A recently published paper by a NASA scientist argues that our moon should in fact be classed as a planet. Alan Stern says the current definition of a planet is only useful for astronomers.

At the moment, what is and isn’t considered a planet is decided by the International Astronomical Union. It has three criteria, all of which must be met:

• is in orbit around the Sun;
• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and
• has cleared the neighbourhood around its orbit.

That definition was passed by the IAU in 2006 with the notable effect that Pluto was reclassified as a dwarf planet, rendering mnemonics outdated and creating havoc in pub quizzes and trivia nights.

Stern was not particularly happy given that he headed the New Horizons project that sent a craft to explore Pluto and its surroundings. The reclassification came seven months after the craft left Earth and almost a decade before it arrived at Pluto.

He says that the public put too much significance into the term “planet” and wrongly assume that anything not a planet isn’t worth exploring.

“’In keeping with both sound scientific classification and people’s intuition, we propose a geophysically based definition of planet that emphasises a body’s intrinsic physical properties over its extrinsic orbital properties.”

Stern’s proposed definition is as follows:

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.

(Or as USA Today put it, “round objects in space that are smaller than stars.”)