Poor Pluto. Fans of the now-dwarf planet may have gotten their hopes up this summer when a small group of astronomers proposed a new definition to decide which celestial bodies deserve the label “planet.” Unfortunately, Pluto, which was famously downgraded from planet to dwarf planet status in 2006, still doesn’t meet the criteria due to its lack of mass.
In 2006, Pluto failed to meet the International Astronomical Union’s updated criteria to be considered a planet. Although it met the first two (orbiting the sun and being “nearly round”), it did not have enough mass to influence debris and other objects in its orbital path.
In July 2024, planetary scientists from the University of British Columbia and UCLA suggested an updated definition in The Planetary Science Journal and presented their proposal at the International Astronomical Union’s General Assembly in August. According to the proposal, one of the biggest problems with the current definition is that it excludes any celestial body outside our solar system. Additionally, the IAU definition does not specify, in numerical terms, how "nearly round" a planet needs to be and how clear of debris its orbital path should be.
The new definition would include any celestial body that orbits a star, brown dwarf, or stellar remnant. The definition suggests that a planet should be more massive than 10^23 kilograms but less massive than 2.5 x 10^28 kg. This range would ensure that the planet is substantial enough to clear its orbit of debris and, due to its gravitational pull, relatively round. The upper end of the range would extend to objects up to 13 times the mass of Jupiter. This would exclude gas giants that have enough mass to undergo deuterium fusion.
Sadly for Pluto, it is too diminutive to qualify under the new definition, as its mass is only 1.31 x 10^22 kilograms. The eight currently recognized planets in our solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) would still be considered planets under the new criteria, a grouping that was confirmed when the scientists applied a mathematical algorithm that identified the shared qualities of those planets.
What makes a planet a planet?
- Notably, the new definition does not focus on how spherical a celestial body is, opening the door to objects in far-off galaxies being considered planets, even if astronomers cannot yet measure their roundness because of how far away they are. According to astronomer and study lead author Jean-Luc Margot, "Roundness is simply not observable. We do not have the technology, and we will not have the technology anytime soon.”
- So far, NASA has identified nearly 6,000 exoplanets beyond our solar system, but there could be billions in the universe. Many could simply be called “planets” if the new definition is adopted.
- Interestingly, there is no scientific consensus on the definitions of some other astronomical objects, including stars, galaxies, and nebulae.
