杏吧原创

Stars are orbited by planets, which are orbited by moons, but what comes next? More moons, according to a new analysis.

Juna Kollmeier at Carnegie Observatories in Pasadena, California, and Sean Raymond at the University of Bordeaux, France calculated whether a moon orbiting a planet could have a moon of its own.

A moon of a moon has no formal name, perhaps because we have never spotted one, but both submoon and moonmoon have been suggested.

Such an object would have to be close enough to its host moon to remain gravitationally bound to the moon instead of the larger planet, but not so close that it would get ripped apart or pulled out of orbit by the moon.

Threading this needle is difficult, but not impossible, Kollmeier and Raymond found. They calculated that there are four moons in our solar system that could theoretically have submoons: Earth鈥檚 moon, Jupiter鈥檚 moon Callisto, and Saturn鈥檚 moons Titan and Iapetus.

These moons are all relatively large and far from their planets, so there is a small area in orbit around each of them where the planet鈥檚 gravity might not steal a moonmoon away.

But even if these moons are able to host a moonmoon, it would be exceedingly difficult to get one in the right place, says Raymond.

鈥淪omething has to kick a rock into orbit at the right speed that it would go into orbit around a moon, and not the planet or the star,鈥� he says. And if that moon moved around over the course of its evolution, like Earth鈥檚 moon has, it鈥檚 unlikely the submoon would stick around.

That may be part of the reason that there don鈥檛 appear to be any submoons in our solar system. 鈥淚 think we can say for sure that there鈥檚 not a moonmoon that鈥檚 kilometres across around Jupiter or Saturn,鈥� says Michele Bannister at Queen鈥檚 University Belfast, UK. 鈥淎 moonmoon down to the size of a skyscraper could exist out there, but I鈥檇 call it moonmoonlet.鈥�

There has also been about a moonmoon orbiting a distant moon that may be the first ever spotted outside our solar system, at a planet called Kepler-1625b. That exomoon, if it exists, is likely a gas giant, orbiting an even larger gas giant.

鈥淭his system where you鈥檝e got a giant planet and a Neptune-sized moon that鈥檚 kind of far away from the planet is sort of the best-case scenario for a moonmoon,鈥� says Raymond.

Unfortunately, it would be difficult to spot a moonmoon outside of our own solar system. 鈥淚t鈥檚 a little blip on top of a blip on top of a blip,鈥� says Raymond. 鈥淚 don鈥檛 have a ton of hope for finding one anytime soon.鈥�

Reference: