West LA Times

Planetary scientists are proposing a new definition of a planet to replace one that many researchers view as sun-centric and outdated. The current definition, established in 2006 by the International Astronomical Union (IAU), specifies that to qualify as a planet, a celestial body must orbit the sun within our solar system.

However, scientists recognize that celestial bodies orbiting stars outside our solar system are fairly common. A forthcoming article in the Planetary Science Journal argues for a new definition of a planet that includes being unrestrained by the bounds of our solar system. The proposal also provides quantitative criteria to further clarify the definition of a planet.

Jean-Luc Margot, lead author of the article and UCLA professor of earth, planetary and space sciences and of physics and astronomy, will present the proposed new definition at the IAU General Assembly in August 2024.

Under the current definition, a planet is a celestial body that orbits the sun, is massive enough that gravity has forced it into a spherical shape, and has cleared away other objects near its orbit around the sun.

“The current definition specifically mentions orbiting our sun. We now know about the existence of thousands of planets, but the IAU definition applies only to the ones in our solar system,” Margot said. "We propose a new definition that can be applied to celestial bodies that orbit any star, stellar remnant or brown dwarf.”

The authors argue that while the requirement to orbit our sun is too specific, other criteria in the IAU definition are too vague. For example, it says a planet has “cleared its orbit” without stating what that means. The proposed new definition contains quantifiable criteria that can be applied for defining planets inside and outside our solar system.

In the new definition, a planet is defined as:

- A celestial body that orbits one or more stars, brown dwarfs or stellar remnants

- More massive than 10^23 kg

- Less massive than 13 Jupiter masses (2.5 x 10^28 kg)

Margot and co-authors Brett Gladman of the University of British Columbia and Tony Yang, a student at Chaparral High School in Temecula, California, ran a mathematical algorithm on properties of objects in our solar system to see which objects clustered together. The analysis revealed groups with distinct qualities shared by planets in our solar system that can be used as starting points for creating taxonomy for planets in general.

For example, if an object has enough gravity to clear a path by accumulating or ejecting smaller objects nearby, it is said to be dynamically dominant.

“All the planets in our solar system are dynamically dominant, but other objects — including dwarf planets like Pluto and asteroids — are not,” Margot said. “So this property can be included in the definition of planet.”

The requirement for dynamical dominance provides a lower limit on mass. However, potential planets can also be too big to fit this new definition. Some gas giants are so large that thermonuclear fusion of deuterium occurs; these become substars called brown dwarfs and therefore do not qualify as planets under this proposal.

The current requirement for being spherical poses another challenge. Distant planets can rarely be observed in enough detail to ascertain their shape with certainty. The authors argue this shape requirement is difficult to implement effectively even though most planets are generally round.

“Having definitions anchored to the most easily measurable quantity — mass — removes arguments about whether or not a specific object meets the criterion,” Gladman said. “This is a weakness of the current definition.”

In practice within our solar system, celestial bodies larger than 10^21 kg appear round; thus all bodies meeting this proposed lower mass limit should also be spherical.

While any official change to IAU's planetary definition may take years to finalize, Margot and his colleagues hope their work sparks discussions leading toward an improved classification framework.