Is Pluto a Planet? You Decide!

Flow charts are all the rage on the internet these days, or so I've been told. They are also used at my new job. Im guessing that is because the engineers we sometimes have to deal with are used to visualizing systems as block diagrams. Anyway, that got me thinking, Could one of the most publicized astronomical controversies of the last few years be clarified using a flow chart? Here is the result: a flow chart to help you decide whether Pluto should be a planet. 

(For instructions on reading a flow chart, see xkcd).

For those seriously interested in the debate, here's what it boils down to. Most astronomers are concerned with very large questions: how the universe evolves from the Big Bang (cosmology), how galaxies form from the cooling plasma produced by the Big Bang, how stars form within Galaxies, and how the debris surrounding newly-formed stars coalesces into planets. 

For planet formation, the broad question is how dust grains that are less than a micron in size coagulate to form bodies thousands of kilometers across --- an increase in size of a factor of 100 trillion. This process probably has two steps, roughly speaking. First, objects the size of boulders and mountains must be formed. This probably happens slowly, as things collide and stick together. Eventually, something will get big enough that its gravity starts collecting other small bodies, accelerating its growth rapidly until it forms a full-fledged planet. 

We now know that Pluto is a member of the Kuiper belt --- a group of bodies of icy material that has stuck together to form objects with sizes ranging from boulders to mountains. Pluto is one of the largest members, and is interesting because it has enough gravity that it pulled itself into a round shape. Unlike the other planets, however, it did not get big enough to collect much of the material in its orbit. Therefore, it exists among a family of bodies that are similar to the asteroids, but that are icy instead of rocky. For this reason, some members of the International Astronomical Union (IAU) voted that Pluto should not be called a planet.

However, as I mentioned, Pluto is unusual among the Kuiper belt objects in that it has enough gravity to make itself round. In this sense, it is similar to other planets. Pluto probably has more interesting geology than other Kuiper belt objects, so it is useful to study it as an exceptional object. For this reason, and for the historical fact that we have called it a planet for about 75 years, many people feel it should keep its status as a planet. 

The IAU compromised to some degree, calling Pluto a "dwarf planet", along with the asteroid Ceres, and (as of 2009) three other Kuiper belt objects, Eris, Haumea, and Makemake. 

Of course, this doesn't make everyone happy. That's why we need a flow chart.

Comments

[Cindy]

I have a question. What consequences await me if I continue to refer to Pluto as a planet? Will I be attacked by a gang of angry astronomers?

[Craig Heinke]

Personally, I like the atmosphere idea (see http://astro.cas.cz/nuncius/appendix.html#heinke), though I think it makes me a "meanie" by your flowchart.
Anyway, I'm happy enough with the current definition. It'll be interesting to see whether enough Pluto-lovers go to Rio this summer to overturn the planet decision.

[Michael Muno]

Yes, Craig, it makes you doubly mean. By disqualifying Pluto's part time atmosphere, you drop both it and Mercury from the list of planets.

By the way, according to Wikipedia (I'm too lazy to use a book), the average pressure of Mars's atmosphere at its surface is less than 1% of the pressure of Earth's. So, if you find yourself really cranky during your next Edmonton winter, you can demote Mars too!