Researchers want to change how the International Astronomical Union defines planets. Until now, astronomers said, it only included worlds orbiting the sun. Under the new definition, planets could orbit other stars as well — and more.
The International Astronomical Union (IAU) defines a planet as a celestial body that orbits the Sun, is massive enough to maintain a spherical shape by gravity, and has “cleaned up” smaller objects in similar orbits by capturing them or throwing them far out into space. But that definition doesn’t include many worlds we know exist: planets orbiting stars that aren’t the Sun. So an article in the Planetary Science Journal calls for a change to that definition.
Mass first and foremost
“We know of thousands of planets, but the IAU definition only applies to those in our solar system,” said Jean-Luc Margot of the University of California, Los Angeles, the paper's lead author. “We propose a new one that can be applied to celestial bodies orbiting any star, stellar remnant, or brown dwarf.”
According to the new definition, a planet is a celestial body that orbits one or more stars, brown dwarfs, or stellar remnants. It is also characterized by a mass between 1023 kilograms – the minimum value that allows for “clearing” the orbit from smaller bodies moving in it – a 2.5*1028 kg. This is 13 times the mass of Jupiter and the limit beyond which a body becomes a star-like object.
The researchers selected these criteria based on a model comparing the characteristics of objects in our Solar System. The analysis revealed typical planetary features that were used as a starting point for creating a taxonomy. For example, all the planets in our Solar System have “cleaned up” their orbits, unlike the dwarf planets, including Pluto and Ceres. In addition, they are not large enough to undergo the processes typical of the smallest star-like objects, brown dwarfs.
The Sphericity Dispute
The researchers added, however, that the sphericity requirement is more problematic. The problem is that distant planets can rarely be observed in enough detail to confidently determine their shape.
“Having a definition anchored in the most easily measurable quantity, mass, eliminates the arguments about whether a particular object meets a given criterion and can be considered a planet,” said Brett Gladman of the University of British Columbia, a co-author of the study. “That's a weakness in the current definition.”
Margot is scheduled to present a proposed modification to the IAU General Assembly in August 2024. While an official change is likely to be at least years away, the researchers hope their paper will spark a discussion that will lead to improved definitions.
Main image source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute