I'm pretty sure that every natural satellite (moon) that we have discovered so far is smaller in size than the planet it orbits. Is it possible for it to be larger and still orbit that planet?
Anonymous
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My first thought was that the larger object would always be the planet, but if one of these objects were much more dense than the other, the more massive object could be smaller than the less massive object. An object with the same size as Neptune and the same composition as Earth would be smaller and yet more massive than an object which has the same size and composition as Uranus.
zahbudar
No. That does not seem possible to me. The question is can a larger body orbit a smaller one. For this to happen, the smaller body must be dense in mass while the larger body is light in mass. This might happen if the orbiting body was mostly gas as in some of our gas giant planets. However, using the Earth as the massive body sort of puts limits upon the size of the gas planet which might orbit it as a moon. Given that sort of constraint, I don't think that the gaseous moon would posses enough gravity to retain its gas molecules. As a result, the gases would eventually bleed off into space and the gaseous moon would evaporate/dissipate.
gintable
My first thought was "no, by definition". But the planet vs moon definition isn't by size, it is by mass. Technically, it is by orbital characteristics. The body that wobbles in a trajectory less than its own size is the planet, and the body that obviously moves around the other is the moon.
In principle it could be possible to have a dense and massive planet with a sparse and geometrically large moon.
For example, suppose Earth were the dense and massive planet, and suppose the hypothetical moon were of the density of Saturn (about the density of wood), a tenth the density of Earth (about that of cast iron). It is reasonable to use this example, because Earth and Saturn have the most dissimilar densities of all the spherical bodies in the solar system.
For this moon to be the mass of Earth, its radius would be about 2.15 Earth radii. So if its radius were only 1.1 Earth radii, then its mass would only be about 13% that of Earth. As long as its orbit were close enough that the two body barycenter were still within Earth, you could call it a moon.
The likelihood of any of this happening is far from possible. Sparse moons like that would need to be gaseous just like Saturn, which would imply that they'd need to form outside of where terrestrial planets form. Also, a gaseous moon like this couldn't remain in-tact, because its own material wouldn't be bound by its own gravity enough to keep it from diffusing away. Plus, the puffy moon damn well better not cross the interlagrangian point between its center and the planet, because if it does, then its gas will fall upward and spiral in toward the planet, much like in star-black hole relationships.
Anonymous
Mass is the key determining factor as to where the center of gravity is located. In order for an object with greater volume to orbit one of lesser volume, the one with lesser volume would need to have more mass... if it has more mass and less volume, then it has a much greater density. All the objects in our solar system fit the model of the larger object having greater mass as well, and with very few exceptions, this barycenter is located within the surface of the primary, while the secondaries orbit around the larger body. If you could somehow find a way to condense the mass of the Earth into a volme less than that of the Moon, and kept the mass of both objects the same, the Moon's orbit would not change. But this smaller Earth of equal mass would have a smaller radius. If this radius was too small, the barycenter would lie closer to the 'New Earth' but not necessarily inside of its surface. The two would orbit the barycenter, just like they do now. But in the case of Pluto and Charon, the barycenter lies outside of both objects.
Irv S
Barely, but it would be extremely odd.
The more massive body would be the 'planet', and the lighter one
would orbit around it.
The densities would have to be very different indeed for a moon to be larger
than it's 'primary'.