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To be or not to be (a planet): The controversial story of Pluto, discovered on this day in 1930

Currently classified as a dwarf planet, Pluto was discovered by astronomer Clyde W Tombaugh on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona.

This is the most accurate natural colour images of Pluto taken by NASA’s New Horizons spacecraft in 2015. (Photo: Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

Today, according to the International Astronomical Union (IAU), our solar system officially has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. But between 1930 to 2006, it had nine. No, a planet did not simply disappear. Rather, scientists took away the planetary status from the ninth, most distant planet from the sun.

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Such is the story of Pluto. Long thought to be the ninth, smallest and outermost planet in our solar system, today it has been relegated to the status of a dwarf planet, much to the chagrin of a small section of astronomers.

We briefly trace the journey of Pluto, from the day of its discovery on February 18, 1930 to today, when we have a better understanding of the planet than ever before.

“Planet X”
In 1906, Percival Lowell, a wealthy businessman, started an extensive project in search of a possible ninth planet, – “Planet X.” The existence of a ninth planet had been theorised by scientists in the late 19th century while observing perturbations in the orbit of Uranus. Astronomers had speculated that some planet, other than Neptune, was behind observed disturbances in Uranus’s orbit. By 1909, Lowell and his collaborator William H Pickering suggested several possible celestial coordinates for Planet X but were unable to prove its existence.

The search for Pluto was paused for fourteen years post Lowell’s passing in 1915, resumed by a 23-year-old astronomer named Clyde W Tombaugh in 1929 at the Lowell Observatory. Tombaugh systematically imaged the night sky in pairs of photographs taken two weeks apart. Then, using 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.

After a year of searching, on February 18, 1930, Tombaugh discovered a possible moving object on his photographic plates. After confirmatory photographs were obtained, the news of the discovery was telegraphed to the Harvard Observatory.

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Interestingly, this was not the first time the still unnamed planet had been photographed. According to researchers today, there are 16 known pre-discoveries of the planet, with the oldest dating as far back as 1909. However, none of the previous astronomers were able to realise just what they had seen.

Headlines and controversies
The discovery of a new planet made headlines across the world with the Lowell Observatory receiving 1000s of letters suggesting possible names. After much deliberation, Pluto was chosen after the Roman God of the underworld. The name was suggested by Venetia Burney, a 11-year-old schoolgirl from Oxford, England. who was fascinated with classical mythology. The name deemed appropriate for presumably the darkest and coldest planet of the solar system, Venetia was given a reward of £5 for her efforts.

But while the general public was excited by the discovery of a ninth planet, the first since Neptune was discovered in 1846, doubts started being raised whether Pluto was indeed Lowell’s “Planet X”. Crucially, Pluto was tiny compared to what was originally predicted, definitely not large enough to cause the observed disturbances in Uranus’s orbit.

That along with the fact that it was more than six times dimmer than what Lowell had predicted, meant that ever since its discovery, scientists questioned Pluto’s status as a planet. Astronomer Armin O Leuschner wrote in The New York Times in April 1930 that Pluto’s dimness and high orbital eccentricity (how much it deviates from being a perfect circle) made it more similar to an asteroid or comet than a planet.

Throughout the mid 20th century, estimates of Pluto’s mass were revised downward. From being considered to be nearly the size of the Earth in 1931, by 1949 Pluto’s size was pegged to be somewhere between Mercury and Mars with its mass being roughly a tenth of that of Earth. In 1978, Pluto’s size was finally determined conclusively – it was one five-hundredth that of Earth or one-sixth that of the Moon, too small to be Lowell’s predicted “Planet X”.

While Lowell’s prediction for the orbit of Planet X was similar to Pluto’s, scientists confirmed that this was purely a coincidence.

Losing planet status
Pluto’s status as a planet really fell into jeopardy 1992 onwards, when the Kuiper Belt was discovered. A circumstellar disc in the outer solar system, the Kuiper Belt contains over 100,000 discovered small objects (diameter over 100km), all revolving around the sun beyond the orbit of Neptune. As more KBOs were discovered, Pluto’s uniqueness as a celestial object quickly diminished.

In 2005, astronomers at Caltech announced the discovery of a new trans-Neptunian object, Eris, which was substantially more massive than Pluto. While some clamoured for it to be called the tenth planet of the solar system, others saw it as the strongest argument for reclassification of Pluto.

Finally, in 2006, the IAU came with a resolution that created an official definition of a “planet”. The resolution outlined three criteria that a planet must meet in order to be so classified:\

The object must be in orbit around the sun,
The object must be big enough to be rounded by its own gravity, and
The object must have cleared the neighbourhood around its orbit.
Pluto failed to meet the third criterion. Pluto is not gravitationally dominant – its mass is substantially less than the combined mass of the other objects in its orbit (0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit, excluding the moon). For celestial objects that satisfy the first two criteria but not the third, the IAU came with the classification of “dwarf planet”.

However, this was not uniformly well received in the scientific community. Many felt that this was an arbitrary classification while others were more emotionally involved. A Bill was passed in California which called IAU’s classification of Pluto to be “scientific heresy”. Many scientists have continued to refer to Pluto as a planet till date. “We are continuing to call Pluto a planet in our papers, we are continuing to call Titan and Triton and some other moons by the term ‘planet’,” said planetary geologist Paul Byrne of North Carolina State University. “Basically, we are ignoring the IAU.”

The New Horizons flyby
On July 14, 2015, NASA’s New Horizons spacecraft made its historic flight through the Pluto system – providing the first close-up images of Pluto and its moons and collecting other data that has transformed our understanding of these mysterious worlds on the solar system’s outer frontier.

The flyby renewed the debate around Pluto’s planet status as well as gave some of the most stunning images of the planet till date, some of which went viral on social media. Regardless of the classification, Pluto remains an object of fascination among the masses.

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