Three astronomers last week proposed expanding the official definition of a planet to include worlds orbiting stars other than our own, a nuance that is currently not included in the formal definition of the term established in 2006 by the International Astronomical Union (IAU). If the trio’s new definition comes to fruition, thousands of bodies in the universe could be confirmed as formal planets.
For a celestial body to qualify as a planet under the current IAU definition, it must orbit the sun, have cleared the area around its orbit, and have enough mass that its gravity would have shaped it into a nearly round shape to be a “planet.” The third requirement is particularly vague because it doesn’t quantify exactly how round the body must be, according to a team led by astronomer Jean-Luc Margot of the University of California, Los Angeles (UCLA).
“The Earth isn’t quite round, so how round does a planet have to be?” study co-author Brett Gladman, an astronomy professor at the University of British Columbia, said in a recent paper. rack“If you look at a world orbiting another star, we can’t measure its shape with current technology.”
The researchers also say that some aspects of the current definition are too specific, such as the definition that requires planets to orbit our sun, because it excludes thousands of worlds around other stars in the universe that otherwise meet the criteria to be called a planet.
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“We now know of thousands of ‘planets’ orbiting other stars, but the IAU definition only applies to those in our solar system, which is clearly a major shortcoming,” Margot said. “We propose a new definition that can be applied to bodies orbiting any star, stellar remnant or brown dwarf.”
In a paper posted to a preprint server on July 10, soon to be published in the Planetary Science Journal, Margot and his colleagues propose to determine the planetary status of a celestial body based on its mass. According to the proposed definition, a world could be called a planet if it was within certain mass limits. For example, it would have to be lighter than 13 Jupiters, at which point nuclear fusion would begin and the object would no longer be a planet, but rather a substar called a brown dwarf.
“By tying definitions to the most easily measurable quantity — mass — debates about whether a specific object meets the criterion are eliminated,” Gladman said. “This is a weakness of the current definition.”
Pluto, which was downgraded to a dwarf planet by the IAU in a highly controversial decision in 2006, would be less massive than the lowest limit suggested by the newly proposed definition and would thus remain a dwarf planet.
Furthermore, the current IAU definition requiring planets to have “nearly round” shapes is difficult to implement, Margot and his team argue, and thus effectively useless because the shapes of many distant worlds cannot be determined with certainty. Instead, using thresholds based on mass “would replace a vague and impractical prescription regarding roundness,” the researchers write in the new paper.
“We draw a line by providing these definitions with numbers, to encourage our community to start the discussion: what exactly is is “A planet?” said Gladman.
The IAU has not yet made any announcements about possible changes to the official definition of a planet. The UCLA statement noted that Margot will present the proposed definition next month at the IAU General Assembly in Cape Town, South Africa. IAU resolutions are typically voted on by members at General Assemblies.