Most people are just getting used to not counting Pluto among the solar system's nine — oops, eight — main planets. Now though, we may have to change our understanding of what planets are (again), as NASA scientists suggest a new definition.
Back in 2006, the International Astronomical Union (IAU) redefined what constitutes a planet in our solar system as follows: "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit." It was the last bit in this definition that effectively kicked Pluto out of the planet roster.
Now, NASA researchers led by by Alan Stern, the principle investigator for NASA's New Horizons Pluto mission, have proposed a geophysical definition. Instead of being dependent on whether a cosmic body orbits the Sun or not, the scientists looked to their intrinsic physical properties. According to the manifesto, "a planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."
A Matter of Opinion?
Stern was particularly adamant about the 2006 decision, as it was proposed by California Institute of Technology (Caltech) astronomer Mike Brown. "Why would you listen to an astronomer about a planet?" Stern told Business Insider's Kelly Dickerson back in 2015. "You really should listen to planetary scientists that know something about this subject," added Stern, who is a planetary scientist himself. "When we look at an object like Pluto, we don't know what else to call it."
Defining what a planet is isn't as arbitrary is it seems. Stern and his team had to come up with a definition that would reinstate Pluto back into full-planetary status, while at the same time leaving out a host of other cosmic objects — like stars and other stellar bodies, including white dwarfs, neutron stars, and black holes. The new definition, however, would include not just all known dwarf planets but also our very own moon.
None of this is final, of course. But if there's one thing we can conclude from this battle of definitions, it's this: there's still so much we don't know about the cosmos. Recent discoveries about Pluto have made it more obvious than ever that it could be defined as a planet, but more research is needed to finally conclude what a planet is.