The debate over whether Pluto should be called a planet or not has raged for years, coming to a head in 2006 when it was reclassified as a dwarf planet by the International Astronomical Union (IAU). Now, that debate is being ignited once again.
Alan Stern from the Southwest Research Institute in Boulder, Colorado, the man who led the groundbreaking New Horizons mission to Pluto in 2015, has co-authored a paper calling for a reclassification of what a planet is. It proposes a new definition that would essentially make any large body in space, including a moon, a planet.
The paper was published by the Lunar and Planetary Science Conference, where it will be presented in Texas next month by lead author Kirby Runyon from Johns Hopkins University in Baltimore, Maryland.
“There would be three criteria [for a planet],” Stern tells IFLScience. “Number one is it’s a body in space. Number two is it's large enough to be rounded by its own gravity. And number three, it’s not so large that it starts fusion in its interior.”
Were this to be adopted, it would essentially increase the number of planets in our Solar System from eight at the moment to approximately 110. It would include the major planets, dwarf planets like Pluto and Ceres, and large satellites like Titan, Europa, and even our own Moon.
Alan Stern (left) was the principal investigator for the New Horizons mission. NASA/Bill Ingalls
The purpose of the proposed reclassification is to highlight that worlds like Pluto are no less important than the supposed major planets. “To mitigate this unfortunate perception, we propose a new definition of planet, which has historical precedence,” the paper notes.
Currently, the IAU definition of a planet requires three criteria to be fulfilled. First, it must be a celestial body in orbit around the Sun. Second, it must have sufficient mass for its self-gravity to make it round. And third, it must have cleared its neighborhood in its orbit.
This latter definition in particular led to the reclassification of Pluto. But it’s also been the most controversial aspect, with many pointing out that worlds like Jupiter and even Earth do not fulfill this criteria. Earth, for example, has many asteroids in its neighborhood that have not been cleared out, as does Jupiter.