Pluto's not coming back, but astronomers want to redefine planets again
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A group of astronomers want to change the definition of a planet.
Why it matters: Their new proposed definition wouldn't bring Pluto back into the planetary fold, but it could reclassify thousands of celestial bodies across the universe.
How it works: The International Astronomical Union's (IAU) current definition of a planet, established in 2006, includes only celestial bodies that are nearly round, are gravitationally dominant and orbit our Sun.
- This Sun-centric definition excludes all of the bodies we've discovered outside our solar system, even if they may fit all other parameters. They are instead considered exoplanets.
- Those behind the new proposal critiqued the IAU's definition in an upcoming paper in the Planetary Science Journal, arguing it's vague, not quantitative and unnecessarily exclusionary.
Zoom out: Their new proposal would instead classify planets based on their mass, considering a planet to be any celestial body that:
- orbits one or more stars, brown dwarfs or stellar remnants and,
- is more massive than 10²³ kilograms (kg) and,
- is less massive than 13 Jupiter masses (2.5 X 10²⁸ kg).
Context: Mercury, the smallest "planet" in the universe under IAU's definition, has a mass of 3.3 x 10²³ kg.
- Pluto would remain a dwarf planet with its mass of 1.31 x 10²² kg.
- The new definition is needed to cap how large a planet may be because nuclear fusion will begin on any body larger than the mass of 13 Jupiters, and the presence of fusion reactions would instead make them brown dwarfs, or substars.
What they're saying: "Having definitions anchored to the most easily measurable quantity — mass — removes arguments about whether or not a specific object meets the criterion," Brett Gladman, a co-author of the paper and an astronomy professor at the University of British Columbia told the University of California, Los Angeles.
- Jean-Luc Margot, the lead author of the article and a UCLA astronomy professor, will present the new proposal at an IAU meeting next month.
- The astronomers noted that any official change to IAU's definition may take years, but they want to spark conversation with their proposal and eventually produce a simpler classification system.
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