Pluto wrongly lost its planet status, say scientists, suggesting that the icy dwarf should be reclassified as a planetary body.
In 2006, the International Astronomical Union (IAU), a global group of astronomy experts, established a definition of a planet that required it to "clear" its orbit, or in other words, be the largest gravitational force in its orbit.
Since Neptune's gravity influences its neighbouring planet Pluto, and Pluto shares its orbit with frozen gases and objects in the Kuiper belt, that meant Pluto was out of planet status.
However, in a study published in the journal Icarus, researchers reported that this standard for classifying planets is not supported in the research literature.
They reviewed scientific literature from the past 200 years and found only one publication -- from 1802 -- that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.
Moons such as Saturn's Titan and Jupiter's Europa have been routinely called planets by planetary scientists since the time of Galileo.
"The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research," said Philip Metzger, planetary scientist from the University of Central Florida in the US.
"And it would leave out the second-most complex, interesting planet in our solar system," said Metzger.
"We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it's functionally useful," he said.
The planetary scientist said that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.
However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger said.
The IAU's definition was erroneous since the literature review showed that clearing orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets, said Kirby Runyon, from the Johns Hopkins University Applied Physics Laboratory in the US.
"We showed that this is a false historical claim. It is therefore fallacious to apply the same reasoning to Pluto," Runyon said.
Metzger said that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet's orbit.
Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.
Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he said.
"It's more dynamic and alive than Mars. The only planet that has more complex geology is the Earth," said Metzger.
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