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An object formed from dust, rock and other materials (small planets) are planetismals. See the Related Link for more information.
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∙ 10y ago
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Why Mercury has no moons?
Because it doesn't. That's just the way it worked out. Mercury has no natural satellites that we know of. Alternatively, Jupiter has at least 62 known moons. It is not for any reason, it's just the way things turn out. I am a different person and say there are none. I think that there is more to it than that. If one was to hypothetically place a natural satellite in any reasonable orbit around Mercury then it becomes possible to calculate the gravitational force between that satellite and firstly Mercury, secondly between the satellite and the Sun. When I did this calculation I learned that the gravitational force between the satellite and the Sun was greater than that between the satellite and Mercury. This tells us that if there had ever been a satellite around Mercury then the Suns influence would have pulled it away from the planet into some sort of orbit around the Sun. This may have happened during the formation of the Solar System or it may not. Any satellite pulled out of such an orbit would have become just another planetismal in the chaos that was the Solar System at the time. The satellite would have collided with some other body, been ejected from the Solar System or been pulled into the Sun itself. It is long gone. Carrying out such calculations for Venus and the Earth is also quite instructive. Venus cannot hold onto a natural satellite either. The Earth and the Moon present a different problem. The gravitational force between the Moon and the Sun is greater than that between the Moon and the Eath. How then is it possible for the Earth to have retained the Moon for over four billion years ? One has to consider that there is another set of related forces acting between bodies as well as the straightforward gravitational force. This is the Tidal Force. Calculating the maximum tidal forces for the above cases shows that it is only in the case of the Earth and the Moon that there is enough force for one of the Solar Systems inner three planets to hang on to a natural satellite. We could easily have lost the Moon long ago had things like the mass of the Moon or the distance from the Earth to the Sun been different. Isaac Asimov wrote a short article in the 1960's on this very topic. He considered all of the planets and their known satellites at the time. It is still relevant and interesting today and well worth reading.
