More on Planets
The answer in short is: the moon doesn’t fit the definition.
I took time to research the issue and found this page http://www.space.com/scienceastronomy/060816_planet_qanda.html
I have taken the liberty of including some quotes from this site to give a proper explanation for the answer I gave.
Q: What is the exact wording of the official IAU proposed definition of “planet” in “Resolution 5 for GA-XXVI”?
A: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
Q: Is Pluto a planet?
A: Yes. In fact, Pluto’s large companion named Charon is also large enough and massive enough to satisfy the definition of “planet”. Because Pluto and Charon are gravitationally bound together, they are actually now considered to be a “double planet.”
So that still sounds like a contradiction of the definition.
Q: What is a “double planet”?
A: A pair of objects, which each independently satisfy the definition of “planet” are considered a “double planet” if they orbit each other around a common point in space that is technically known as the “barycentre”. In addition, the definition of “double planet” requires that this “barycentre” point must not be located within the interior of either body.
Ok so Charon is not a satellite (moon) then…
Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”?
A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth.
Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the “barycentre”) that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be “satellites” rather than planets.
Q: Why is Pluto-Charon a “double planet” and not a “planet with a satellite”?
A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double planet.”
So there you have it.
Later-
XR4-IT
