why only some planets have moons?

Question

mercury venus no moons

Answer 1

Wow Goffyhero, you've got some interesting answers here !

The actual answer is partially down to chance and partially down to the mass of the planet.

Very basically, the larger the planet, the more the mass, the greater the gravitational field, the larger amount of space it "controls" and therefore the higher likelihood of it having moons. Good examples of this are the gas giants (Jupiter and Saturn) who both have many moons.

There is also an element of chance involved. The positions of the bodies that are to become planet and moon have to be correct for the gravitational effects to result in an orbit. The best example of this is our own Moon, which has the largest mass ratio against its host planet (ie. Earth) in the Solar System. This could be explained by the Moon having been split off from the Earth very early in their geological history and not achieving a high enough velocity to escape gravity. This did not occur with Venus, for instance, despite the fact that its size and mass is similar to Earth's.

The reasons do get a lot more complicated than this, but I hope this brief and simple overview will be of some interest.

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Hi Mez - thanks for your extra comment. I agree that the Earth and the Moon have a mass ratio of 0.01230, and Pluto and its moon Charon have a mass ratio of 0.147, but I think the consensus of opinion is that the Pluto-Charon system is defined as a double planet rather than a planet-satellite system, as the size and mass are so close.

Answer 2

Gravity of the planet, which in turn depends on the mass of the planet and the volume are the parameters which contribute to the fact whether a planet has a moon or not.. the more the volume and mass, the more likely that the plantet has more moons.. I totally agree with the answer of "the_lipsiot"

Answer 3

You could also ask why some people don't have babies. The moons are the babies of the planets.

To the_lipsiot, what about Pluto and Charon? Charon has a much larger mass ratio to its host (Dwarf) planet, Pluto than the Earth and moon!

Answer 4

Dear Sir

As all of us might have few ideas about the universe. But is has a lakhs and lakhs of miracles. Everything is a molecular formation, only molecular atoms creates everything, you, me, the world etc., etc.,

For example if you keep a bread in a closed condition for few days, after you can see the fungus around the bread, and after few days the fungus become worms, and after few days it becomes insects,., like this there is lot planets in the universe, while molecular formation after lots of light years it will burst and splilts and hanging in the universe based on the gravity, few things may cool and few things may hot and too hot, where there is a cool atomosphere. (we will name it as moon, earth, etc,.)

So there is no specified reasons as you asked. Even after a long time our moon will change or divert to other place.

Hope you can understand this ?

Thanks and best regards

M.Vasanth