A new study has revived the vexed question of whether our Moon qualifies as a planet based on a new definition of what a planet is. It also makes the case for reinstating Pluto as a planet.

Pluto was demoted from a planet to a "trans-Uranian dwarf planet" in 2006 by the International Astronomical Union (IAU).

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The published technical research paper that supports these claims, "A Geophysical Planet Definition," was written by a five-person team that includes Sol Alan Stern, an American engineer and planetary scientist.

Stern achieved fame as the principal investigator of the New Horizons mission that made its spectacular flyby of Pluto in July 2015, and is the Chief Scientist at Moon Express, an American start-up that wants to mine the Moon.

Stern is adamant the Moon should be upgraded to a planet. He also believes two of Jupiter's moons and two of Saturn's moons should also be classified as planets. Pluto should be made a planet once again.

Stern et al believe the existing criteria for determining what constitutes a planet is obsolete and needs to be rewritten. Their scientific paper argues the geophysics of a planetary body should determine whether it is a planet. This determination shouldn't be based on whether or not this body orbits the Sun.

Stern contends that in the mind of the public, the word "planet" carries a significance lacking in other words used to describe planetary bodies. The corollary is that "non planets" cease to be interesting enough to warrant scientific exploration.

He revealed that a question asked frequently of him is "Why did you send New Horizons to Pluto if it is not a planet?"

A lasting answer to this perplexing query, it seems, is developing a new definition of planet.

"In keeping with both sound scientific classification and people's intuition, we propose a geophysically based definition of planet that emphasizes a body's intrinsic physical properties over its extrinsic orbital properties," he said.

"Why would you listen to astronomers about a planet (instead of) planetary scientists that know something about this subject?" asked Stern.

Tagsmoon, planet, Pluto, trans-Uranian dwarf planet, International Astronomical Union, Alan Stern, New Horizons mission, planetary scientists