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20 January 2016

The Long and Winding Road to Pluto

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Ten years ago —on 19 January 2006—, the New Horizons probe took off towards the edge of the Solar System. Its main mission was to study Pluto, which was still considered a planet at that time. But that same year, it was demoted to the dwarf planet category, in the latest twist in a story that began as the obsession of an astronomy loving businessman.

Sunset on Pluto, taken by the New Horizons probe. Credit: NASA / Johns Hopkins

In 1906, American entrepreneur Percival Lowell launched an ambitious project to discover a possible ninth planet —tentatively naming it “Planet X”—, which according to some astronomers would have to exist in order to explain certain disturbances in the orbit of Uranus. Similar speculation had led to the discovery of Neptune, the eighth most distant planet from the Sun, so Lowell wanted to repeat the formula by making a huge effort and providing the necessary means —i.e. his own astronomical observatory— to hunt for “Planet X”, which he said ought to be a large planet (equivalent to as much as seven times the size of the Earth).

Although he died in 1916 without fulfilling his dream, the Solar System’s ninth planet was indeed finally discovered in his own Lowell Observatory (Arizona, USA) on 18 February 1930. This was achieved by Clyde Tombaugh, a 24 year-old astronomy apprentice who was given —as soon as he started working at the observatory— the task of looking for “a needle in an astronomical haystack”. He had to compare pairs of photographs of the night sky taken on different days in order to see if any light spots shifted their position, which is actually what planets do. After nearly a year comparing photographs with a blink comparator, Tombaugh finally found a planet contender.

“The arrows mark the different position of Pluto in both images”. Credit: Lowell Observatory Archives

Following Lowell and Tombaugh, the third person to become a protagonist of this story was 11-year-old Venetia Burney. The news of the discovery circled the globe and Venetia, a passionate follower of classical mythology stories, came up with the idea of naming it after the Roman god of the underworld, Pluto. Venetia’s grandfather, a former librarian at the University of Oxford, suggested the name to an astronomy professor, who in turn sent it to his American counterparts. In the end, it reached the Lowell Observatory, which had the right to name the new planet. In the end the observatory received more than 1,000 proposals from many countries. In the final vote though, Pluto won after defeating —by unanimous decision— two other gods (Minerva and Chronos).

The name soon permeated popular culture. That same year, Walt Disney created a new character to accompany Mickey Mouse: the dog Pluto, although he never confirmed it was a tribute to the new planet. But what was indeed a tribute, was the name of a new radioactive element discovered in 1941: plutonium (which followed the tradition of uranium and neptunium).

A history of astronomical denials

Size comparison of Earth, Moon and Pluto. Credit: NASA

With such popularity, the fact that Pluto had disappointed in astronomical terms went unnoticed for the general public, bearing in mind Lowell’s predictions on the supposedly large “Planet X” altering the orbit of Uranus. In 1931, the first real calculations estimated that its size was similar to that of the Earth. Yet every new calculation gradually made Pluto smaller and smaller until 1978, when it became known that the Earth was 650 times heavier than the new planet. Pluto was too small to disrupt Uranus’s orbit: if Lowell’s “Planet X” existed at all, it was surely not Pluto.

In 1979, Pluto ceased to be the ninth planet. It became the eighth because its orbit crosses Neptune’s and for 20 years (until 1999) its position was closer to the sun than that of Neptune. It was around that time that Pluto’s planetary status began to be questioned, since in 1992 similar objects in the Kuiper belt were discovered.

Pluto’s surface has an extraordinary variety of colors. Credit: NASA

To settle the argument, the International Astronomical Union finally decided to define the term planet, which had no formal definition previously. An agreement was reached on August 24 2006 and the definition left Pluto out. Hence, it became a “dwarf planet” six months after the New Horizons spacecraft had set out in search for this strange world of ice and rock. Six times smaller than our moon, tiny Pluto has his own five moons and does nos cease to amaze us. When the NASA probe finally reached Pluto on 14 July 2015, it showed us some puzzling red regions and what appeared to be large active ice volcanoes.

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