My Very Eager Mother Just Served Us Nine Pizzas” – If you went to an elementary high school before 2006, you probably had to memorize this mnemonic to learn the order of the nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, And Neptune. But in 2006, everything changed. Exactly 14 years ago on this day, Pluto was removed from the list of our nine planets. But what happened to the smallest member of our planetary family? If it’s still up there, orbiting the Sun, why did we drop it from our list? For that, let us go back in time.

The Four New ‘Planets’

The problem started long before the discovery of Pluto. In the era of Kepler and Galileo, any object orbiting the Sun was considered to be a planet. Things went smoothly. We knew of 7 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus. In 1801, astronomers discovered Ceres. This rocky sphere orbited the Sun between Mars and Jupiter. Ceres was predicted to exist in the year 1776 because astronomers noticed a huge gap between the orbits of Mars and Jupiter. The mathematics tells us that there should be another planet between them. When Ceres was discovered, it was labeled as a planet – a new entry in the list of planets.

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Next year, in 1802, astronomers discovered another large object orbiting the Sun – Pallas. In 1804, the third object was discovered – Juno and then in 1807 – Vesta was discovered. All the four objects lie between Mars and Jupiter. For about 50 years, these four objects were considered as planets. However, in the 1850s, after thousands of such objects were discovered, they were classified as asteroids. One thing common between all these objects was that they were barren and rocky. Some of them did not even have a spherical shape. After this declassification, our list was back to the original 7 planets. In 1846, Neptune was the eighth entry in the list.

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The Discovery of Pluto

Pluto was discovered on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering. This period in astronomy was one of intense planet hunting, and Pickering was a prolific planet predictor.

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Lowell Observatory, Pluto Dome, Flagstaff, AZ. Historic American Buildings Survey (Library of Congress).

In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X.” By 1909, Lowell and Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted the search until his death in 1916, to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but they were not recognized for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909.

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Clyde Tombaugh (Image: NASA)

The search for Planet X did not resume until 1929 when the job was handed to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory.  Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs.

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On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The discovery made headlines across the globe.

Pluto Over The Years

When Pluto was discovered, it was thought to be as big as Uranus and Neptune. In 1931, its size was revised down to that of the Earth. Its size kept shrinking after new revisions in 1948, 1958, 1993, 1994, and 1995. Finally, astronomers made an accurate measurement in the year 2006. Today we know Pluto is even smaller than our Moon and only twice the size of the formerly discovered Ceres. In 1978, astronomers discovered Pluto’s moon, Charon. At first, it seemed to be strong evidence in favor of Pluto as a planet – if it is big enough to have a natural satellite, it should be a planet. But Charon was half as big as Pluto! In fact, due to Charon’s gravitation pull, Pluto’s orbit wobbles considerably.

These facts raised some eyebrows on the planetary status conferred on Pluto. Things would get worse every time a new Pluto like object was discovered beyond Neptune’s orbit.

Enough, Pluto!

Though questions were being raised on the status of Pluto as a planet, it remained on our list of nine planets for decades. However, the final strike against Pluto was the discovery of Eris in 2005. Though it is a bit smaller than Pluto, initial observations placed it somewhere more massive than the ninth planet. Following this discovery, In 2006, the International Astronomical Union revised the definition of a planet. According to the new criteria, for an object to be considered as a full-sized planet, it should:

  1. be spherical
  2. orbit the Sun
  3. have cleared its neighborhood

Pluto fulfils the first two conditions but runs into trouble in the third one. Clearing the neighborhood implies that there should be no objects around it, other than its natural satellites.

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Reaction To This Decision

Not everyone was happy with the demotion of Pluto. Gail Oxton, who built the software for New Horizons (the first spacecraft to visit Pluto) and has been on the mission since 2002, says she and her colleagues were “devastated” when Pluto got demoted. “And it still stings a little, especially when we hear yet another joke about poor Pluto. While there are other exciting objects in the solar system, this was the last unexplored planet, those things everyone had to memorize when they were in grade school. We were making history. When Pluto was demoted 7 months after launch, it immediately robbed us of some of that excitement.” – she said.

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Pluto captured by the New Horizons space probe

NASA’s New Horizons spacecraft was launched on January 19, 2006, with the objective of becoming the first spacecraft to explore Pluto and its moons up close.

While Pluto was still a planet during the launch, it was soon demoted to a dwarf planet.

NASA, however, went ahead with its mission objective, studying the dwarf planet Pluto, its moons, and other objects in the Kuiper Belt.

The Current Status of Pluto

Currently, Pluto is a dwarf planet. A “dwarf planet,” as defined by the IAU, is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus ellipsoid in shape), but has not cleared its neighboring region of other objects. The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets (the big eight), dwarf planets and satellite planets.

Pluto could not even complete a full revolution around the Sun since its discovery. It takes 248 years to orbit the Sun. All we can say is, “Revolve in Peace, Pluto”.

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