I heard the Moon one day might be declared to be a planet. When and Why?

Answer 1

This has been one of my favorite questions to debate about for years!!! The earth and moon really even as defined today are a binary planetary system.

Above someone said that the moon orbits around a point on the Earth while from our perspective here on Earth that looks to be true. Move above the solar system and look at it and you see it doesn’t the moon itself orbits around the sun. We never make the moon (Luna) move backwards in it’s orbit, the moon continues on a counter clockwise orbit around the sun at all points. What the Earth does to the moon is effect it’s orbital speed by our own mass. We speed up and slow down the moon, we draw it closer and then farther away from the sun. Our effect on the moon is to give it a scalloped looking shape as it orbits around the sun. Not the curly cue look that would be needed to say it orbit’s the Earth. This same curly cue shape the moon does not make as it orbits around the sun EVERY other object classified as a moon in the solar system does make.

So you say well I can see the moon orbits around the Earth so it is a satellite. If we were on the moon we would see the same thing. It looks like the Earth orbits around the moon if you are on the lunar surface. If you made this assumption when you were on the lunar surface you would be just as correct as making the assumption we all were taught that the moon orbits around the Earth. Why? Because the moon wobbles the Earth in it’s orbit around the sun the same way the Earth wobbles moon in it’s orbit around the sun. If you plot either the moons orbit around the sun or the Earths orbit around the sun both are scalloped shaped and not elliptical. The moon wobbles the Earth much less then the Earth wobbles the moon, but we are both orbiting the sun yet are so close that we effect each others orbit.

This is very similar to the way stars rotate around each other in a binary star system. Neither star orbits the other, but they both dance around each other.

If you are familiar with how Galileo and Copernicus figured out using retrograde motion that the Sun is the center of the solar system, then this might make sense as proof that the moon does not orbit around the Earth. The sun never shows retrograde motion if you observer it from the moon.

Answer 2

Under the original IAU draft definition, any body that was held in a spheroidal shape by gravity (as the Moon is) would have been considered a planet if it was neither a star nor a satellite of another body. A satellite was to be defined as a small body orbiting a barycenter inside of a larger body. The barycenter of the Earth-Moon system is not at the center of the Earth, but is inside of it. On the other hand, the barycenter of the Pluto-Charon center is actually outside of both bodies, and that's why that definition would have christened Charon as a planet.

However, momentum exchange is very gradually causing the Moon to move further from the Earth. Eventually, billions of years from now, the Moon will have moved so far from the Earth that their barycenter will actually be outside of the Earth, and then the Moon would qualify as a planet under this definition. This is a moot point, though, because that definition was not adopted. Instead, a different definition was made official, and it includes the requirement that the planet has cleared the neighborhood of its orbit. If the potential planet meets all other requirements, but not this last one, it is instead called a "dwarf planet." Thus, the Moon could possibly become a dwarf planet in the distant future, and it would be binary with the Earth instead of being its satellite. However, the new definition does not preserve the definition of satellite that was used with the previous draft, so it's not clear if it still applies.

However, all of this is still most likely a moot point, because by the time the Moon would have moved far enough from the Earth for this to be a relevent discussion, the Sun will have entered its red giant phase and probably swallowed both the Earth and the Moon.

Answer 3

Scott * is absolutely correct. As anyone who has studied astronomy on a college level knows, the moon does NOT orbit the earth, it orbits the sun. The fact that the moon and the earth are gravitationally locked together is a matter of coincidence only and results from the moon's gravitational capture.

That being said, I doubt the moon will be widely accepted as a planet simply because it would be too difficult to buck thousands of years of institutionalization. There may come a time when the classification of the moon as a planet is passed within the scientific community and becomes a point of passing trivia (perhaps a question in the Trivial Pursuit game).

But here is a point to ponder. As anyone can see (if they care to work out the math) the moon actually orbits the sun. Therefore, it is not actually a moon, and the earth has failed to clear its own region of space of debris (in this case, the moon). So does this mean that (based on the present new definition of a planet) the earth itself may lose its classification of a planet?

Something to think about.....

Answer 4

The only way I'd think the Moon would get in is because it's so close to the size of the Earth that it doesn't really orbit the Earth... rather, both spin around an imaginary epicenter, a point in space closer to the Earth but that's between the two bodies while they're both orbiting the Sun at the same time. One could then declare both "twin" planets.

Answer 5

not going to happen. to be considered a planet (a double planet : earth-moon), the moon would need to rotate not around a pooint located inside earth, but outside of it (in space). In that case, earth and the moon would both be rotating around each other, and not only the moon around the earth like right now.

BUT: the moon is slowly going away from us... And we know the moon would be far enough from earth to rotate slightly outside of earth in a LONG LONG time... so it would become a planet, in a LONG LONG time...

HOWEVER, I've been using the conditional form "would" and not "will" in the last paragraph for a reason: the moon will be far enough of earth only after the sun changes into another type of star and fry earth and the moon in 5 billion years. So, earth and the moon will be taken inside the sun long before they are far enough apart to become double planets.

Answer 6

That may happen since the moon is very close to the earth in size to where it could be called a binary planet system and that the moon does in a way not orbit the earth per se and orbits the sun in addition to that. When it will happen nobody knows for sure.

Answer 7

I don't think so. One of the requirements of a planet is that it be orbiting a sun. Unless they want to declare the Earth - Moon system to be a double planet.

Answer 8

The moon can't be a planet since it orbits a planet not a star.

Answer 9

If you like, and for a small fee, I will declare the moon to be a planet for you. You can choose the time and the reason.

Answer 10

The IAU has now come up with a standardized definition of a planet, and the moon does not fit the criteria. It does not orbit the Sun, it orbits the Earth which orbits the sun. I doubt that will ever happen.