We has eight planets instead of nine.

We all remember learning about the nine planets throughout our school years.

Some of us older individuals also remember when Pluto was reclassified as a dwarf planet by the decision of the International Astronomers Union (IAU). It is now taught in schools that our Solar System has eight planets instead of nine. Some people welcomed the reclassification as a display of what was possible with our  advancements in science and technology. Others were not so welcoming and believed that the reclassification was unfair and unjustified. Owen Gingerich, an astronomer from Harvard University, said “I think the IAU voted primarily on scientific grounds and were not sensitive to the historical and cultural role Pluto has played” (Inman, 2006).

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In 1929, Vesto Slipher instructed Clyde Tombaugh, of the Lowell Observatory, to search for Planet X. Planet X was a planet believed to exist by Percival Lowell. Tombaugh’s job was to take paired photos of the night sky. He would use a blink comparator to flip between the two images taken. This would create the “illusion of movement”of any object that had changed its relative position. On February 18, 1930, Pluto was discovered as the ninth planet from the Sun. Many people submitted suggestions for what to call this newly discovered planet.

Venetia Burney, an eleven-year-old girl from Oxford, England, was interested in classical mythology. She suggested they name the planet after Pluto the god of the underworld. (Pluto, 2018) The staff at Lowell Observatory chose that name because the planet’s sign would become PL which would also stand for the initials of Perceval Lowell. Walt Disney named his new character Pluto although it is unsure if there is a connection to the planet.

Keeping with the tradition of naming new elements after planets, Plutonium was named after Pluto (Llewellyn, Wong, & Ortega, 2015, p. 2). Pluto’s strange orbit takes it on a 248 Earth-year journey around the Sun (Why is Pluto no longer a planet?, 2017). One AU (astronomical unit) is defined as the average distance between the Earth and the Sun.

Pluto’s orbit ranges from 29.658 AU to 49.305 AU; this is about 4 billion miles from the Sun.

Pluto is about one-third the size of Earth’s moon and is made primarily of ice and rock (Pluto, 2018). It has an estimated temperature of about -330 degrees F (Llewellyn, Wong, ; Ortega, 2015, p. 2). It has five moons of its own: Charon, Styx, Nix, Kerberos, and Hydra. “Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body” (Pluto, 2018). In 1992 objects began to be discovered that were similar in size to Pluto. They were discovered in an area of comets and other bodies beyond Neptune called the Kuiper Belt. It is similar to the Asteroid Belt between Mars and Jupiter.

This caused Pluto’s planet status to be questioned. As larger and larger objects were discovered many people thought Pluto should be reclassified as a Kuiper Belt object. In 2005 Eris was discovered and was the biggest object to be discovered since Triton in1846 (Pluto, 2018). In 2006 at the 26th General Assembly the IAU defined the term ‘planet’. There are three prerequisites for being classified as a planet. First, a body must orbit the Sun.

Second, it must be massive enough to be pulled by its gravity into a round shape. Third, a body must have cleared the neighborhood of its orbit (Pluto, 2018). This last condition means that a planet sweeps out comets and debris in its orbit. These conditions established by the IAU were originally meant to apply to the Universe but were later changed to apply only to our Solar System (Inman, 2006). Pluto does orbit the Sun. It also is massive enough to have been pulled into a round shape by its gravity.

However, it does not meet the third requirement for planet status. Pluto’s mass is only .07 times the total mass of other objects in its orbit. In contrast, Earth is 1.

7 million times the total mass of other objects in its orbit (Pluto, 2018). The third requirement implies that a planet must be gravitationally dominant. There can be no other bodies in its orbit other than its moons and other bodies that are under the planets gravitational influence (Why Is Pluto No Longer a Planet, n.d.). Another observation that does not directly contradict the three requirements is that Charon is half the size of Pluto.

Other planets are much larger than their moons (Inman, 2006). “The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criteria 3 would be called dwarf planets” (Pluto, 2018). Pluto is now classified as a dwarf planet in the Kuiper Belt. It also has the distinction of being the first Kuiper Belt object ever discovered (Pluto, 2018). As of 2006, over 44 dwarf planets had been classified. Hundreds more since then and hundreds more going forward will be discovered because it is such a large category (Inman, 2006). In January 2006, before the reclassification, NASA launched the New Horizons spacecraft. It’s destination was Pluto, over 4.

5 light years from Earth. “Solar panels are no use at this distance, so New Horizons runs on heat from the radioactive decay of a lump of plutonium” (Battersby, 2015). It travelled at 37,000 miles per hour and took over 9 years to reach Pluto. New Horizons will study Pluto’s atmosphere, surface structure, and its five orbiting moons. Clyde Tomdaugh’s ashes were on the New Horizons spacecraft as they flew by the planet he discovered 85 before (Llewellyn, Wong, & Ortega, 2015, p. 3). Like all scientific discoveries, Pluto is subject to the guidelines that scientists create to classify things. These guidelines were clarified  after new discoveries had been made and the term ‘planet’ was more strictly defined.

“Science is a human endeavor in which classification of our natural world and interpretation of evidence can result in the coexistence of opposing, het valid, points of view” (Llewellyn, Wong, & Ortega, 2015, p. 3). We will continue to tell our children and grandchildren of the days when there were nine planets but we must also teach them that all scientific theories and discoveries are subject to re-evaluation in light of new information. “For the 76 years between Pluto being discovered and the time it was reclassified as a dwarf planet it completed under a third of its orbit around the Sun” (Pluto Facts – Interesting Facts about Dwarf Planet Pluto, 2017).


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