More than a decade after it was demoted, the debate over whether Pluto is a planet has been reignited by a new study.
New research from the University of Central Florida in Orlando claims the reason Pluto lost its planet status is "not valid".
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, the Daily Mailreports.
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, the new 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.
"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," said UCF planetary scientist Philip Metzger, who is with the university's Florida Space Institute.
"And it would leave out the second-most complex, interesting planet in our solar system."
Metzger said moons such as Saturn's Titan and Jupiter's Europa have been routinely called planets by planetary scientists since the time of Galileo.
"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."
"It's a sloppy 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."
According to Metzger, 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, he believes.
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.
"We showed that this is a false historical claim," Runyon said.
"It is therefore fallacious to apply the same reasoning to Pluto," he said.
Metzger said 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's orbit.
"Dynamics are not constant, they are constantly changing," Metzger said.
"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 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," Metzger said.
"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 said.
"It's more dynamic and alive than Mars," Metzger said.
"The only planet that has more complex geology is the Earth."
Last year astronomers proposed a new way to define planets based on "the physics of the world itself," citing technical flaws in the definition adopted by the International Astronomical Union in 2006 as the reason for the possible overhaul.
If accepted, the geophysical definition would essentially classify all "round objects in space that are smaller than stars" as planets, including Pluto, other dwarf planets, and even moons.
Scientists from NASA's New Horizon's mission will make their proposal at the Lunar and planetary Science Conference in March.
The team argues that the IAU definition is flawed in several ways, including that it only recognises as planets those which orbit our sun.
This leaves out objects orbiting other stars or those orbiting freely through the galaxy.
Along with this, they say there are parameters which even the planets in our solar system cannot satisfy.
The new definition, they argue, would meet the needs of both scientific classification and "peoples' intuition."
By the proposed geophysical definition: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."
Or, simply put, "round objects in space that are smaller than stars."
This definition holds the physics of the planet itself to more importance than the physics of its interactions with other objects, the researchers explain.
Adopting this definition would see roughly 110 objects in the solar system classified as "full-fledged" planets, including dwarf planets and moon planets such as Ceres, Pluto, Charon, and our own moon.
According to the proposal, the new definition would be better for scientists, educators, and students alike, as it is more intuitive and emphasises the intrinsic physical properties of a planetary body.
And, it speaks to a practice that is already in use.
"In keeping with emphasising intrinsic properties, our geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects," the authors explain.
"Our definition captures the common usage already present in the planetary science community.
"In peer-reviewed planetary science publications and talks, the world 'planet' often substitutes for the given name of the world, even if the world is a moon or a dwarf planet."