# Is the moon a planet?

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.

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newscientist.com/article/… – barrycarter Nov 16 '15 at 17:34

## 2 Answers

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.

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+1. I don't know why the official definition of a planet isn't just given as "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune or Pluto". It's an unambiguous definition that would end all controversy for ever, and would be easily justified by noting that those bodies don't fundamentally have much in common apart from their historical connection to the word planet, as you pointed out. – Nathaniel Feb 15 '13 at 7:49
If we discovered, say, a Neptune-sized body in a nearly circular orbit at 150 AUs, I think we should call it a "planet". And we've detected hundreds of "planets" orbiting other stars. If we're going to use the word, I think we should have a consistent definition for it. – Keith Thompson Feb 15 '13 at 17:05
I heard an interview with a planetologist (i.e. generalized geologist) on the matter of Pluto's status at one point, and he commented that his field has a rather different view of the question than observational astronomers: i.e. that Pluto has the characteristics that make it interesting to planetologists (self gravitational roundness and stratification, the ability to hold an atmosphere of sorts and so on). In other words, there may be good reason to have multiple definitions; say, no to Pluto as far as planetary system studies go but yes for planetology. – dmckee Feb 15 '13 at 17:51
Pluto is also interesting, especially because its heaviest moon, Charon, has a weight ratio of about 9 and a barycenter outside of Pluto. – fibonatic Aug 16 '13 at 1:42

Based on what the Moon is: size, composition, physical properties, etc., the Moon is certainly a planet.

Per the definition adopted by the IAU in 2006, it is excluded as such by the fact that it does not go where a "planet" should go. In other words it has not met the criterion of being located in a primary solar orbit having cleared it's orbit. But what about Earth? The same argument used to exclude the Moon could also be used to exclude the Earth, could it not?

But why the question?

Astronomers have debated and proposed their definition of the word "planet" with its narrow definition that works for them.

But the general public now needs a "word" to refer to such objects as the Moon and the Earth in a general sort of way. Of course they would go to the world of science in search for this word. And science has replied with a specific definition that is too narrow, and then has the adacity to scoff at the question as being "silly".

We live in an exciting time of discovery. Clearly a few new words are warranted. For example, the gas giants are definitely a different kind of object than the likes of Earth, Venus, or Mars, but it seems OK to describe these differing things with the adjectives jovian and terrestrial applied to the general noun, "planet". Earth is a terrestrial planet. Saturn is a jovian planet. What about the Moon? Is it proper to call it a lunar planet?

It is obvious to me that the public will, during the void, continue to use the word planet in its general meaning. The real "silly" ones are those who steal words from the common vocabulary, re-define them, insist upon (with precision) that their use be predicated only upon their new definition, and call as silly any question that arises with regard to the ensuing confusion.

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## protected by Qmechanic♦Aug 15 '13 at 23:49

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