In 2006, the International Astronomical Union (IAU) downgraded Pluto’s status to “dwarf planet.”
The IAU defines a planet as a celestial body that “has cleared the neighborhood around its orbit,” which means that it must be the largest gravitational force in its orbit.
However, Neptune's gravity influences its neighbor Pluto. The dwarf planet also shares its orbit with frozen gases and objects in the Kuiper belt, researchers note.
Metzger, lead author of the study, reviewed scientific literature from the past 200 years and found only one publication, from 1802, that employed the clearing-orbit requirement to classify planets. The 1802 study, he argues, was based on “since-disproven reasoning.”
Set against this backdrop, he believes that the IAU definition of what constitutes a planet needs to be rethought. "It's a sloppy definition," he said in a statement. "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."
Moons such as Saturn's Titan and Jupiter's Europa have been routinely called planets by planetary scientists since the time of Galileo, according to Metzger. "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 added.
The definition of a planet should be based on its intrinsic properties, the scientist said, as opposed to properties that can change, such as the dynamics of its orbit.
"Dynamics are not constant, they are constantly changing," he explained. "So, they are not the fundamental description of a body, they are just the occupation of a body at a current era."
Instead, Metzger recommends a planet classification based on whether it is sufficiently large so that its gravity allows it to become spherical in shape.
"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,” he said.
Pluto, he notes, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons.
The study was co-authored by Mark Sykes of the Planetary Science Institute; Alan Stern, of the Southwest Research Institute; and Kirby Runyon of Johns Hopkins University Applied Physics Laboratory.
The IAU told Fox News that it has not yet received a formal proposal to change its definition of a planet. “There is a very clear, and known, way to table motions in the IAU, which is to propose an IAU Resolution through the relevant Working Group(s) and Division. So far, no such Resolutions have been proposed,” a spokesman said, via email. “It is nevertheless good and healthy to debate these topics.”