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 official definition of the term established in 2006 by the International Astronomical Union, or IAU. If the trio’s new definition is perfect, thousands of celestial bodies throughout the universe should be confirmed as official planets.
For a celestial body according to the IAU’s current definition of a planet to be able to orbit the sun as a planet, it would have to clear the area around its orbit and have enough mass to sculpt gravity into a nearly round shape to be… a “planet.” According to a team led by astronomer Jean-Luc Margot of the University of California, Los Angeles (UCLA), the third condition is particularly ambiguous because it simply does not quantify how round the celestial body is.
“Earth isn’t exactly round, so how round must a planet be?” Study co-author Brett Gladman, a professor of astronomy at the University of British Columbia, said recently statement. . . . “If you look at a world orbiting another star, with current technology, we can’t measure its shape.”
Researchers also say some aspects of the current definition are too specific, such as requiring planets to orbit our sun, because it excludes thousands of worlds around other stars in the universe that otherwise meet the criteria for being called planets.
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“We now know there are thousands of ‘planets’ orbiting other stars, but the IAU definition only includes those in our solar system, which is obviously a major flaw,” Margot said. “We propose a new definition applicable to celestial bodies orbiting any star, stellar debris, or brown dwarf.”
in a paper Published July 10 on a preprint server, soon to be published in the journal Planetary Science , Margot and her colleagues propose determining a celestial body’s planetary status based on its mass. According to the proposed definition, a world can be called a planet if it is within a certain mass limit. For example, it must be lighter than 13 Jupiter, beyond which nuclear fusion begins and the object is no longer a planet but a secondary star called brown dwarfs.
“Having definitions anchored by the most easily measurable quantity — mass — removes arguments about whether a particular thing meets the criteria,” Gladman says. “This is the weakness of the current definition.”
Pluto, downgraded to a dwarf planet by the IAU in a highly controversial decision in 2006, will be lighter than the lowest limit proposed by the newly proposed definition, and thus will continue to remain a dwarf planet.
Nevertheless, the IAU’s current definition of requiring planets to have “nearly round” shapes is difficult to apply, Margot and her team say, and thus effectively useless because the shapes of many distant worlds cannot be resolved with certainty. Instead, the use of mass-based thresholds “replaces a vague and impractical prescription about variability,” the researchers write in the new paper.
“We’re drawing a line in the sand by putting some numbers to these definitions, to encourage our community to start the conversation: what exactly.” is A planet?” Gladman said.
The International Organization for Migration (IOM) has not yet made any announcement about a possible change in its official definition of a planet. Margot is scheduled to present the proposed definition at the IAU General Assembly in Cape Town, South Africa, next month, the UCLA statement noted. IAU resolutions are typically voted on by its members during plenary sessions.
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