In 2006, during their 26th General Assembly, the International Astronomic Union (IAU) passed a resolution to adopt a formal definition for the term “planet”. According to this definition, bodies that orbit the Sun, are spherical, do not orbit other bodies, and have cleared their orbits were designated planets. Pluto, and other such bodies that did not meet all of these requirements, would thereafter be designated as “dwarf planets”.

However, according to a new study led by Philip T. Metzger – a planetary scientists from the Florida Space Institute (at the University of Central Florida) – the IAU’s standard for classifying planets is not supported by the research literature on Pluto, and is therefore invalid. For those people who have maintained that “Pluto is still planet” for the past twelve years, this is certainly good news!

Their study – titled “The Reclassification of Asteroids from Planets to Non-Planets” – was recently published in the scientific journal Icarus. The study was led by Metzer and was co-authored by Mark V. Sykes of the Planetary Science Institute, Kirby Runyon of the Johns Hopkins University Applied Physics Laboratory (JHUAPL), and Alan Stern – the principal investigator of the New Horizons mission from the Southwestern Research Institute (SwRI).



For the sake of their study, the team reviewed scientific literature from the past 200 years to look for instances where the clearing of orbit was used as a requirement for classifying planets. They found only one publication – from 1802 – that relied on this , and the reasoning behind it has since been disproven. In addition, Saturn’s moon Titan and Jupiter’s moon Europa have been routinely referred to as planets by scientists since the time of Galileo.

As Metzger explained in a recent UCF Today news release:

“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.”

According to their study, the literature review showed that the real division between planets and other celestial bodies (such as asteroids and planetoids), occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed. However, this reason is no longer considered a factor when attempting to determine if a celestial body is a planet.

“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,” said Metzger. “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 Kirby Runyon, the IAU’s definition was erroneous since their review showed that clearing an orbit has not been standard practice when attempting to distinguishing asteroids from planets – as the IAU did for the definition they adopted in 2006. Since this is a false historical claim, he said, it should not have been applied to Pluto.

As an alternative, Metzger and his colleagues claim that the definition of a planet should be based on its intrinsic rather than extrinsic properties (such as the dynamics of its orbit), which are subject to change. In short, they recommend that classifying a planet should be based on whether or not it is large enough that its gravity allows for it to achieve hydrostatic equilibrium (i.e. becomes spherical). As Metzger explained:

“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… 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’s active geology and dynamism is what allows for it to have an interior ocean, a multilayered atmosphere, organic compounds and evidence of ancient lakes and multiple moons. According to Metzger, the only planet that has more complex geology is planet Earth.

This is not the first time that Runyon and Sterns have recommended that the classification of planets be based on intrinsic properties. Last year, Runyon (then a final-year PhD student at Johns Hopkins University) was the lead author on a study that was prepared in anticipation of the 48th Lunar and Planetary Science Conference. Titled “A Geophysical Planet Definition“, Runyon and his colleagues (which included Alan Stern) offered the following definition for a planet:

“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.”

In short, their definition would apply to any astronomical body that has achieved hydrostatic equilibrium, which not only applies to Pluto but most of the Solar System’s largest moons. All told, this definition would result in a Solar System of 110 planets instead of 8. Ergo, Ceres, Pluto, Eris, Haumea, Makemake, Titan, Europa, Ganymede, et al. – all planets!

Ever since the IAU adopted their formal definition, several alternative definitions have been proposed that emphasize things other than orbital characteristics. If nothing else, this has indicated that the “great planet debate” is far from over. In the future, and with additional exoplanet discoveries, it is entirely possible that a consensus will be reached on how we classify astronomical bodies. Until then, we can expect that this issue will remain a controversial one.

Further Reading: UCF Today, Icarus