What is the true definition of a planet, and could there be a more refined definition in the future? This is what a recently published study suggests Journal of Planetary Science, We hope that a team of researchers from the United States and Canada will explore the potential of a new definition of “planet.”

This research has the potential to challenge the long-standing definition of Pluto by the International Astronomical Union (IAU), which adopted IAU Resolution B5 in 2006, downgrading Pluto from a “planet” to a “dwarf planet.”

Here, Universe Today discusses this incredible study with lead author Dr. Jean-Luc Margot, a professor in the Department of Earth, Planetary, and Space Sciences at UCLA, about the motivation behind the work, the key implications, and what steps need to be taken for the IAU to implement its new definition, and whether Dr. Margot thinks Pluto should be reclassified as a planet.

So what was the motivation behind this work?

“The IAU’s B5 decision is problematic; it is vague and excludes exoplanets, and the problems are not going to go away on their own,” Dr Margot told Universe Today.

“Our community and the public deserve better definitions of important astrophysics terms like ‘planet’ and ‘moon’. We’ve had 18 years to identify problems and consider possible ways to improve. We have good reason to believe that we are better equipped to deliver a good result in 2024 than in 2006.”

According to IAU Resolution B5, the current definition of a planet is that it: (a) orbits the Sun, (b) has sufficient mass to overcome the forces of gravity of a solid body to assume a hydrostatic equilibrium (nearly circular) shape, and (c) has cleared the neighborhood around its orbit. Unfortunately, this resulted in Pluto being demoted from the dwarf planet category because it did not meet criteria for (c).

IAU Resolution B5 also limits itself to planets in our solar system, specifically those orbiting a single star. In contrast, about 50 exoplanets have been confirmed to orbit a single star in binary (binary) systems. Therefore, the new IAU definition of exoplanets could help create a better framework for planets in the universe.

While IAU Resolution B5 is considered a qualitative (non-mathematical) definition of planets, this recent work attempted to develop a more quantitative (mathematical) definition or planetary taxonomy (classification) that could encompass planets and satellites (moons) both within and outside our Solar System.

To achieve this, the researchers used a set of equations to calculate the “area-clearing capacity of a planetary body,” which coincides with criterion (c) of IAU Resolution B5 and determines the approximate extent to which a planetary body is required to “clear the planetary area.”

Additional equations were also used to determine the difference between a planet and a moon. So what are the most important findings of this study?

“We assume that planetary bodies can be classified by easy-to-measure properties: orbital elements and masses,” Dr. Margot tells Universe Today.

“We found that uncontrolled clustering of Solar System objects according to orbital elements and masses produces distinct groups. The clustering shows that the moons are distinct from the planets and the 8 planets are distinct from all other objects. The existence of these groups and the gaps between these groups provide natural dividing lines for planetary classification.”

Dr. Margo continues: “We emphasize the ability to clear an area over a period of time, not the state of clearing the area. The former is reliable, easily measured and observable; the latter is difficult to implement and measure.

“We propose a single cleanup timeline that applies to all stars, stellar remnants and brown dwarfs. We propose a definition consistent with the IAU guidelines, but we also explore potential difficulties with those guidelines. We propose a simpler proposal based on mass that removes some of these difficulties.”

This proposal consists of the following definition:

A planet is a celestial body that (a) orbits one or more stars, brown dwarfs, or stellar remnants and (b) weighs more than 1023 kg and (c) has a mass less than 13 Jupiter masses (2.5×1028 kg).

A satellite is a celestial body that orbits a planet. Headquartered in Paris, France, the IAU was founded on July 28, 1919, in Brussels, Belgium, with the aim of promoting and advancing all aspects of astronomy, including scientific research, public outreach, and global education.

As of May 2024, the IAU consists of 92 countries and has 12,738 members. The IAU has held 32 General Assemblies in its history, with the aim of establishing a scientific protocol or, in the case of planets, providing a new definition during the 26th General Assembly in 2006.

Examples of recent resolutions include the Protection of Radio Astronomy from Radio Interference and Advances in Ultraviolet Astronomy, both at the 31st General Assembly in 2021. So what steps need to be taken for the IAU to apply these three criteria to define planets?

“The IAU has an established process for reviewing and voting on proposed resolutions,” Dr. Margo told Universe Today.

“In my view, the IAU should follow its own established process and consider all reasonable applications. We were not successful in 2024 but we will most likely resubmit it to the 2027 General Assembly, hoping that a more positive outcome will emerge at that time.”

As noted, the motivation for this study was derived from the IAU’s 2006 Resolution B5, which established a new planet definition that resulted in Pluto being downgraded from planet to dwarf planet according to new criteria.

The scientific community, including Dr. Alan Stern, principal investigator of the New Horizons mission that visited Pluto in 2015, was met with skepticism, and Pluto was demoted in early 2006, less than a year after New Horizons launched.