What moons have moons

Space: can moons have moons?

Our moon, Callisto of Jupiter and the Saturn satellites Titan and Iapetus - only these four moons could theoretically have their own companions in our solar system. Nothing is known about this so far, and if there is no astronomical sensation, then we must turn our eyes to exomoons that are yet to be discovered in order to discover any moons of moons. Juna Kollmeier from the Carnegie Observatories in Pasadena and Sean Raymond from the Université de Bordeaux explain in an arXiv paper that they are not excluded in principle. For this purpose, the respective moon and its satellite - which could be called the sub or lunar moon - must be certain Meet prerequisites. The corresponding celestial body must therefore be so close to its moon that it is not attracted by the neighboring planet, but that gravity binds it to the actual satellite. At the same time, the lunar moon must not get too close to it, because otherwise gravity could tear it apart or throw it out of lunar orbit.

At the same time, the moons would have to have a sufficiently large gravity, which only occurs above a certain mass. In addition, they would have to be so far away from their home planet that a second moon could orbit the first moon without complications. In our solar system this would only apply to the four named moons - all others would be too small or too close to their planet. In addition, another requirement would have to be met. The submoon would have to get into the area of ​​influence of the actual moon with exactly the right speed so that it gets into an orbit around this satellite and not into one around the planet. "We can pretty much assume that there are no larger lunar moons at Saturn or Jupiter," explains Michele Bannister from Queen’s University Belfast to the NewScientist. And if the earth's moon ever had a submoon, it would have been thrown out of its orbit by the slow movement of our satellite away from the earth.

So moon moon friends must direct their hope to exomoons. And anyway: the first candidate for such an exomoon may have been discovered; it should be the size of Neptune. Compared to exoplanets, however, their detection is even more difficult - and according to the current state of technology it then seems almost impossible to detect an exomoon moon.