However, scientists recognize the prevalence of celestial bodies orbiting stars outside our solar system. An upcoming article in the Planetary Science Journal advocates for a new definition of a planet, unbound by our solar system, along with quantitative criteria for further clarification.

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

The current definition describes a planet as a celestial body that orbits the sun, is massive enough for gravity to form it into a spherical shape, and has cleared its orbit of other objects.

"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 the sun is overly specific, other criteria in the IAU definition are too vague, such as the term "cleared its orbit." The new proposed definition includes quantifiable criteria that apply to planets both inside and outside our solar system.

Under the new definition, a planet is a celestial body that:

- Orbits one or more stars, brown dwarfs, or stellar remnants

- Is more massive than 10^23 kg

- Is 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, used a mathematical algorithm to analyze the properties of objects in our solar system. They found distinct qualities shared by planets that could serve as a basis for defining planets more generally.

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

"All the planets in our solar system are dynamically dominant, but other objects - including dwarf planets like Pluto, which is not a true planet, and asteroids - are not," Margot said. "So this property can be included in the definition of planet."

The requirement for dynamical dominance sets a lower mass limit. However, potential planets can also be too large. Some gas giants, for example, are so massive that thermonuclear fusion of deuterium occurs, transforming them into substars known as brown dwarfs, which are not considered planets. This limit is set at 13 Jupiter masses.

The current spherical shape requirement is problematic since distant planets are rarely observed in enough detail to confirm their shape. The authors argue that the shape criterion is practically useless for definitions, even though planets are typically 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."

Fortunately, in our solar system, celestial bodies larger than 10^21 kg tend to be round, so objects meeting the proposed lower mass limit of 10^23 kg are expected to be spherical.

Although an official change to the IAU definition of a planet may be years away, Margot and his colleagues hope their proposal will spark a conversation leading to an improved definition.

Research Report:Quantitative Criteria for Defining Planets

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