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Pluto Should Be Reclassified as a Planet, Experts Say (sciencedaily.com)
The reason Pluto lost its planet status is not valid, according to new research from the University of Central Florida in Orlando. From a report: 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. Since Neptune's gravity influences its neighboring 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 new study published online Wednesday in the journal Icarus, UCF planetary scientist Philip Metzger, who is with the university's Florida Space Institute, reported that this standard for classifying planets is not supported in the research literature. 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. He said 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," Metzger said. "And it would leave out the second-most complex, interesting planet in our solar system." "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. "It's a sloppy definition," Metzger said of the IAU's definition. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."
"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," Metzger said. "And it would leave out the second-most complex, interesting planet in our solar system." "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. "It's a sloppy definition," Metzger said of the IAU's definition. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."
Jerry Smith.
The redefinition of the phrase presented in the summary is silly. “Clearing its orbit” means just what it says. But then Neptune also fails that test, since it hasn’t “cleared its orbit” of Pluto - and therein lies the problem.
If scientists had meant a planet should “be the largest gravitational force in its orbit”, they would have said exactly that. The phrasing is clear, concise, and unambiguous.
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Additionally, its mass should be enough to form a spheroid. Pluto looks like a goddam potato.
You may want to check that again before you say it around anyone in person. Pluto is nicely spherical. Perhaps you're thinking of Vesta?
Its not about emotions, it's about useful terminology. To a planetary scientist, a planet is a body that's large enough to be in hydrostatic equilibrium, which creates a fundamental distinction between two very different types of bodies. A body in equilibrium is differentiated, has been altered by fluids, has experienced geothermal heat over long time periods, etc. A body not in equlibrium is undifferentiated and - if old - comprised of the primitive relics of our solar system. To a planetary scientist, the large moons are likewise "planetary" moons, in that they're gone through the same sort of differentiating processes as planets; their position and path of motion does not change what they are.
The IAU, which is primarily astronomers and not planetary scientists, made a big mess for actual planetary scientists. Rather than creating their own new orbital classification for their needs, they took away a term in widespread use from another set of scientists. The latter have been the leading voices for overturning the IAU decision since then.
The fact that the planetary science definition is much closer to the popular definition than the IAU definition is is actually a side point. Although one worth bringing up nonetheless. New Horizons lead Alan Stern likes to bring up the "Captain Kirk Test", when discussing the popular usage of the term, as distinct from either scientific usage of the term. That is, if the Starship Enterprise was in orbit around it, and Captain Kirk said "Beam us down to that (blank)", would that word be "planet", or something else? As humans, we automatically recognize "object in space so large that its gravity has pulled it into a sphere" as a planet. To the point that we sometimes struggle when discussing science fiction when such bodies are presented as moons. Think of how many times you've heard Star Wars fans refer to the Forest Moon of Endor as a planet or whatnot.
Of course, all *this* is tangential to the fact that the IAU definition is a completely self contradictory minefield based on false premises (foremost of which is that planets actually clear their own neighborhoods - Mars's neighborhood, for example, was primarily cleaned by Jupiter, not Mars***), but that's an entirely different story....
*** No, the Stern-Levison parameter doesn't help. It's based on a current body's ability to clear asteroids, not a planetisimal's ability to clear other planetisimals. And indeed, Stern is as mentioned one of the biggest opponents of the IAU definition. Basing your argument on a guy who disagrees with it is never a good start!
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