What is a planet? How would you describe a planet? What makes you a human or a dog a dog, do we have a definition or are we just what we are because we look like what we are? When ancient astronomers started looking at the sky, what they saw was a blanket of twinkling lights, a sea a stars. Some believed Earth was the center of all known as the Geocentric Model, until astronomers started noticing that some of the twinkling lights were transiting quickly across the sky. As astronomers discovered planets, some, like Galileo Galilei (though he was not the first and did not suffer as those before him for going against the church’s view of a Geocentric Model) dared to say that the the solar system was heliocentric and everything revolved about the Sun and Earth was not the center of all things.
Discoveries such as the moons around Jupiter helped prove to society that our solar system was heliocentric. However, from our perspective here on Earth things do seem to orbit with Earth at the center. The stars throughout the galaxy seem to be fixed on a spherical background as the sphere rotates slowly around Earth. For us it appears as if the stars do not move, there change in position is so small over time that they seem to be immobile in the sky. So when an early astronomer saw what appeared to be a star change position drastically over a few nights, they discovered it was planets and not distant stars. Over the years with just the naked eye planets were identified and than after the invention of the telescope more distant planets were discovered like Neptune, Uranus, and the last being Pluto.
For years we learned about our nine-planet solar system in order as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Sometimes it could be Pluto than Neptune, since Pluto has an extensively elongated orbit when it reaches its closest approach to the Sun, Pluto’s orbit actually crosses into Neptune’s making it closer to us than Neptune. It was these years when Pluto was as it closest that scientist tried to learn more about it, difficult still because even at its closets its still really small and really far away. Much of what we thought of Pluto was guesswork and when New Horizons arrived it showed us things that were surprising and unexpected.
As technology advanced so did our view of the sky, things that were far were becoming clearer. New planets were being discovered similar to Pluto, which led to the discovery of the Kuiper Belt, an incredibly enormous system of celestial bodies in the outer solar system similar to our beloved Pluto. The Kuiper Belt was hiding Pluto-like planets just waiting to be discovered, one significant discovery was Eris, an object that has been argued whether or not it was bigger or smaller than Pluto. Either way the Cal-Tech team led by Mike Brown had an exciting discovery a new planet, only it was not classified a tenth planet, it instead was dubbed a dwarf planet.
So this brings about a question if Eris and Pluto are similar than why do we call Pluto a planet and Eris a dwarf planet? The reason is simple, up to this point–the discovery of Eris in 2005–we had no formal definition of a planet. Planets were found, and named after Greek gods it seemed the only definition for a Planet was that it was a big round object orbiting the Sun. With these discoveries of new dwarf planets rivaling Pluto in size it raised a red flag, that maybe an official definition of what a planet is should be established.
The International Astronomical Union or IAU met, discussed, and set a definition for what a planet is. This was quite controversial to some because planetary scientists thought it was odd that astronomers would decide this and not them. Either way the definition was set and in order for an object to be classified a planet it has to meet these three criteria: 1) Be in orbit around the Sun; 2) Maintain hydrostatic equilibrium, which is a fancy way of saying its large enough that its gravity causes it to be a nearly-round object; 3) Clear its orbital neighborhood, meaning that it is not manipulated by another objects gravity other than the Sun, and other objects are not in its orbital path. Every object has gravity and all the objects in space have some sort of small effect on one another. Some of these effects though are so minute, they are practically non-existent. We know the Moon has its effect on Earth, it stabilizes the Earth’s spin and causes tides in her oceans. However, the Moon does not have an effect on the Earth’s orbital path and the Earth keeps the Moon tidally locked in an orbit about her. So the Earth meets all three criteria as does all the others–minus Pluto.
Unfortunately, Pluto does not meet criteria number three. A big part of this reason is the discovery of the Kuiper Belt, Pluto moves in an orbital path with all these objects. Pluto also has an interesting relationship with its moon Charon, as the two act as a binary system–meaning one does not orbit the other, but they orbit around a fulcrum or a fixed point between each other. If you were standing on Pluto looking a Charon you would always see the same face, and if you were standing on Charon you would always see Pluto’s same face. They are locked onto each other that in theory you could build a bridge between the two.
With the clarification of what a planet is, Pluto was reclassified a dwarf planet. Of course, there was a mass hysteria uproar over it, but, when you look at the facts and examine the evidence, it truly makes sense to define a planet as such. Currently, there has been five objects classified as dwarf planets including Pluto and they are all smaller than our Moon.
Featured Image Courtesy of: https://www.mirror.co.uk/science/gallery/stunning-pictures-planets-jupiter-mars-12626611. From top left to bottom right; Mars, Uranus, Earth, Mercury, Neptune, and Venus.
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