Astronomers are advocating for a revised definition of planets, marking a potential shift nearly two decades after Pluto was reclassified as a dwarf planet. The proposed definition introduces measurable criteria, notably including the planet’s mass, while affirming Pluto’s status as a dwarf planet.
The current definition by the International Astronomical Union (IAU) defines a planet based on three characteristics: orbiting the sun, achieving a nearly round shape due to gravitational forces, and clearing its orbit of smaller debris, except satellites and moons.
“The problem previously was the lack of a quantitative definition,” explained Brett Gladman, an astronomer at the University of British Columbia, who, with colleagues, authored the proposal. Their findings were detailed in a paper released on July 10 via arXiv and are slated for presentation at the IAU General Assembly in August.
Gladman highlighted shortcomings in the existing definition, citing vague criteria regarding planetary size and orbital clarity, particularly concerning bodies like Earth and Jupiter, which have intersecting asteroid paths. Moreover, the current definition overlooks exoplanets orbiting stars beyond our solar system.
Jean-Luc Margot, lead author and UCLA astronomer, emphasized the impracticality of the roundness criterion, stressing the lack of observational technology to verify it in practice.
In place of these criteria, the new proposal prioritizes mass as a measurable attribute. According to the revised definition, a planet is a celestial object meeting specific criteria:
To ensure objectivity, the astronomers employed unsupervised clustering, an algorithm grouping similar objects, effectively classifying the eight planets in our solar system.
Critically, dynamical dominance—the ability to clear orbital paths—is correlated with a planet’s mass. For instance, each of the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) exceeds the mass threshold necessary for dynamical dominance, distinguishing them from less massive dwarf planets like Pluto.
Despite the emphasis on mass, the proposal acknowledges that celestial bodies larger than 10^21 kilograms typically assume a spherical shape due to gravitational forces. This clarification aims to refine, rather than contradict, the current definition.
“Humans are inherently bound to language, names, and classification as means to comprehend complexity,” noted Gladman. “Precise categorization is crucial for scientists and the public alike.”
Margot anticipates deliberations and potential revisions during the IAU General Assembly, viewing the proposal as a step toward improved astrophysical terminology. He stressed the community’s responsibility to establish clearer definitions for these fundamental astronomical concepts.