The reason Pluto lost its planet status is not valid, according to new research from the 麻豆原创.
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鈥檚 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, 麻豆原创 planetary scientist Philip Metzger, who is with the university鈥檚 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 鈥� from 1802 鈥� that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.
He said moons such as Saturn鈥檚 Titan and Jupiter鈥檚 Europa have been routinely called planets by planetary scientists since the time of Galileo.
“[Pluto is] more dynamic and alive than Mars. The only planet that has more complex geology is the Earth.鈥�
鈥淭he 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,鈥� Metzger says. 鈥淎nd it would leave out the second-most complex, interesting planet in our solar system.鈥�
鈥淲e 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鈥檚 functionally useful,鈥� he says.
鈥淚t鈥檚 a sloppy definition,鈥� Metzger says of the IAU鈥檚 definition. 鈥淭hey didn鈥檛 say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.鈥�
The planetary scientist says 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 says.
Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, says the IAU鈥檚 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.
鈥淲e showed that this is a false historical claim,鈥� Runyon says. “It is therefore fallacious to apply the same reasoning to Pluto.”
Defining “Planet”
Metzger says 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鈥檚 orbit.
鈥淒ynamics are not constant, they are constantly changing,鈥� Metzger says. 鈥淪o, they are not the fundamental description of a body, they are just the occupation of a body at a current era.鈥�
Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.
鈥淎nd that鈥檚 not just an arbitrary definition,” Metzger says. 鈥淚t 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.鈥�
Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he says.
鈥淚t鈥檚 more dynamic and alive than Mars,鈥� Metzger says. 鈥淭he only planet that has more complex geology is the Earth.鈥�
Co-authors on the research included Mark Sykes, of the Planetary Science Institute; Alan Stern, of the Southwest Research Institute; and Runyon of Johns Hopkins University Applied Physics Laboratory.
Before joining 麻豆原创, Metzger worked at NASA鈥檚 Kennedy Space Center from 1985 to 2014. He earned both his master’s (2000) and doctoral (2005) degrees in physics from 麻豆原创.
