The Definition of a Planet Could Change (Again)

The classification of celestial bodies has long been a subject of debate within the scientific community, particularly when it comes to defining what constitutes a planet. This discussion has resurfaced with new proposals challenging the current definition, prompting reconsideration of how we categorize these distant worlds.

The 2006 Definition and Its Controversies

In 2006, the International Astronomical Union (IAU) introduced a definition that demoted Pluto from its status as the ninth planet of our solar system. This decision was based on three criteria:

1. A planet must orbit the Sun.
2. It must be massive enough to assume a spherical shape through hydrostatic equilibrium.
3. It must have cleared its orbit of other debris.

According to the IAU, Pluto failed to meet the third criterion, as it shares its orbital zone with other objects in the Kuiper Belt. The IAU's decision was influenced by the discovery of numerous Pluto-like objects, raising concerns about an ever-expanding list of planets that might overwhelm educational and classification systems.

Inconsistencies with the "Clearing the Orbit" Criterion

The requirement that a planet must have "cleared its orbit of other debris" has been a point of contention and inconsistency. This criterion implies that the planet must be the dominant gravitational force within its orbital path, effectively absorbing or ejecting smaller objects. However, this concept is not as clear-cut as it seems, and even some of the current planets in our solar system do not strictly adhere to this rule.

1. Jupiter and Its Trojans: Jupiter, the largest planet in our solar system, shares its orbit with a significant number of Trojan asteroids. These are groups of asteroids that remain in stable Lagrangian points, leading and trailing Jupiter along its orbital path. Despite this, Jupiter's status as a planet is unquestioned, highlighting a discrepancy in the application of the "clearing the orbit" criterion.
2. Neptune and Pluto: Neptune and Pluto have an orbital resonance, where Pluto's orbit is in a 3:2 resonance with Neptune. This means that for every three orbits of Neptune around the Sun, Pluto completes two. Despite this overlap, Neptune is considered to have "cleared its orbit," while Pluto is not, which raises questions about the consistency of this criterion.
3. Earth and Near-Earth Objects (NEOs): Earth, too, shares its orbital space with various near-Earth objects (NEOs), including asteroids and comets that cross its path. These objects occasionally pose collision risks, suggesting that Earth has not entirely cleared its orbit of debris.

Alan Stern, a leading expert on Pluto and the principal investigator of NASA's New Horizons mission, stated:

“It's patently clear that the definition is flawed, and much of the world agrees with us.”

– Alan Stern

Implications for Current Classifications

While the new definition attempts to resolve ambiguities, it introduces its own set of challenges. The mass criterion, though scientifically grounded, may be contentious, especially since some brown dwarfs have masses similar to large planets. Moreover, the concept of dynamical dominance, while robust for our solar system, may be less applicable in exoplanetary systems with complex gravitational interactions, such as the TRAPPIST-1 system.

Critics argue that the current and proposed definitions both overlook the geological and compositional aspects that could further refine planetary classification. For example, planets are often geological entities, characterized by processes such as volcanism, tectonics, and atmospheric dynamics, which are not necessarily tied to their orbital mechanics or mass.

Potential Impact on our Solar System

The debate over how to define a planet is far from settled. The upcoming proposal to the IAU in August 2024 will likely spark further discussion and research, potentially leading to a more universally accepted definition. As our observational technologies and understanding of celestial bodies advance, the criteria for planetary classification will undoubtedly evolve.

In the meantime, this ongoing discourse highlights the dynamic nature of scientific inquiry and the importance of continually reassessing our frameworks as new discoveries emerge.

Whether Pluto regains its planetary status or remains a dwarf planet, the pursuit of a clearer and more inclusive definition underscores our quest to understand the cosmos more comprehensively.

Author's Background:

I first became interested in astronomy and astrophysics around 25 years ago, watching TV shows such as Modern Marvels: Space Exploration and Space Odyssey: Voyage to the Planets with my father, who is now an amateur astronomer and optical engineer. I have long followed the news and exploration surrounding the former planet of Pluto. Back in 2006, I started a 9-year countdown clock for the New Horizons interplanetary space probe to Pluto on my MySpace page, the mission of which lasted three times longer than the popularity of MySpace. Here is a list of resources I recommend, if you are interested in the topic:

Further Resources

?? Books:

• "How I Killed Pluto and Why It Had It Coming" by Mike Brown: This book by the astronomer who discovered Eris, a dwarf planet, offers an insider's view on the decision to reclassify Pluto.
• "Pluto Confidential: An Insider Account of the Ongoing Battles over the Status of Pluto" by Laurence A. Marschall and Stephen P. Maran: This book provides a detailed account of the controversies and discussions around Pluto’s planetary status.

?? Scientific Papers and Articles:

• Stern, S. A., & Levison, H. F. (2002). "Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes." This paper discusses the criteria for classifying planets and offers insights into the debate.
• Runyon, K. D., Stern, S. A., et al. (2017). "A Geophysical Planet Definition." This paper proposes a definition of a planet based on geophysical characteristics rather than orbital dynamics.
• Russell, D. (2017). The Moon Meets All Requirements of the IAU Definition for “Planet”. International Journal of Astronomy and Astrophysics, 7(4), 291-302

?? Websites and Online Resources:

• NASA’s New Horizons Mission: New Horizons: The official NASA page for the New Horizons mission, which provides detailed information about Pluto and its exploration.
• International Astronomical Union (IAU): IAU: The organization responsible for the definition of planetary bodies, including Pluto. Their site includes articles and resources on the topic.

?? Videos and Documentaries:

• PBS NOVA: Pluto and Beyond: A documentary exploring the New Horizons mission and the reclassification of Pluto.
• YouTube Channels: Anton Petrov: Provides updates and detailed explanations on astronomical discoveries. Definition of "Planet" May Change Again!
• YouTube Channels: Star Talk Neil DeGrasse Tyson's official YouTube channel on astrophysics and astronomy. Neil deGrasse Tyson Debates a Pluto Expert

?? Humor and Pop Culture

• Rick and Morty: Pluto is a Planet
• The Big Bang Theory: I liked Pluto

Comment Your Thoughts!

What do you think about the current definition of a planet? Should Pluto be re-established as the 9th Planet of Our Solar System?

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