Astronomers Reveal the Reason Pluto Lost Its Planet Status is Not Valid

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In 2006, the International Astronomical Union (IAU) 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 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 paper published August 29 in the journal Icarus, Florida Space Institute researcher Philip Metzger and co-authors reported that this standard for classifying planets is not supported in the scientific literature.

This Pluto mosaic was made from New Horizons LORRI images taken on July 14, 2015, from a distance of 49,700 miles (80,000 km). This view is projected from a point 1,118 miles (1,800 km) above Pluto’s equator, looking northeast over the dark, cratered Cthulhu Regio toward the bright, smooth expanse of icy plains called Sputnik Planum. Pluto’s North Pole is off the image to the left. This mosaic was produced with panchromatic images from the New Horizons LORRI camera; with color overlaid from the Ralph color mapper onboard New Horizons. Image credit: S.A. Stern et al.

Dr. Metzger and his colleagues from the Planetary Science Institute, the Southwest Research Institute and the Johns Hopkins University Applied Physics Laboratory reviewed the scientific literature from 1801 to the present.

They found that William Herschel’s 1802 paper is the only case in the literature that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

The researchers found 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.

Dr. Metzger said, “Moons such as Titan and 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. 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.”

However, this reason is no longer considered a factor that determines if a celestial body is a planet.
“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. We showed that this is a false historical claim. It is therefore fallacious to apply the same reasoning to Pluto.” said Dr. Kirby Runyon, from the Johns Hopkins University Applied Physics Laboratory.

According to the team, 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.


Dr. Metzger said, “Dynamics are not constant, they are constantly changing. So, they are not the fundamental description of a body, they are just the occupation of a body at a current era. We recommend 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. 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 added. “It’s more dynamic and alive than Mars. The only planet that has more complex geology is the Earth.”
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