Will our moon join the planet club?

Contributed by
Mar 27, 2017

For a relatively small celestial object, Pluto has been spotlighted in an enormous ongoing debate about what constitutes planetary status (or the lack thereof)—could our moon be next?

Planetary scientist Alan Stern, Pluto’s #1 fan and now possibly the moon’s, thinks so. Both famous and infamous for sending NASA’s New Horizons mission to Pluto, his probe only reached it the once-planet by the time it got demoted by the International Astronomical Union (IAU). This prompted backlash from Stern that may or may not have involved a certain eight-letter word regarding his opinion on the validity of Pluto’s downgraded status, along with the pressing question of why New Horizons was even sent to study Pluto if its subject was no longer a planet.

“To mitigate this unfortunate perception,” declared Stern and NASA colleagues in a controversial proposal, “we propose a new definition of ‘planet’, which has historical precedence. In keeping with both sound scientific classification and people’s intuition, we propose a geophysically-based definition of 'planet' that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties."

So what exactly does this mean for the moon?

Backtrack several thousand years. Stern evidently had supporters even before humanity could launch anything into space. Even Aristotle believed the moon to be a planet. While the mystery factor of the “man in the moon” enticed the ancients into observing it, an anomalous astronomical coincidence convinced them it was more than just a satellite in Earth’s shadow. We apparently have a rebel moon, because it doesn’t orbit in (or extremely close to) the equatorial plane of our planet as other moons do. Even the Greek word for moon has nothing to do with sub-planetary objects; it literally translates to “thing that shines”, referring to the sunlight it reflects in the night sky.

18th Century map showing the moon’s peculiar orbit.

Unfortunately for Aristotle (and Stern), the Renaissance saw a rebirth of ideas as to what the moon should be classified as. Copernicus’ telescope told of a heavenly body that orbited Earth rather than the sun. This is how it first got downgraded to a satellite, which derives from “satelles”, meaning “servant”. Arguments between astronomical purists who believed moons were planets and more radical thinkers who insisted they were satellites hardly helped. Galileo’s then-groundbreaking discovery of Jupiter’s moons made it lose even more of its luster, because it was no longer "the" moon.

Stern believes that astronomers’ opinions of planets are too obscured by their studies of every other floating object and cosmic phenomenon in space, while planetary scientists such as himself zero in exclusively on moons, planets, and planetary systems. He even likened asking an astronomer for an opinion on Pluto or the moon’s planetary status to expecting brain surgery from a podiatrist. An apology to podiatrists everywhere, but the IAU’s definition of planethood is relatively narrow. Planets have to orbit the sun, need enough gravity to assume a round shape and should be gravitationally dominant enough to clear their orbital zone. Now, wait for the irony. Comets and other smaller cosmic objects are always shooting through planetary orbits—which makes absolute zone clearing impossible.

Taking the obvious zone clearing flaw into account, Stern proposes thata 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.” Read: planets are any round objects in space smaller than stars. That would qualify both Pluto and the moon.

 Whether or not the IAU will accept this proposal, the planet Luna is now at least a possibility.

(via Space.com)