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A Changing Understanding of The Solar System and Why Pluto is Not a Planet
The Solar System of Ancient Greece
The truncated etymology of the word, "planet," (planete in middle English) derives from the Latin word planeta which derives from the Greek word planetai meaning literally, "wandering star."
In BCE Greece, before the invention of sophisticated telescopes and the endeavors of careful observation and mathematic computations in regard to the heavenly bodies, the only bodies that appeared to the lay observed to wander the sky, and indeed the only bodies that wandered in a non-uniform proximal relationship were Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. It was to these seven bodies that the label, "planet," was originally applied. (They also incidentally formed a matrix for the seven days of the week.)
This view of our Solar System persisted until Copernicus.
The Copernican Model of Heliocentricty
The Ptolemaic System, or Geocentricity, persisted until 1543 when Nickolai Copernicus published his, "De Revolutionibus Orbium Coelestium." Despite the Theological opposition to this theory it eventually gained traction through the Empirical consensus provided by Galileo, Kepler, and later others.
This new model, which placed the Sun at the center of the Solar System and the Earth as an orbiting body, required the subtraction of the sun and the addition of the Earth as Planets. The calculations also revealed that the Moon indeed was only in orbit around the Earth and thus it was likewise removed from the list of, "planets." The Solar System planetary model as we knew it now consisted of Mercury, Venus, Earth, Mars, Jupiter, and Saturn.
Uranus and Neptune
John Flamsteed first observed Uranus in 1690. Pierre Lemonier likewise observed it a number of times in the mid 1750's. it was reported as a comet by William Herschel in 1781. The Astronomer Royal and Herschel himself remained unsure of how to classify the observed body but they did observe that it lacked the customary, "tail," seen in other comets.
Further observation of the body's rotation by Anders Lexell and Johan Bode led them to conclude that it was indeed "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn." By 1783 the consensus that a new planet had been discover was accepted.
Hershel originally named the new planet, "Georgium Sidus," (George's Star) in a genuflection to his patron the King of England. The unpopularity of this name outside of Britain, coupled with the tradition of naming planets after the Roman equivalents of Greek gods later caused the name to be changed to Uranus after the Roman god of the sky.
Irregularities in Uranus's Orbit, not accountable for by Newtonian Laws, led Astronomers Le Verrier in France and Adams in England to postulate another planet beyond Uranus. This planet was first seen and recognized for what it was in 1846 by Johann Galle and Heinrich d'Arrest in Berlin.
Le Verrier, presented a memoir to the Academie des Sciences in 1845 predicting the position of Neptune within one degree. These calculations done independently verified the unpublished predictions of Adams. A later Memoir describing the orbit and mass of Neptune by Le Verrier was sent directly to Galle. And so just after midnight on September 24th 1846 with the help of Le Verrier's calculations Neptune was discovered within an hour of searching.
This important discovery was a vindication of Newtonian laws of Motion and thus an important moment in 19th century science.
Though not visible without the aid of a telescope, a scouring of Galileo's records seems to suggests that he observed this Planet beyond Uranus in 1613. After re-reviewing a number of Astronomical records from 1781, 1795, and 1847 by various European Astronomers the confirmation that Neptune sightings had been made numerous time and simply not recognized for what they were was confirmed.
With the addition of Uranus and Neptune the solar System now had eight observed Planets.
The Asteroid Belt
In the large gap between Mars and Jupiter, during the first half of the 19th century, 5 planets were discovered and named. First Ceres, then Vesta, Pallus, and later Juno (between Vesta and Pallus) and Astraea finally in 1845. These were all included in School texts as planets. The increasing discovery of more and more bodies in this region began to make the assumption that they were planets untenable.
Hershel's words upon initially discovering Ceres and Pallus were, "Neither the appellation of planets, nor that of comets, can with any propriety of language be given to these two stars." The term, "Asteroid belt" came into popular use in the 1850s and with over a hundred such bodies having been discovered by 1868 (over 100,000 to date) these bodies previously labeled, "planets," were relegated to the status of Asteroids.
From the same observations of the gravitational irregularities observed in Uranus's orbit which led Astronomers to speculate that the body Neptune existed and was responsible for these irregularities, further calculations following the discovery of Neptune showed that Neptune alone was not entirely responsible for these orbital aberrations in Uranus. This led to a hypothesized, "planet X," which was purportedly discovered in 1930 by The Lowell Observatory in Arizona. This body was named Pluto after the suggestion of an 11-year-old English girl for the God of the Underworld.
When Calculations reveled that Pluto, with it's very small mass, could not be the imagined "planet X," the idea of planet X's existence fall out of favor for hypotheses involving the effect of Pluto's atypical orbit and the collective effect of other bodies in the Kuiper Belt as a means to account for Uranus's rotational anomalies.
The Downgrading of Pluto
With the discovery of several other large icy-bodies, past Neptune, in an area called the Kuiper Belt, Pluto was demoted in 2006 to a Dwarf Planet or a Plutoid, a term encompassing all the icy, rocky small orbital bodies beyond Neptune that do not meet the defining characteristic of a planet as established by the International Astronomical Union in 2006.
The definition for qualification as a planet were set out (for the first time since the Greeks) as follows;
1. They must have Hydrostatic Equilibrium (a near spherical shape)
2. They must be the primary object in orbit around the Sun (thus disqualifying moons which are more strongly effected by the gravitational field of the planet they orbit than by the gravitational field of the Sun)
3. They must have cleared their orbit of debris. (meaning that the body is gravitationally dominant and has causes smaller objects to accrete with it, be pushed into another orbit, or caused them to fall into resonant orbit)
Pluto, like other Plutoids, does not satisfy this third condition and is consequently now classified as a Dwarf Planet, leaving us with a current Solar System model including 8 planets. The four small dense solid planets, Mercury, Venus, Earth, and Mars and the four gaseous giants, Jupiter, Saturn, Uranus, and Neptune.
As for the future, we'll have to wait and see what is in store for our Solar System. Many details about Pluto will become known in 2015 when the, "New Horizons," spacecraft arrives there collects pictures and data as it passes by and sends information back to Earth. When we consider the revisions made based on increased discovery and understanding just concerning our solar system within the last 150 years, it truly makes the future of space exploration an intellectually exciting prospect.