October 11, 2018

• Moons can have moons and they are called moonmoons

  Iapetus, a moon of Saturn, could have its own moon

  By Leah Crane of New Scientist

  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’s moon, Jupiter’s moon Callisto, and Saturn’s 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’s gravity might not steal a moonmoon away.

  Read more: First known exomoon could be a baffling monster the size of Neptune

  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.

  “Something 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,” he says. And if that moon moved around over the course of its evolution, like Earth’s moon has, it’s unlikely the submoon would stick around.

  That may be part of the reason that there don’t appear to be any submoons in our solar system. “I think we can say for sure that there’s not a moonmoon that’s kilometres across around Jupiter or Saturn,” says Michele Bannister at Queen’s University Belfast, UK. “A moonmoon down to the size of a skyscraper could exist out there, but I’d call it moonmoonlet.”

  There has also been some speculation 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.

  “This system where you’ve got a giant planet and a Neptune-sized moon that’s kind of far away from the planet is sort of the best-case scenario for a moonmoon,” says Raymond.

  Unfortunately, it would be difficult to spot a moonmoon outside of our own solar system. “It’s a little blip on top of a blip on top of a blip,” says Raymond. “I don’t have a ton of hope for finding one anytime soon.”

  Reference: arxiv.org/abs/1810.03304

  • 7:47 am
  • 1 Comment