Saturn’s moons Titan and Iapetus, Jupiter’s moon Callisto, Earth’s Moon, and the recently-discovered extrasolar moon Kepler 1625b-i are capable of hosting their own moons (submoons), according to a study published in the Monthly Notices of the Royal Astronomical Society: Letters.
An artist’s impression of an extrasolar submoon. Image credit: Sci-News.com.
Each of the Solar System’s giant planets has large moons but none of these moons have submoons.
“Planets orbit stars and moons orbit planets, so it was natural to ask if smaller moons could orbit larger ones,” said co-author Dr. Sean Raymond, from the University of Bordeaux.
Dr. Raymond and his colleague, Dr. Juna Kollmeier from the Observatories of the Carnegie Institution of Washington, investigated the stability of hypothetical submoons.
Their calculations show 10 km-scale submoons can only survive around large (1,000 km-scale) moons on wide orbits.
They also found that a handful of known moons (Callisto, Titan, Iapetus, and the Moon) are theoretically capable of hosting long-lived submoons.
“Callisto fits the bill of a satellite that could host its own satellite, although none have been found so far,” they said.
“However, further calculations are needed to address possible sources of submoon instability, such as the non-uniform concentration of mass in our Moon’s crust.”
“The lack of known submoons in our Solar System, even orbiting around moons that could theoretically support such objects, can offer us clues about how our own and neighboring planets formed, about which there are still many outstanding questions,” Dr. Kollmeier said.
The moons orbiting Saturn and Jupiter are thought to have been born from the disk of gas and dust that encircle gas giants in the later stages of their formation.
Our own Moon, on the other hand, is thought to have originated in the aftermath of a giant impact between the young Earth and a Mars-sized body.
The lack of stable submoons could help planetary researchers better understand the different forces that shaped the satellites we do see.
“Of course this could inform ongoing efforts to understand how planetary systems evolve elsewhere and how our own Solar System fits into the thousands of others discovered by planet-hunting missions,” Dr. Kollmeier said.
For example, the possible moon orbiting the Jupiter-sized exoplanet Kepler 1625b is the right mass and distance from its host to support a submoon. Although, the inferred tilt of its orbit might make it difficult for such an object to remain stable. However, detecting a submoon around an exomoon would be very difficult.
Given the excitement surrounding searches for potentially habitable exoplanets, the team calculated that the best case scenario for life on large submoons is around massive stars.
Although extremely common, small red dwarf stars are so faint and their habitable zones so close that tidal forces are very strong and submoons — and often even moons themselves — are unstable.
“An artificial submoon may be stable and thereby serve as a time capsule or outpost,” the researchers said.
“On a stable orbit around the Moon — such as the one for NASA’s proposed Lunar Gateway — a submoon would keep humanity’s treasures safe for posterity long after Earth became unsuitable for life.”
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Juna A. Kollmeier & Sean N. Raymond. 2019. Can moons have moons? MNRASL 483 (1): L80-L84; doi: 10.1093/mnrasl/sly219
