It is proposed to overcome the heliocentric definition of a planet

12 Jul. () –

Planetary scientists are proposing a new definition of a planet to replace one that many researchers consider heliocentric and obsolete.

The current definition, established in 2006 by the International Astronomical Union (IAU), the organization that officially names objects in space, specifies that to qualify as a planet, a celestial body must orbit the sun within our solar system.

But scientists know that celestial bodies orbiting stars outside our solar system are fairly common, and an upcoming paper in the Planetary Science Journal argues for a new definition of a planet that includes one that is not bound by the boundaries of our solar system. The proposal also provides quantitative criteria to further clarify the definition of a planet. The paper is currently available at the arXiv preprint server.

Jean-Luc Margot, senior author of the paper and professor of Earth, Planetary and Space Sciences and of Physics and Astronomy at UCLA, will present the proposed 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 that gravity has forced it into a spherical shape, and has cleared other nearby objects from its orbit around the Sun.

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

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

In the new definition, a planet is a celestial body that: orbits one or more stars, brown dwarfs or stellar remnants and has a mass greater than 10 kilograms to the power of 23 and has a mass less than 13 times that of Jupiter (2.5 X 10 kilograms to the power of 28).

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

DYNAMIC DOMAIN

For example, if an object has enough gravity to clear a path by accumulating or ejecting nearby smaller objects, 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, which is not a true planet, and asteroids, are not,” Margot said. “Therefore, this property can be included in the definition of a planet.”

The dynamical domain requirement provides a lower limit on mass. But potential planets may also be too large to fit the new definition. Some gas giants, for example, are so large that thermonuclear fusion of deuterium occurs and the object becomes a substar called a brown dwarf and therefore not a planet. This limit has been determined to be the mass of 13 or more Jupiters.

The current requirement of being spherical, on the other hand, is more problematic. Distant planets can rarely be observed in sufficient detail to determine their shape with certainty. The authors argue that the shape requirement is so difficult to implement that it is effectively useless for definitional purposes, even though planets are generally round.

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

The good news is that in the solar system, celestial bodies larger than 10 kilograms to the power of 21 appear to be round. Therefore, all bodies that satisfy the proposed lower mass limit of 10 kilograms to the power of 23 are expected to be spherical.

While any official changes to the IAU’s definition of a planet are likely to be years away, Margot and her colleagues hope their work will start a conversation that will result in an improved definition.