The research, published recently in the Planetary Science Journal, has the potential to challenge the long-standing definition put forth by the International Astronomical Union (IAU), which established IAU Resolution B5 in 2006, which resulted in the demoting of Pluto from a ‘planet’ to a ‘dwarf planet’.
The IAU was founded on 28 July 1919 in Brussels, Belgium with the goal of promoting and improving all aspects of astronomy, including scientific research, public outreach and global education.
As of May 2024, the IAU consists of 92 countries and 12,738 members. The IAU has held 32 General Assemblies throughout its history with the aim of establishing scientific protocols. In the case of planets, the IAU adopted a new definition during its 26th General Assembly held in 2006. Since that definition, NASA has confirmed the existence of 5,690 exoplanets to date and this number continues to increase at a steady rate every day, which means that the number of exoplanets orbiting multiple stars will also increase.
The 2006 definition has since become problematic. A new definition of a planet could provide a better framework for identifying and characterizing exoplanets and their corresponding moons (exomoons) as we continue to explore the universe.
In light of that, Universe Today spoke with the lead author of the study on the new definition for planet, Dr. Jean-Luc Margot, professor in the Department of Earth, Planetary and Space Sciences at the University of California, Los Angeles, about the importance, the meaning behind the work, and the steps that need to be taken for the IAU to implement the new definition.
Why is a new definition needed?
“The IAU Resolution B5 has problems – it is vague and doesn’t account for exoplanets – and the problems won’t go away on their own,” Dr Margot told Universe Today.
“The astronomical community and the public deserve better definitions for important astrophysical terms like ‘planet’ and ‘satellite.’ We have had 18 years to identify the problems and consider possible solutions. There are good reasons to believe that in 2024 we are better equipped than in 2006 to produce good results.”
According to IAU Resolution B5, the current definition of a planet is as follows:
(a) in orbit around the Sun:
(b) has sufficient mass so that internal gravitational forces can help force it into hydrostatic equilibrium (spherical) form;
(c) glows in the area around its orbit.
Unfortunately, this resulted in Pluto being demoted from a planet to a dwarf planet because it did not meet criterion (c).
In addition, IAU Resolution B5 limits itself to planets in our solar system, meaning that it only includes planets in single-star systems. In contrast, about 50 exoplanets have been confirmed orbiting their host stars in binary star systems. Therefore, a new IAU definition that includes exoplanets could help establish a better set of planets in the universe.
While IAU Resolution B5 is considered a qualitative definition for planets, this recent study attempted to develop a more quantitative definition that applies both within and beyond our Solar System.
To do this, the researchers used a series of equations to calculate the “region-clearing potential” of a planetary object that matches criterion (c) in IAU Resolution B5, with the goal of determining the approximate size a planetary object would need to have to “clear a region.”
Additional equations were also used to determine the difference between a planet and a satellite. So what are some of the most important results from this research?
“We propose that planetary objects can be classified based on properties that can be easily measured: orbital factors and mass,” Dr. Margot told Universe Today.
According to the new proposal, a planet is a celestial body that meets the following three criteria:
(a) orbits one or more stars, brown dwarfs, or stellar remnants;
(b) weighs more than 10^23 kg;
(c) has a mass less than 13 times that of Jupiter (2.5 × 10^28 kg).
A satellite is a celestial body that orbits a planet.
So what is the significance behind this research?
As noted, the motivation for this study stemmed from the 2006 IAU Resolution B5 to establish a new definition of planet, which resulted in Pluto being downgraded from a planet to a dwarf planet based on new criteria at the time.
This was immediately met with skepticism from the scientific community. This resistance was also seen in politics, with several state governments, including California, New Mexico and Illinois, publicly opposing Pluto’s demotion. So, in Dr. Margot’s view, should Pluto be reclassified as a planet?
“Pluto is a wonderful planet to explore,” Dr. Margot told Universe Today. “However, it is pointless to distinguish Pluto from the other eight planets in the Solar System. One may have legitimate concerns about the scope and precision of the IAU’s 2006 definition of a planet. However, our work is not focused on Pluto, but rather on quantifying and generalizing the definition of a planet.”
According to Dr. Margot, the new research could help establish principles by developing a quantitative method for identifying planets and satellites both in our Solar System. What’s more, this 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 in identifying planets.