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.
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 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.
Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, 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.
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.
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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 UCF, Metzger worked at NASA's Kennedy Space Center from 1985 to 2014. He earned his doctorate in physics from UCF in 2005.
With more than 66,000 students, UCF is one of the nation's largest universities, offering more than 200 degree programs at its main campus in Orlando and more than a dozen other locations in Central Florida and online.
UCF was ranked by U.S. News & World Report as among the nation's top 25 most innovative universities along with Harvard, Stanford and Duke, and has been described by The Washington Post as "part of a vanguard that is demolishing the popular belief that exclusivity is a virtue in higher education." For more information, visit ucf.edu.
Icarus