Officially, Only the Sun Can Have Planets. Is it Time to Fix the Definition of “Planet”?

What is the true definition of a planet, and could there be a more refined definition in the future? This is what a recent study published in The Planetary Science Journal hopes to address as a team of researchers from the United States and Canada investigated the potential for a new definition of a “planet”. This study holds the potential to challenge the longstanding definition outlined by the International Astronomical Union (IAU), which established IAU Resolution B5 in 2006, resulting in demoting Pluto from a “planet” to a “dwarf planet”.

Here, Universe Today discusses this incredible research with study lead author, Dr. Jean-Luc Margot, who is a professor in the Department of Earth, Planetary & Space Sciences at the University of California, Los Angeles, regarding the motivation behind the study, significant results, what steps need to be taken for the IAU to implement his new definition, and whether Dr. Margot thinks Pluto should be reclassified as a planet. So, what was the motivation behind this study?

“IAU Resolution B5 is problematic – it is vague and excludes exoplanets – and the problems will not go away on their own,” Dr. Margot tells Universe Today. “Our community and the public deserve better definitions for such important astrophysical terms as ‘planet’ and ‘satellite’. We have had 18 years to identify the problems and consider possible ways forward. There are good reasons to believe that we are better equipped in 2024 than in 2006 to produce a good outcome.”

According to IAU Resolution B5, the current definition of a planet is as follows:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it   assumes a hydrostatic equilibrium (nearly round) shape, and 

(c) has cleared the neighborhood around its orbit.

Unfortunately, this resulted in Pluto being demoted from a planet to a dwarf planet since it did not meet criteria (c). Additionally, IAU Resolution B5 limits itself to planets within our solar system, specifically pertaining to planets orbiting a single star. In contrast, approximately 50 exoplanets have been confirmed to orbit one star within a binary (two-star) system. Therefore, a new IAU definition that encompasses exoplanets could help establish a better framework for planets throughout the cosmos.

While IAU Resolution B5 is considered a qualitative (non-mathematical) definition for planets, this recent study attempted to develop a more quantitative (mathematical) definition, or planetary taxonomy (classification), that could encompass planets and satellites (moons) both within and beyond our solar system. To accomplish this, the researchers used a series of equations to calculate a planetary body’s “ability to clear a zone” which coincides with criteria (c) of IAU Resolution B5, with the goal of ascertaining the approximate size a planetary body needs to be to “clear a zone”. Additional equations were also used to ascertain the difference between a planet and a satellite, as well. Therefore, what were some of the most significant results from this study?

“We suggest that planetary bodies can be classified on the basis of properties that are easily measurable: orbital elements and masses,” Dr. Margot tells Universe Today. “We find that unsupervised clustering of solar system bodies according to orbital elements and masses yields distinct groups. The clustering reveals that satellites are distinct from planets, and it reveals that the 8 planets are distinct from all other bodies. The presence of these groups and the gaps between these groups provides natural dividing lines for planetary taxonomy.”

Dr. Margot continues, “We emphasize the focus on the ability to clear a zone in a specified timescale as opposed to the state of having cleared a zone. The former is robust and easily quantifiable and observable, the latter is difficult to implement and difficult to quantify. We propose a single clearing timescale applicable to all stars, stellar remnants, and brown dwarfs. We propose a definition that is aligned with IAU recommendations, but we also examine potential difficulties related to these recommendations. We propose a simpler, mass-based proposal that avoids 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) is more massive than 10²³ kg, and

(c) is less massive 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 formed on July 28, 1919, in Brussels, Belgium with the goal of promoting and improving all facets of astronomy, including scientific research, public outreach, and global education. As of May 2024, the IAU is comprised of 92 countries and a membership of 12,738 individuals. The IAU has held 32 General Assemblies during its history with the goal of establishing scientific protocol, or in the case of planets, providing a new definition during the 26^th General Assembly in 2006. Examples of more recent resolutions include protections of radio astronomy from radio interference and advancements in ultraviolet astronomy, both at the 31^st General Assembly in 2021. Therefore, what steps need to be taken for the IAU to implement these three criteria for defining planets?

“The IAU has an established process for considering and voting on proposed resolutions,” Dr. Margot tells Universe Today. “In my opinion, the IAU ought to follow its established process and consider all reasonable submissions. We were unsuccessful in 2024, but we are likely to resubmit in advance of the 2027 General Assembly, and we are hoping for a more positive outcome at that time.”

As noted, the motivation of this study was derived from IAU Resolution B5 in 2006 that established a new definition of a planet, which resulted in Pluto being demoted from planet to dwarf planet based on the new criteria. This was immediately met with skepticism from the scientific community, including from Dr. Alan Stern, who is the Principal Investigator of the New Horizons mission that visited Pluto in 2015, with Pluto being demoted less than a year after New Horizons launched in early 2006. This pushback was also seen in politics as several state governments, including California, New Mexico, and Illinois, publicly denounced the demotion. Therefore, in Dr. Margot’s opinion, should Pluto be reclassified as a planet?

“Pluto is an amazing planetary body that is worthy of exploration,” Dr. Margot tells Universe Today. “However, it does not make sense to classify Pluto with the eight planets. One may have legitimate concerns about the scope and precision of the 2006 IAU planet definition, but at least the outcome of the IAU resolution was sensible: Pluto did not belong with the eight planets and needed to be assigned to a different class. Our work is not focused on Pluto but instead on quantifying and generalizing the definition of planet.”

As of this writing, NASA has confirmed the existence of 5,690 exoplanets and this number continues to grow at a steady rate daily, which means the number of exoplanets orbiting multiple stars will grow, as well. Therefore, a new definition of a planet could provide a better framework for identifying and characterizing exoplanets and their respective satellites (exomoons) as we continue to explore the cosmos. This recent study could help establish that framework by developing a quantitative approach to defining planets and satellites both in our solar system and beyond, which could help shape our understanding of the universe and our place in it. Additionally, using mathematics to establish a new definition could also remove any subjectivity from defining planets, as well.

“Nothing is engineered in our proposal to keep the number of planets small,” Dr. Margot tells Universe Today. “The clustering analysis is completely agnostic about human feelings. It could have resulted in a group of 8 planets, 12 planets, or 50 planets, and we would have done exactly the same thing. It identified 8 planets. Readers who are chagrined that smaller bodies are not recognized as planets should take comfort in the fact that these bodies are no less worthy of exploration. A taxonomic classification in one group or another is not an indicator of scientific importance.”

How will this new definition help scientists better understand planets and exoplanets in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!