Safety zone saves giant moons from fatal plunge

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Numerical simulations showed that the temperature gradient in the disk of gas around a young gas giant planet could play a critical role in the development of a satellite system dominated by a single large moon, similar to Titan around Saturn. Researchers found that dust in the circumplanetary disk can create a “safety zone,” which keeps the moon from falling into the planet as the system evolves.

Safety zone saves giant moons from fatal plunge
An artist’s impression of a satellite forming around a giant gas planet
which is itself still forming around a star
[Credit: Nagoya University]



Astronomers believe that many of the moons we see in the Solar System, especially large moons, formed along with the parent planet. In this scenario, moons form from the gas and dust spinning around the still forming planet. But previous simulations have resulted in either all large moons falling into the planet and being swallowed-up or in multiple large moons remaining. The situation we observe around Saturn, with many small moons but only one large moon, does not fit in either of these models.

Simulation results of the migration map for a disk. The direction of the migration is inward/outward in the blue/red 
regions. (Video) Black points represent the satellites. All but one of these satellites move toward the planet 
and fall into it. The outermost object moves to a red “safety zone” allowing it to survive until the disk gas
 dissipates. You can see that the positions of the “safety zones” change over time 
[Credit: Fujii & Ogihara, A&A, 2020]




Yuri Fujii, a Designated Assistant Professor at Nagoya University, and Masahiro Ogihara, a Project Assistant Professor at the National Astronomical Observatory of Japan (NAOJ), created a new model of circumplanetary disks with a more realistic temperature distribution by considering multiple sources of opacities including dust and ice. Then, they simulated the orbital migration of moons considering pressure from disk gas and the gravity of other satellites.

Safety zone saves giant moons from fatal plunge
Simulation results showing the orbital radii vs. time of 7 hypothetical moons with Titan mass. As the simulation
progresses, almost all of these satellites fall into the planet, however, the outermost satellite alone survives
until the disk gas dissipates. This satellite temporarily resides in the “safety zone”
 [Credit: Fujii & Ogihara, A&A, 2020]

Their simulations show that there is a “safety zone” where a moon is pushed away from the planet. In this area, warmer gas inside the orbit pushes the satellite outward and prevents it from falling into the planet.




“We demonstrated for the first time that a system with only one large moon around a giant planet can form,” says Fujii. “This is an important milestone to understand the origin of Titan.”

Safety zone saves giant moons from fatal plunge
A scenario for the formation of a single large moon. (1) As a planet forms, a disk containing gas and dust rotates
 around the planet. Solid materials condense in this disk. (2) Solid components grow to the size of the satellite
 in the circumplanetary disk. The simulations in this research started from this stage. (3) The orbits of these
 satellites in the disk change gradually due to the influence of the gas. Many satellites approach the planet
while orbiting, and eventually fall into the planet. However, a satellite with an orbit located in a “safety zone”
does not fall into the planet, but maintains its distance from the planet. (4) As the gas in the disk dissipates,
the satellite which survives in the “safety zone” will remain until the end with a stable orbit
[Credit: NAOJ]

But Ogihara cautions, “It would be difficult to examine whether Titan actually experienced this process. Our scenario could be verified through research of satellites around extrasolar planets. If many single-exomoon systems are found, the formation mechanisms of such systems will become a red-hot issue.”

These results were published in Astronomy and Astrophysics Letters.

Source: National Institutes of Natural Sciences [March 09, 2020]

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