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Where is the Submoon of Earth?



The earth's moon. Credit: Public Domain

"Can moons have moons?"

This simple question posed by Carnegie's four-year-old son, Juna Kollmeier, has started it all. Not long after this first bedtime co-ordination, Kollmeier coordinated a program at Kavli Institute of Theoretical Physics (KITP) on the Milky Way, while her former college classmate Sean Raymond from the Université de Bordeaux participated in a parallel KITP program on KITP dynamics Earth-like planets. After discussing this very simple question in a seminar, the two teamed up to solve it. Their findings form the basis of an article published in the Monthly Notice of the Royal Astronomical Society .

The duo launched an internet firestorm late last year when they released a draft of their article that tested the possibility of moons orbiting other moons on a preprint server for physics and astronomy manuscripts.

The online conversation was obsessed with the best term to describe such phenomena with options such as Moon Moon and Mini Moon. The nomenclature, however, was not the subject of Kollmeier and Raymond's investigations (though they have a preference for submonsibles). Rather, they wanted to define the physical parameters for moons that would be able to be stably circumnavigated by other, smaller moons.

"Planets orbit stars and moons orbiting planets, so it was natural to ask if smaller moons could be larger," Raymond explained.

Their calculations show that only large moons in broad orbits can pick up submunitions from their host planet. Tidal forces from both the planet and the moon destabilize the orbits of submobos orbiting smaller moons or moons closer to their host planet.

They found that four moons in our own solar system are theoretically able to record their own satellite submamoises. The Jupiter's moon Callisto, Saturn's moons Titan and Iapetus, as well as Earth's own moon, all fit into the bill of a satellite that could host its own satellite, though no such has ever been found. However, they add that further calculations are needed to address possible sources of sub-fund instability, such as the uneven concentration of mass in the lunar crust of our moon.

"Theoretically, one could support such objects, give us clues as to how our own and neighboring planets have formed, to which there are still many open questions," said Kollmeier.

It is believed that the moons orbiting Saturn and Jupiter were born from the disk of gas and dust surrounding the gas giant planets in later stages of their formation. It is believed that our own moon arose after a huge impact between the young Earth and a body in Mars size. The lack of stable submamoises could help scientists better understand the various forces that shaped the satellites.

Kollmeier added, "This, of course, could affect ongoing efforts to understand how planetary systems evolve elsewhere and what our own solar system is like. The system fits in with thousands of others discovered by planetary hunting missions."

For example, the newly discovered potential Exomoon orbiting the Jupiter-sized Kepler 1

625b found the proper mass and distance from its host to support a submono, Kollmeier and Raymond. The derivative inclination of its orbit, however, could make it difficult such object remains stable. However, it would be very difficult to discover a submoon around an Exomoon.

Faced with the excitement of finding potentially habitable exoplanets, Kollmeier and Raymond calculated that the best case for living on large submoons are the stars. Although very common, small red dwarf stars are so weak and their habitable zones so close that tidal forces are very strong and submoons (and often even moons themselves) are unstable.

Finally, the authors point out that an artificial submoon is possible stable and serve as a time capsule or outpost. In a stable orbit around the Moon – like the one proposed by NASA for the Lunar Gateway – a Submoon would keep mankind's treasures safe for posterity long after Earth was unsuitable for life. Kollmeier and Raymond agree that there is still much to do (and to have fun) to understand sub-moons (or their absence) as a rocky record of the history of the planet-moon systems.


Explore further:
Searching for other lives on other moons

Further information:
Juna A Kollmeier et al. Can Moons Have Moons?, Monthly Notices from the Royal Astronomical Society: Letters (2018). DOI: 10,1093 / mnrasl / sly219


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