You'll remember the International Astronomical Union's redefinition of a planet in 2006 as the move that demoted Pluto to a mere dwarf planet. But scientists recall the change for another reason: The new classification is incredibly vague. Basically, any nearly round celestial body that orbits the sun and has sucked in or flung away all objects around its orbit is now deemed a planet, reports Motherboard. While the latter requirement is what felled Pluto, the IAU definition as a whole refers only to planets in our solar system, notes UCLA planetary scientist Jean-Luc Margot. "I want a classification that applies both to the solar system and to exoplanets," he tells New Scientist, and he's come up with a mathematical formula, based on the size of the planet's orbit and the mass of its sun, to make it happen.
The formula determines if a celestial body has the mass necessary to clear its orbit and "when a body has sufficient mass to clear its orbital neighborhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape," Margot says in a release. The formula can be applied to 99% of 5,000 known exoplanets, he adds, and would allow scientists to identify a new exoplanet without the need for high resolution images. Oddly enough, if the IAU adjusts its definition to include Margot's formula at the next general assembly in 2018, the moon would become a planet because it has the critical mass necessary. Keep in mind, however, that "the IAU has not defined the term 'satellite,'" Margot says. "When they do, that will affect what they might decide about double planets versus satellites." (This astronomer thinks Pluto should be a planet again.)