Explained: Why Pluto Is Not A Planet

In August of 2006, the International Astronomical Union (IAU) made a textbook altering decision. The IAU is the main naming and definition-making organization in the astronomical community. After much debate and discussion, scientists at the IAU meeting collectively decided that Pluto's planetary title would be removed, and it would be labeled as a dwarf planet.

While some people applaud the IAU's decision, others refuse to accept it. The decision was made in 2006, and although people know Pluto is not a planet, most people fail to understand why Pluto is not a planet.

The common belief is that the IAU simply stripped Pluto's planet status. However, in reality, the IAU just issued a new set of requirements that would define if an object were a planet or not. Pluto failed to meet these new requirements. Pluto is not a planet because it does not meet all of the criteria that it takes for an object to be labeled as a planet.

The IAU stated that a celestial body must meet the following conditions to be called a planet:

1. Orbits the Sun - The object should be orbiting the Sun. It cannot be orbiting another planet, or another object. It can only be a satellite of the Sun.
2. Be a sphere - The object's 'self-gravity' should be strong enough that it smooths out any (major) bumps or ridges to become a mostly spherical body.
3. Cleared its orbital neighborhood - There should not be any other bodies in the object's orbit. During the object's formation, it should have absorbed and cleaned out any debris in its orbit (with the exception of moons, because moons are gravitationally 'caught').

Pluto is not a planet because it fails to meet the third condition.

• Compared to Pluto, Pluto's moon Charon, is pretty large because it is only about half Pluto's size. Both objects orbit a common center of gravity, but Pluto orbits this center of gravity at a much close distance than Charon, so that's why Charon is considered Pluto's moon.
• For every three times Neptune orbits the Sun, Pluto orbits it only twice. This is called a 3:2 orbital resonance. In addition, there is a whole category of objects that do exactly this; they're called Plutinos. Pluto is also a Trans-Neptunian Object (TNO) and these objects orbit the Sun at a farther distance than Neptune does.
• Pluto is also on the borderline of a region in our Solar System known as the Kuiper Belt where many icy bodies (both big and small) orbit the Sun at a very large distance.

Pluto falls into a whole range of objects that it can be included into. It can be considered a Trans-Neptunian Object, Kuiper Belt Object, and a Plutino. Basically, Pluto has not really cleared its orbit. There are too many objects that are similar to Pluto and are both larger and smaller than it is, that share common characteristics.

Therefore, instead of being a planet, Pluto is a dwarf planet. Dwarf planets orbit the Sun, are nearly round, have NOT cleared its orbital neighborhood, and does not orbit any other body (not a satellite).

-----
Image Credits: IAU, The IAU Votes; NASA, Pluto's newly discovered Moons; Wikimedia, the New Solar System.
Additional Resource: AstronomyCast: Pluto's Planetary Identity Crisis (Why Pluto isn't a planet)

It's Important to Capitalize in Astronomy

A small pet peeve of mine is capitalization. Many people fail to realize that astronomical objects such as the Earth or Sun should have their first letter capitalized. The most plain and obvious reason is that both of those words are proper nouns, and proper nouns should be capitalized because they can be seen as either a place or a thing.

I understand it is fairly simple to write something like, "The sun is at the center of the solar system." But I just feel those are large errors to commit when writing. Again, both the Sun and Solar System are proper nouns so they should automatically be capitalized.

From the IAU Manual

The International Astronomical Union (IAU), which is the official authority on naming celestial things, the structure of astronomical papers, and astronomical grammar, does not have the authority to fully demand capitalization all the time. Different countries have different rules in remarks to their grammar. The IAU Style Manual states, "The use of capitals for the initial letters of words is much more common in English (and German) than in French," but they do recommend that astronomical objects should be capitalized in both languages.

This is what the IAU recommends in reference to astronomy capitalization, "The initial letter of a word should be typed or printed as a capital in the following cases...individual astronomical objects (such as Earth, the Solar System, Orion, the Crab Nebula, Galactic Centre)...[and] names of individual objects or instruments (Voyager 2)."

I just wanted to address this because it is important to capitalize the Andromeda Galaxy just as its important to capitalize the White House. I am not going to end on a note saying thay, "To keep astronomy sacred and precious we must first correct our grammar." All that I am going to say is just capitalize those astronomical objects because all things in astronomy are wonders, are proper, and are worth the extra effort to be capitalized.

Afterthoughts:

• Is it just me, or did I say 'astronomical objects' a lot?


• I noticed that major writing applications such as Word, Open Office, and other online spell-check features do not automatically capitalize astronomical objects.


• I recently discussed astronomical capitalization with Helium.com and since then, they have changed most of their space-related articles so words like the Sun and Solar System are capitalized.


• The IAU does not say anything about capitalizing astronomical objects that are plural

----------
Links:
IAU Recommendation of Spelling (Online)
IAU Full Recommendation of Capitalization (In Print)

The Universe

The History Channel is premiering an episode of their 13 week series entitled, "The Universe." The series contains different documentaries about the Universe. Tonight's episode is, "Secrets of the Sun." Future episodes discuss "Mars," and "Deep Space Threats to the Earth." Tune in to learn about the secrets, mysteries, and the new discoveries in this series. Read the introduction to, "The Universe," on the official website here. It's very motivational. The Universe is on the History Channel every Tuesday 8pm/9c.

Image Credit: Universe Today

The Universe Made Simple

In this blog, I will explain everything about the Universe very simply. I will deliver space news, discoveries, and the basic fundamentals of space and astronomy wihtout all of the confusing technicalities. I will explain the universe as a person, I will try my best not to sound like an encyclopedia. Please bookmark this blog, and I hope you will be reading my future posts!