Is the moon a planet?

Question

Can our moon qualify as a planet? With regard or without regard to the exact definition of the planet, can the moon be considered as planet as Mercury, Venus and Earth etc. not as the satellite of the planet Earth.

Answer 1

Officially, no -- but there is a weak case to be made that the Moon orbits the Sun rather than the Earth.

If you trace the Moon's path in a Sun-centric frame of reference, that path is completely convex. Quoting this Wikipedia article:

Unlike most other moons in the Solar System, the trajectory of the Moon is very similar to that of its planet. The Sun's gravitational pull on the Moon is over twice as great as the Earth's pull on the Moon; consequently, the Moon's trajectory is always convex (as seen when looking inward at the entire Moon/Earth/Sun system from a great distance off), and is nowhere concave (from the perspective just mentioned) or looped.

If the Earth suddenly vanished, the Moon would continue in its orbit around the Sun (the same is true of any satellite), and it's big enough that it would be considered a planet (if there were anyone left to consider it at all).

On the other hand, in an Earth-centric frame of reference, the Moon is in an elliptical orbit around the Earth (with some significant, but not overwhelming, perturbations caused by the Sun's gravity).

From the same article:

Some consider the Earth–Moon system to be a double planet because of the relatively large size of the Moon; the Earth–Moon mass ratio, about 81:1, is much smaller than that of most other natural satellites in the Solar System. An informal criterion for a double planet system is that its barycentre must be exterior to both bodies. By that criterion, the Earth and Moon are an ordinary planet–moon system, because their mutual distance is about 60 Earth radii but their mass ratio is 81:1 so their centre of mass always lies well within the Earth.

But that seems like a very arbitrary criterion. If the Moon were either about a third farther away or about a third more massive, the barycenter would be above the Earth's surface. Intuitively, it doesn't seem (to me) that that should be enough to make it a planet. But if the Earth and the Moon were of roughly equal mass, orbiting each other at about the same distance, then we'd probably call them a double planet system -- and the official definition of a planet would probably have been written a bit differently. (Then again, any such criterion probably has to be arbitrary.)

This article also discusses the controversy over whether the Earth-Moon system might be considered a double planet. The conclusion is that it isn't -- and the 2005 IAU resolution that established the current definition of "planet" explicitly lists the eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

So no, the Moon is not officially considered to be a planet. But it certainly has some of the characteristics of a planet. And really, the issue is more about the meaning we assign to the word than about objective astronomical or physical reality.

Any definition of the word "planet" is necessarily arbitrary. Consider three Solar System bodies: Jupiter, Mercury, and Ceres. Mercury and Ceres are much more similar to each other than either is to Jupiter, yet Jupiter and Mercury are both classified as "planets", while Ceres is a "dwarf planet" (formerly merely an asteroid). If we were assigning names starting with our current knowledge, I suspect the classification would be quite different, but the name "planet" goes back to the time when Jupiter and Mercury were both nothing more than dots of light in the night sky, and Ceres was unknown. The word "planet" comes from a Greek word meaning "wanderer", used for the few "stars" in the sky that move relative to the others.