A planet can have a moon, so could that moon have its own moon? In principle, yes—under suitable conditions. Astronomers use the term submoon; “moonmoon” is a cheerful informal nickname. The difficult part is keeping the smaller object in a stable orbit for a long time.
The paper’s limits and the internet’s names
Juna Kollmeier and Sean Raymond explored the question in their paper Can Moons Have Moons?, first posted in October 2018 and subsequently published in Monthly Notices of the Royal Astronomical Society. Their analysis describes why the host moon’s size and distance from its planet matter:
“Tidal [energy flux] destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar System's moons.”
“A handful of known moons are, however, capable of hosting long-lived submoons: Saturn's moons Titan and Iapetus, Jupiter's moon Callisto, and Earth's Moon.”
The suggested hosts are examples within a stability calculation. A favorable orbit is not the discovery of a body occupying it. The paper also considered the proposed exomoon Kepler-1625b-I. A submoon could be possible under some assumptions about that candidate’s properties, but unknown parameters limited what the calculation could establish. This was an extension of the theoretical question to a possible extrasolar system, rather than a detection of a moon orbiting another moon.
Meanwhile, the naming conversation took on a life of its own. “Submoon” competed with moonmoon, moonette, moonito, grandmoon and other playful suggestions. In the October 2018 posts below, Kollmeier favoured “submoons” while welcoming the enjoyment generated by the alternatives. The exchange captures the public conversation around the paper and the appeal of an easily grasped astronomical puzzle.
In the authors’ tidal-stability model, a submoon about 10 kilometers in radius requires a large host moon, roughly on the 1,000-kilometer radius scale, sufficiently far from its planet. This is a model-dependent scale: orbital distance, density and tidal properties also enter the calculation, so it is not a universal size test.
The naming discussion was deliberately lighter. The earlier interview quoted Kollmeier’s response, “IAU [the International Astronomical Union] will have to decide!” “Moonettes,” “moooons” and “moonitos” were part of the playful exchange, not an announcement that any of those terms had been officially adopted.
More than one body is pulling
A submoon would feel the gravity of its immediate host and the influence of the larger planet. Tidal interactions can change its orbit over time. A location that seems possible in a simple sketch may therefore fail when the system’s long-term evolution is considered.
In their study of submoon stability, Juna Kollmeier and Sean Raymond found that large moons on sufficiently wide orbits could offer favorable conditions. Their examples included the Moon, Callisto, Titan and Iapetus, subject to the assumptions of the model.
Possible does not mean discovered
The study investigated what could survive, not a confirmed natural submoon orbiting one of those bodies. An object also has to form or be captured in the first place. Demonstrating a stable region is one part of the story, rather than a guarantee that something occupies it.
Artificial spacecraft orbiting the Moon are a familiar comparison, but they are not evidence that a naturally formed lunar satellite exists.
Why the question is useful
A playful name can lead to a serious investigation of how planetary systems form and change. When reading about a “new moon,” ask whether the claim concerns a detected body, a tentative candidate or a theoretical possibility.
Even an apparent companion may follow a different kind of orbit. Kamoʻoalewa, Earth’s quasi-satellite, circles the Sun rather than orbiting Earth like the Moon does.
The 2018 naming debate
The term "moonmoon" has been put forth as a potential name. Other suggestions include mini-moon, second-order moon, and nested moons. Atlas staffers’ contributions: Meta-moon? Moon squared? Moony McMoonface? Tell us what you think it should be called. https://t.co/1jY4YPuQ6z
— Atlas Obscura (@atlasobscura) October 10, 2018
If you ask me, they are "Submoons" (hence the title of our article!!!) But the internet has brought forth a wide variety of names and associated joy and for this, I'm delighted: Moonlet, Moonmoon, Moonito, Moonette, Moonlet, Grandmoon, Metamoon...#letsasklevi
— Juna A. Kollmeier (@thejunaverse) October 10, 2018
(This post is no longer available on X.)
Credits and references: arxiv.org (the paper) · Kepler-1625b · New Scientist · Quartz.
I am cured from herpes
ReplyDeleteHis result is 100% guarantee.
Robinsonbuckler11 ((@gmail. com ))……