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Pluto a Planet? New Research from UCF Suggests Yes
The reason Pluto lost its planet status is not valid, according to new research from the University of Central Florida in Orlando. In 2006, the International Astronomical Union, 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. [...] [Philip] Metzger, who is lead author on the study, 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.
[...] 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.
[...] Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape. "And that's not just an arbitrary definition, Metzger said. "It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body." 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," Metzger said. "The only planet that has more complex geology is the Earth."
Planet Ceres, please.
The Reclassification of Asteroids from Planets to Non-Planets (DOI: 10.1016/j.icarus.2018.08.026) (DX)
Related: Pluto May Regain Status as Planet
Earth is a "Dwarf Planet" Because it has not Cleared its Orbit
(Score: 4, Informative) by toddestan on Monday September 10 2018, @02:16AM
Actually, most people seem to be arguing for the hydrostatic equilibrium definition, which is a consistent, clear-cut, and a very scientific definition of what is and is not a planet. And yes, we all know that means there are actually far more than 9 planets by that definition - I don't see anyone here arguing that that "planet" should include Pluto but not objects like Ceres.
It's the people who have decided that there should only be eight planets in the solar system that are dancing around trying to create a definition that includes those eight objects and excludes all other objects. Of course, you can work backwards to create some arbitrary definition to do so for that works for the solar system, but you'll more than likely run into problems with other solar systems where things may not be so clear-cut. I mean, those objects orbiting other stars that Kepler keeps finding we have been calling planets, but that's a bit presumptuous because we can't actually know if they really are planets because we have no way to know if they have actually "cleared" their orbits - which just shows how poorly thought out the IAU definition is. But using hydrostatic equilibrium as the definition of a planet can be applied consistently everywhere, and with that definition there's no question those objects are planets. Of course, the IAU gets around that problem by defining a planet as something that only orbits our Sun, which is also ridiculous.