Astronomers may (again) change the definition of "planet": here's why

Scientists are proposing a new definition for the term "planet" to replace the one they believe is outdated and centered around our Sun. The current definition, established in 2006 by the International Astronomical Union (IAU), states that a planet must orbit the Sun, excluding many celestial objects discovered outside our Solar System.


A study set to appear in the Planetary Science Journal argues for a broader definition, including bodies orbiting other stars. This proposal includes clear quantitative criteria to better define what a planet is, not limited to our Solar System. Professor Jean-Luc Margot from UCLA, the lead author of the paper, will present this new definition at the IAU General Assembly in August 2024.

According to the current definition, a planet is a celestial body that orbits the Sun, is massive enough to be spherical, and has cleared its orbit of debris. However, this definition excludes exoplanets and lacks precision, according to Margot and his colleagues. They propose a new definition based on mass, eliminating subjective debates about shape.

The proposed new definition states that a planet is a celestial body that:
- Orbits one or more stars, brown dwarfs, or stellar remnants;
- Has a mass greater than 10²³ kg;
- Has a mass less than 13 Jupiter masses (2,5 x 10²⁸ kg).

The authors used a mathematical algorithm to analyze the properties of objects in our Solar System, identifying distinct groups. They suggest that dynamic dominance and mass are key criteria for defining a planet. Objects massive enough to trigger deuterium nuclear fusion, thereby becoming brown dwarfs, would not be considered planets.

Although a spherical shape is commonly associated with planets, it is difficult to confirm for distant objects. Therefore, the authors find this characteristic impractical for the definition. They consider mass, which is easily measurable, to be a more robust criterion.

Lastly, the authors hope their work will spark a discussion leading to an improved definition of what a planet is, although any official change to the IAU's definition could take several years.