But wait there's more

Planets. What are they? How many are there? What are they plotting? These questions have plagued mankind for millennia. OK maybe not the last one, but still the quest to understand the mysteries of space and the movements of the planets has been a driving force behind scientific endeavour for thousands of years.

Solar system, music by Queensland Symphony Orchestra

https://www.youtube.com/watch?v=dkHMOW4mfgY

The first descriptions of the inner planets (Mercury and Venus) and the closest outer planets (Mars, Jupiter, Saturn) were made by ancient Babylonian astronomers around 4000 years ago. Uranus would not be discovered for another ~3800 years, in April of 1781. Neptune would follow relatively soon after in September, 1846. Finally Pluto would join the ranks of the planets following its discovery in January, 1930.

Diagram of the solar system.
http://spaceplace.nasa.gov/ice-dwarf/en/

The first explanations for the movements of the sun, moon, stars and planets throughout the night sky were developed approximately 2600 years ago with the creation of the geocentric model. There were many variations of this model throughout the ages but they all feature the Earth as the center of a revolving universe. These models were based on the observable fact that the sun, moon and stars all appear to circle the earth. An opposing theory was put forth approximately 300 years later which placed the sun at the center of the universe with all the planets, Earth included, revolving around it. This heliocentric model was disregarded, and often violently opposed by supporters of the geocentric model (particularly the Catholic Church and other religious bodies who believed, for obvious reasons, that the Earth and humanity were the center of creation) for hundreds of years. It wasn't until the reintroduction of this model in the 16th century by Nicolas Copernicus that it began to spread and overtake the old geocentric model. Eventually it was accepted that the planets, including Earth, did in fact orbit around the sun while various moons orbited their respective planets. The principle idea of the sun as the center of the universe was more easily discarded as advances in technology aided in the shift in scientific thought. Now we know that the sun is simply one star among the estimated 100-300 BILLION stars in our galaxy, the Milky Way. In turn our galaxy is only one among an estimated 225 BILLION galaxies in the observable universe. 

Monty Python's Galaxy Song from The Meaning of Life. Performed by Eric Idle

https://www.youtube.com/watch?v=buqtdpuZxvk

However, today we are going to scale back and focus solely on the planets, dwarf planets, moons, asteroids, and various other satellites of our solar system. Our story opens on Pluto. Once the proud 9th and final planet in our solar system it has since fallen on hard times.

Sad Pluto the dog from Disney
http://www.disneyclips.com/imagesnewb/pluto3.html

76 years after its discovery Pluto was officially demoted from planet to dwarf planet on September 13, 2006. While Pluto is hardly the first stellar body to be reduced in status (Solar Bodies Previously Considered Planets) it is the most controversial (because it is the most recent). While many people were unhappy with Pluto's demotion (mainly due to sentimental reasons) the International Astronomical Union (IAU) officially finalized the criteria for planethood in a resolution ratified near the end of 2006. The resolution criteria read as follows:

'The IAU...resolves that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories in the following way:

(1) A planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."'

The resolution continues by stating that according to the above definition Pluto is a "dwarf planet" and is placed into the new category, Trans-Neptunian Objects. Additionally planets and dwarf planets are two separate classes of objects despite the similarity in names, and so dwarf planets are not simply smaller planets.

Credit: by Karl Tate, SPACE.com contributor
http://www.space.com/18584-dwarf-planets-solar-system-infographic.html

This classification and ordering of the objects in our solar system is relevant because of a recent publication by Konstantin Batygin and Dr. Michael Brown. Dr. Brown was one of the leaders of the Pluto demotion movement in 2006. Now in the first 2016 issue of The Astronomical Journal, Batygin and he describe a new addition to the list of planets, the mysteriously titled Planet 9. While there is no direct evidence of this planet its existence can be inferred from the motion of several smaller objects in the Kuiper Belt. The Kuiper Belt is a ring of dwarf planets and small solar system bodies that orbit the sun at the outer edges of the solar system (similar to the Asteroid Belt between Mars and Jupiter). Specifically the researchers noted that a KBO (Kuiper Belt Object) named Sedna exhibited strange orbital characteristics. Aside from the extreme range (it takes 11,400 years to complete an orbit, 69 times longer than Neptune's ~165 years) it is one of several KBOs which share an almost identical perihelion (the point of orbit closest to the sun), both in angle and distance. Batygin & Brown calculated that the likelihood of these objects sharing the same angle and distance in their perihelion was approximately 0.007%. In light of this they determined that the most plausible explanation for this oddity was that Sedna and its contemporaries were being influenced by a large unseen object. This unknown planet has a mass ~10 times that of Earth and orbits the Sun approximately once every 20,000 years. Batygin and Brown have crowdsourced their observation team, hoping that some observatory in the world will be able to provide the visual confirmation of their, as yet, theoretical planet.

Credit: By Karl Tate, infographics artist
http://www.space.com/31671-planet-nine-discovery-explained-infographic.html

So while Pluto is out of luck with regards to planethood we may soon have a new 9th planet to take its place. Its just unfortunate that Pluto was demoted because how cool would it have been to name this new discovery Planet X! Anyway as always thanks for stopping by and be sure to come back soon!

Various Sci-Fi novels and films involving Planet X
(Clockwise from left): http://bookreviewbuzz.com/science-fiction-clouds-over-planet-x/
http://ufodigest.com/article/man-planet-x-science-fiction-movie-1951-review-links-and-comparison-real-planet-x
http://www.demonoid.pw/files/details/2478312/?load_bal=002014149902&show_files=1&page=2
http://www.sffaudio.com/review-of-brad-lansky-and-the-alien-at-planet-x/

Sources:

https://en.wikipedia.org/wiki/Pluto#IAU_classification

https://en.wikipedia.org/wiki/IAU_definition_of_planet

https://en.wikipedia.org/wiki/Planet_Nine

http://time.com/4184942/planet-9-new-pluto-solar-system/

http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22/pdf

https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons

http://www.skyandtelescope.com/astronomy-resources/how-many-galaxies/

http://www.universetoday.com/36610/how-many-galaxies-have-we-discovered/