that this proposal, should it be approved, will answer the question. Until now, we had Pluto and 2003 UB313 as questionables. Well, if they had decided that a body had to be 4000km or large to be a planet, then the Solar System would have 8 definite planets. Drawing an arbitary line with some objects just bigger than that size and some just smaller isn’t fair, and would not really clear things up. So, they came up with a definition that makes 12 planets for sure, and about a dozen other objects that may or may not be planets. Personally, I don’t think that this is an improvement.
I rather hope that they vote this proposal down, and try for another one. Of course, it doesn’t really matter. The reason that astronomers have never really bothered with a definition of “planet” before is that we don’t really care. The label doesn’t matter. We know what we are talking about.
-Astroprof
(Image credit: NASA and International Astronomical Union)
Astroprof’s Page » Plutoids on June 11, 2008 at 12:41 pm: 1
[…] to finally devise a new definition for the term planet. A committee for the IAU worked on a proposed definition for about a year. Unfortunately, that definition did little to clear things up. Bowing to popular […]