Three astronomers last week proposed expanding the official definition of a planet to include worlds orbiting stars other than our own, a nuance not currently included in the formal definition of the term established in 2006 by the International Astronomical Union (IAU). If the trio’s new definition pans out, thousands of celestial bodies in the universe could be confirmed as formal planets.
To qualify as a planet under the current IAU definition, a celestial body must orbit the Sun, clear the region around its orbit, and have enough mass that its gravity will shape it into a nearly round shape. to be a “planet”. The third requirement is particularly vague because it doesn’t quantify how round the celestial body should be, according to a team led by astronomer Jean-Luc Margot of the University of California, Los Angeles (UCLA).
“The earth is not perfectly round, so how round does the planet have to be?” Study co-author Brett Gladman, a professor of astronomy at the University of British Columbia, said recently declaration. “If you look at a world orbiting another star, we can’t measure the shape with current technology.”
The researchers also say that some aspects of the current definition are too specific, such as the one 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 labeled 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 obviously a big mistake,” Margot said. “We propose a new definition that can be applied to celestial bodies orbiting any star, stellar remnant, or brown dwarf.”
IN paper Sent to the preprint server July 10, soon to be published in the Planetary Science Journal, Margot and his colleagues propose to determine the planetary status of a celestial object based on its mass. According to the proposed definition, a world could be called a planet if it is within certain mass limits. For example, it must be lighter than 13 Jupiters, beyond which nuclear fusion begins and the object is no longer a planet, but rather a substar called a brown dwarf.
“Anchoring the definitions to the most easily measurable quantity—weight—eliminates arguments about whether or not a particular object meets the criterion,” Gladman said. “This is the weakness of the current definition.”
Pluto, which was downgraded to a dwarf planet in 2006 by a much-debated decision by the IAU, would be lighter than the lowest limit proposed by the newly proposed definition and would thus continue to be a dwarf planet.
In addition, the current IAU definition requiring planets to have “nearly round” shapes is difficult to implement, Margot and his team say, and is therefore practically unusable because the shapes of many distant worlds cannot be resolved with certainty. Instead, using thresholds based on weight would “replace a vague and impractical prescription for roundness,” the researchers write in the new paper.
“Let’s draw a line in the sand by putting some numbers in these definitions to encourage our community to start a discussion: What exactly Yippee planet?” Gladman said.
The IAU has not yet made any announcements about possible changes to its official definition of a planet. The UCLA statement noted that Margot is scheduled to present the proposed definition next month at the IAU General Assembly in Cape Town, South Africa. Resolutions of the IAU are usually voted on by its members in general meetings.