Scientific definition of a planet says it must orbit our sun: A new proposal would change that

Planetary scientists are proposing a brand new definition of a planet to exchange one which many researchers view as sun-centric and outdated. The present definition—established in 2006 by the Worldwide Astronomical Union (IAU), the group that formally names objects in space—specifies that to qualify as a planet, a celestial physique should orbit the sun inside our solar system.

However scientists know that celestial our bodies orbiting stars exterior our solar system are pretty widespread, and a forthcoming article within the Planetary Science Journal argues for a brand new definition of a planet that features being unrestrained by the bounds of our solar system. The proposal additionally supplies quantitative standards to additional make clear the definition of a planet. The paper is at the moment available on the arXiv preprint server.

Jean-Luc Margot, lead creator of the article and UCLA professor of Earth, planetary and space sciences and of physics and astronomy, will current the proposed new definition on the IAU Common Meeting in August 2024.

Underneath the present definition, a planet is a celestial physique that orbits the sun, is very large sufficient that gravity has compelled it right into a spherical form, and has cleared away different objects close to its orbit across the sun.

“The present definition particularly mentions orbiting our sun. We now know in regards to the existence of hundreds of planets, however the IAU definition applies solely to those in our solar system,” Margot mentioned. “We suggest a brand new definition that may be utilized to celestial our bodies that orbit any star, stellar remnant or brown dwarf.”

The authors argue that whereas the requirement to orbit our sun is simply too particular, different standards within the IAU definition are too obscure. For instance, it says a planet has “cleared its orbit” with out stating what meaning. The proposed new definition accommodates quantifiable standards that may be utilized for outlining planets inside and out of doors our solar system.

Within the new definition, a planet is a celestial physique that:

• orbits a number of stars, brown dwarfs or stellar remnants and
• is extra large than 10²³ kg and
• is much less large than 13 Jupiter lots (2.5 X 10²⁸ kg).

Margot and co-authors Brett Gladman of the College of British Columbia and Tony Yang, a pupil at Chaparral Excessive Faculty in Temecula, California, ran a mathematical algorithm on the properties of objects in our solar system to see which objects clustered collectively. The evaluation revealed teams of distinct qualities shared by planets in our solar system that can be utilized as a place to begin for making a taxonomy for planets usually.

For instance, if an object has sufficient gravity to clear a path by accumulating or ejecting smaller objects close by, it’s mentioned to be dynamically dominant.

“All of the planets in our solar system are dynamically dominant, however different objects—together with dwarf planets like Pluto, which isn’t a real planet, and asteroids—will not be,” Margot mentioned. “So this property could be included within the definition of planet.”

The requirement for dynamical dominance supplies a decrease restrict on mass. However potential planets will also be too huge to suit the brand new definition. Some gas giants, for instance, are so giant that thermonuclear fusion of deuterium happens, and the article turns into a substar referred to as a brown dwarf and due to this fact not a planet. This restrict has been decided to be the mass of 13 or extra Jupiters.

The present requirement to be spherical, however, is extra problematic. Distant planets can not often be noticed in sufficient element to determine their form with certainty. The authors argue that the form requirement is so tough to implement that it’s successfully ineffective for definitional functions, though planets are typically spherical.

“Having definitions anchored to probably the most simply measurable amount—mass—removes arguments about whether or not or not a particular object meets the criterion,” Gladman mentioned. “This can be a weak spot of the present definition.”

The excellent news is that within the solar system, celestial our bodies bigger than 10²¹ kg seem like spherical. So all our bodies that fulfill the proposed decrease restrict on mass of 10²³ kg are anticipated to be spherical.

Whereas any official change to the IAU definition of planet is probably going a number of years away, Margot and his colleagues hope their work begins a dialog that ends in an improved definition.