Beyond Pluto: Redefining "Planet" in the Age of Exoplanets
For many of us, the definition of a planet was definitively, albeit controversially, established in 2006 with the demotion of Pluto to a dwarf planet. Now, a group of astronomers is pushing to redefine the term, not just for the sake of Pluto, but to encompass the vast, diverse population of exoplanets – worlds orbiting stars beyond our own Sun.
Their paper, currently available on the preprint server arXiv, comes ahead of the International Astronomical Union’s (IAU) General Assembly next month. This gathering, where the current definition of a planet was established in 2006, could see a major shift in our cosmic vocabulary.
Why the Fuss Over Pluto?
It’s important to note that this redefinition isn’t about restoring Pluto’s planetary status. The team’s focus is on the thousands of exoplanets that are currently excluded from the definition of planet. With NASA having confirmed nearly 6,000 exoplanets, and billions more expected, these distant worlds offer invaluable insights into planetary evolution, the formation of star systems, and even the search for life beyond Earth – a field known as astrobiology.
"All the planets in our solar system are dynamically dominant, but other objects—including dwarf planets like Pluto, which is not a true planet, and asteroids—are not," explained Jean-Luc Margot, lead author of the study and a planetary scientist at UCLA. "So this property can be included in the definition of planet."
A Broader Horizon: The Proposed Definition
The 2006 IAU definition stipulated that a planet in our solar system must:
- Orbit its host star.
- Be large enough to be mostly round.
- Possess sufficient gravitational dominance to clear its orbital neighbourhood.
Pluto failed to meet the last criterion, hence its demotion.
The new proposal, however, casts a wider net. It defines a planet as a celestial body that:
- Orbits one or more stars, brown dwarfs, or stellar remnants.
- Possesses a mass greater than 1023 kg.
- Has a mass less than 13 Jupiter masses.
This definition emphasizes mass as a primary criteria, rather than the specific clearing of an orbital neighbourhood. This is important because unlike our own solar system, where planets dominate their orbital zones, exoplanet systems often exhibit complex and dynamic interactions, making the "clearing" criterion less applicable.
The upper mass limit is also significant. While massive planets can form through accretion – the gradual accumulation of smaller bodies – exceeding 13 Jupiter masses triggers nuclear fusion, transforming the object into a brown dwarf.
Pluto, with a mass of approximately 1.31 x 1022 kg, would still fall under the new definition, but it’s the vast population of exoplanets that this proposal aims to include.
Why Change?
The team argues that a mass-based definition is more straightforward and objective. "Having definitions anchored to the most easily measurable quantity—mass—removes arguments about whether or not a specific object meets the criterion," stated Brett Gladman, a planetary scientist at the University of British Columbia and co-author of the study. "This is a weakness of the current definition."
Impact and Implications
The proposed definition won’t be implemented immediately. The team intends to present their proposal at the IAU General Assembly next month, where it will be subject to rigorous debate and scrutiny. However, if successful, this shift could usher in a new era of planetary exploration and understanding.
The proposed change would not just redefine exoplanets as "planets", but shift our perspective on our place in the cosmos. Instead of looking at planets as the exclusive neighbors of our Sun, we would consider "planet" as a more universal label, encompassing countless worlds throughout our universe.
This change can have a profound impact on scientific discourse and public perception of our place in the cosmos. The inclusion of exoplanets under the broader definition of "planet" will undoubtedly lead to new avenues of research and inspire further exploration, while fostering a sense of wonder and connection to the vastness of the universe.
A Broader Perspective
While some might consider this redefinition as an attempt to "undo" Pluto’s demotion, it is much more than that. It’s a reflection of the expanding scope of astronomy and the realization that our own solar system is just a tiny nook in a universe teeming with planetary systems.
By embracing a more inclusive definition of "planet," we acknowledge the diversity and vastness of our cosmic backyard. We acknowledge that, the search for life, whether here or elsewhere, might not be restricted to just those bodies in our solar system.
The proposed redefinition of "planet" is not just a change in terminology, but a shift in our perspective on our place in the universe and the potential for life beyond Earth. It’s a reminder that our knowledge of the cosmos is constantly evolving, and that even deeply ingrained concepts can be reshaped by new discoveries and a thirst to understand our place in the grand tapestry of existence.
