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.
It happened between the 1910s and 1950s, when the decline in the number of papers on planetary science coincided with the rise of publications such as almanacs, Metzger told UCF Today.
“There were enough almanacs being sold in England and in the US that every household could get one copy every year.”
Those almanacs provided its readers with a whole range of information – from calendars of astronomical events to cooking recipes and fiction. But there was also a heavy emphasis on astrology, including astrological weather forecasts that can only be made if there was a limited number of planets.
“This was a key period in history, when the public accepted that the Earth orbits the Sun instead of the other way around, and they combined this great scientific insight with a definition of planets that came from astrology,” Metzger said.
And those views that moons and satellites shouldn’t be considered planets then made their way into scientific literature. But this definition doesn’t work anymore as astronomy relies on advanced technology that allows it to study space much more thoroughly, he added.
“There’s an explosion in the number of exoplanets that we’ve discovered over the last 10 years, and that’s only going to increase as we put better telescopes in space.”
“We need to fix this [the definition of a ‘planet’] now before we get too far in this revolution with exoplanets. We want to be doing excellent science because this great influx of data is making it vastly more important to define our new discoveries correctly.”
Metzger and his colleagues are pushing for the return to Galileo’s definition, and if their call is heard Pluto will become a planet again, with numerous other celestial bodies joining it.
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