What is a Planet?

Some very specific factors are used to determine which celestial bodies qualify as planets and which do not. One of the organizations involved in this area is the International Astronomical Union. *The International Astronomical Union (IAU) is an organization of over 9,000 professional astronomers. It is the only community of experts that has the legal and scientific authority to define the word “planet”. The IAU has been in charge of planetary nomenclature since 1919. It holds a General Assembly every three years. (Wikipedia).

Just as science determines what a “triangle” is or what “bacteria” are scientists determine what a “planet” is. According to official classification there are eight planets in our solar system, a few “dwarf” planets and many minor planets. Planets are defined by dynamics and motion, which is appropriate since the word “planet” is based on a Greek word for “wanderer.” Scientists have come to a general agreement that classifying celestial objects based on movement and dynamics is much easier than defining them based on their physical characteristics.

What About Pluto?

Maybe the best way to figure out what a planet is would be to determine what a planet is not. In 2006 astronomers decided that Pluto was not really a planet. When the scientists discovered there were celestial bodies orbiting stars other than the sun and discovered some small celestial objects beyond Neptune that were orbiting the sun they decided it was time to find a more accurate definition for the word “planet.”

Based on information available in 1930 Pluto was determined to be exceptional and was given status as a planet. In 2006 there was a lot more information available. Based on charts of the orbits of Jupiter, Saturn, Uranus and Neptune as compared to the orbit of Pluto, a change was called for. It seems Pluto is one of several hundred small bodies orbiting beyond Neptune.

The key to all this is that Pluto is deeply embedded in a swarm of orbiting bodies similar to Pluto itself. It seems that this tiny, one-time planet isn’t so exceptional after all. It certainly isn’t considered one of a kind, like the other planets now recognized by the IAU.

This may lead some people to think that the classification “planet” is based on size but this simply isn’t true. To repeat: it’s based on movement and dynamics. Apparently, if Pluto were the only tiny object orbiting in that location, in that way, it would qualify as a planet.

One more interesting item should be addressed when we try to determine what a planet is. At one time only the celestial bodies in our solar system were considered planets. Things got a bit confusing when astronomers discovered objects orbiting those other stars. This increased the number of potential planets from eight or nine to hundreds. The IAU first addressed the problems with defining Pluto as a planet. The scientists may have to study each object now and make a similar determination. What is a planet? The question is answered, for now.