Tatarstan Academy of Sciences

Pluto was discovered in the ring of bodies beyond the orbit of Neptune, known as the Kuiper belt, back in 1930, being declared the ninth planet in the Solar System.

However, its status was questioned after several other objects of the same size were found in the Kuiper belt, with the International Astronomical Union (IAU) eventually downgrading Pluto to “dwarf planet” in 2006.

It happened in line with the new definition of a “planet” adopted by the IAU, which stated that a celestial body has to orbit the Sun, have a nearly round shape and be gravitationally dominant, clearing its own orbit, to meet the criteria.

Pluto ended up being disqualified because its orbit intersects with that of Neptune and because it shares its orbital neighborhood with other objects in the Kuiper belt.

The rule demanding a planet to clear its own orbit “was really developed post facto to keep an orderly, small number of planets,” Philip Metzger, from the Florida Space Institute at the University of Central Florida (UCF), said.

And the very idea that there should be a limited number of planets has little to do with science, stemming from folklore and astrology, he pointed out.

In order to prove that point, Metzger and his team studied a massive bulk of planetary literature from the past 400 years, sharing the results of five years of their work in the Icarus astronomical magazine.

According to the paper, entitled ‘Moons are planets,’ the definition introduced by Galileo in the 1600s – that a planet only needed be a geologically active body in space – had been used by scientists throughout much of history and only eroded in the 20th century.

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