Before answering the question, I first have to explain how satellites or moons are formed.
There are two ways.
In some cases, when a solar system is forming, there are collisions between objects, between dwarf planets and other dwarf planets, or between planets of relatively similar sizes.
In these collisions, clouds of debris are generated that are gravitationally linked to the planet and it is in them that moons are formed.
These moons are associated with the planet by gravity and it is very unlikely that one moon is linked to another, unless it is a very binary system, since, due to gravity, the two will be most attached to the planet with the greatest mass. and, therefore, its gravitational attraction is greater.
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The other way moons are formed is by capture.
When solar systems are forming there is a super chaotic state of many bodies colliding with each other.
What could occur here would be captures of larger bodies that, in turn, had smaller bodies captured, that is, planets with their moons.
But what happens in these captures is that we find ourselves with the problem of the three bodies, which we already know is a problem in all areas.
When a capture occurs, it is highly probable that the smaller of the bodies will be ejected because the two larger bodies act as if they were a slingshot.
So although in this type of satellite formation, in theory, it could be the case of the capture of a planet with a satellite that, when captured, would become a moon and a mini-moon associated with it, what happens is that the less massive body it's the one that takes basically all the gravitational pull and shoots out.
A very specific configuration would be needed in which the moon was a certain distance away and the moon's satellite was of such a mass that it would still be attracted to the moon and not the planet.
But it would be an extremely unlikely range of parameters.
The most probable thing is that either the body with the least mass will be thrown away or in the end everything will be dominated by the planet's gravity, so that, in the end, the satellite and its mini-satellite become two moons of the planet.
The question of the moons is very interesting.
In fact, in our Solar System, almost all the planets, even dwarf planets like Pluto, have satellites.
But there are also exceptions to highlight such as Venus and Mercury that do not have.
The rest of the bodies in the Solar System all have many moons but they are always dominated by the planet.
We are not aware of any cases, neither in our Solar System nor among extrasolar planets, in which moons with their own natural satellites have been detected.
Yes, these possible configurations have been studied in theory and what has been seen is that they are not stable configurations.
What can be given is moons with rings.
They must be very, very compact rings because if not, due to the tides, everything would deform too much.
But it has been seen, theoretically not observed, that a moon could have rings.
Also in a very restricted range.
But in theory this would be possible.
Amelia Bayo
is a doctor in astrophysics and the scientist responsible for the MOONS instrument at the European Southern Observatory (ESO) in Germany.
Question submitted by Hanin Ben Ayad
Coordination and writing:
Victoria Toro
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