Most planets in our solar system have orbiting moons and even some asteroids have their own moons. But do any moons have moons ? Is it possible? Could there be so-called Submoons ? Juna Kollmeier of Carnegie Science said her 4-year-old son had sparked her interest in the subject when she asked this seemingly logical question. It's a simple question. If most other objects in the solar system can have moons, then why not moons themselves?
Kollmeier and her colleague Sean Raymond of the Université de Bordeaux decided to try to answer the question. Their findings have now been published in a new peer-reviewed article in the February 2019 issue of the Monthly Notices of the Royal Astronomical Society .
As explained by Raymond in a statement by Carnegie Science:
] Planets orbit stars and moons orbiting planets, so it was obvious to ask if smaller moons could orbit larger ones.
So far, no submobos have been found orbiting any of the moons most likely to support them – the Jupiter's moon Callisto, Saturn's moons Titan and Iapetus, and Earth's own moon. Kollmeier:
The absence of known submoons in our solar system, even orbiting moons that could theoretically support such objects, may provide clues to the emergence of our own and neighboring planets, for which many questions remain.
The researchers found that only large moons in broad orbits of their host planets can take submoons. Normally, any submoons orbiting smaller moons closer to their planet would destabilize their orbits by tidal forces. Jupiter's big moon Callisto, Saturn's big moon Titan, another moon of Saturn named Iapetus, and the earth's moon could theoretically have all the submoons. Why not?
According to the researchers, there may be other sources of submoon instability, such as the uneven mass concentration in the earth's crust.
Part of the answer may also be related to the formation of the primary moons. It is believed that the Earth's moon was created by a collision between the Earth and another body about the size of Mars – and that collisions may have contributed to life on Earth. Some other moons, such as Jupiter and Saturn in orbit, are from the same cloud of gas and dust that formed the planets themselves. Kollmeier added:
This, of course, could influence ongoing efforts to understand how planetary systems are evolving elsewhere and how our own solar system fits in with the thousands of others discovered by planetary hunting missions.
Maybe that's the case In many, or even most, cases, there are several factors that make the orbits of Submoons inherently unstable. Knowing if this is true or not may have to wait for the discovery of moons orbiting distant exoplanets. Moons themselves are much harder to discover and so far only one promising candidate has been found – a possible exomoon orbiting the Jupiter-sized exoplanet Kepler-1625b. This possible moon – about the size of Neptune – is big enough and far enough away from its planet that Submoons should be possible. Astronomers must first check the primary moon, if any, before looking for submonsons.
Even though the Earth Moon does not have a Submoon now, according to the researchers, it could be an artificial in the future, perhaps the NASA's planned Lunar Gateway. The Lunar Gateway would help to establish humanity's presence in space, as stated by William Gerstenmaier, Director of Human Exploration and Operations at NASA Headquarters:
The Gateway will be a strategic presence in the Cislunar Give room. This will drive our activities with commercial and international partners and help us explore the moon and its resources. We will eventually transfer this experience to human missions to Mars.
Raymond has also written a cool poem about moons with moons that you can enjoy here on his blog.
Conclusion: The possibility that moons have their own moons is fascinating, although we have not found any examples yet. This new research by Carnegie Science shows that it is indeed possible but only in the right circumstances.
Source: Can moons have moons?
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