What exactly qualifies as a planet? Astronomers have disagreed on this seemingly basic point for years, and the disagreement has led to clear battlelines being drawn.
On the one side is the International Astronomical Union (IAU), which recognises only the existing eight planets within our Solar System. But on the other side is a group of astronomers headed by US planet researcher Philip Metzger from the University of Central Florida, who has worked more than 30 years with NASA. His group aims to simplify the definition of a planet, with the single qualification that a planet is a heavenly body which is now or has been geologically active. That’s it.
“We all know what a planet is,” Philip Metzger tells Science Illustrated. “It’s a geologically complex world.”
Metzger’s definition is simple, but the consequences are large. Such a definition would include not only currently-excluded Pluto, but also the Moon
