What do you call a moon that orbits another moon? A moonmoon of course!

It has long been known that planets orbit stars and that moons orbit planets. A new analysis suggests that there may be another level to this structure — moons that orbit moons.While a moon orbiting another moon is yet to be observed by astronomers, researchers argue that a moon orbiting a larger moon, which in turn revolves around a planet, could exist.Two astronomers, Dr Juna Kollmeier at Carnegie Observatories in California and Sean Raymond from the University of Bordeaux, used calculations and simulations to analyze just that possibility. Their findings about the same were pre-published on arXiv for peer-review.The pair found that ‘moonmoons’, moons that orbit moons, are, in fact, possible under certain conditions. A considerable difference in the size of the two moons and a large distance between them and the planet would be two such requirements.Saturn’s moons Titan and Iapetus, Jupiter’s Callisto and Earth’s own Moon could theoretically have had a moonmoon or a ‘submoon’ at some point in the past, a Gizmodo report says.[caption id="attachment_5359791" align="alignnone" width="1280"]

Janus and Tethys, two of Jupiter's moons, hint at one of the key requirements for a submoon to exist - that a moonmoon be considerably smaller than the moon it orbits. Image courtesy: NASA[/caption]There also isn't an official name that these objects have been given so far, and aren’t recognized by the International Astronomical Union (IAU). Though moonmoon is apt enough, methinks.Kollmeier and Raymond are far from discouraged by the lack of evidence for the existence of these hypothesized moons of moons. A tantalizing recent discovery made by Hubble and Kepler of a peculiar, enormous exoplanet called Kepler 1625-b beyond Neptune in nearby galaxy hints at the possibility that moonmoons could very well exist.The enormous exoplanet Kepler 1625-b was detected as it passed in front of its host star, Kepler 1625. Shortly after this, another, fainter dimming of the light from Kepler-1625 was detected as it reached the Kepler telescope's detectors.The discovery of the exoplanet's Kepler 1625-b and its moon used a method called the 'transit method', where light signals from the star it orbits are observed for interference. These light signals are interrupted as a celestial objects passes in front of the star, which researchers back on Earth use to gauge an object's distance, length of its orbit and its location in our galaxy, in some cases.Kepler 1625-b is ten times that of Jupiter and its moon likely the size of Neptune, as a recent discovery has pointed out.Kollmeier and Raymond view the Kepler 1625 system as a good candidate in which to explore the possibility of moonmoons, their study suggests.While it could still be years before we see a decisive answer, the moonmoon theory has inspired many, who have taken the liberty of crafting memes and proposing names for the hypothesized moonmoon.