March 10, 2020 : A New Study Involving Yuri Fujii Demonstrates That A Large Exomoon Similar To Titan Can Take Shape In A Safety Zone Around A Gas Giant

A new study entitled « Formation of single-moon systems around gas giants », published in Astronomy and Astrophysics in March 2020 and proposed by a team composed of Yuri Fujii and Masahiro Ogihara demonstrates the potential development of a large moon similar to Titan in the safety zone of a circumplanetary disk. The researchers performed several simulations of the formation process of moons around a Gas Giant on the basis of the PC Cluster operated by the National Astronomical Observatory of Japan (NAOJ). Numerical simulations reveal that the temperature gradient in the gas disk around any Gas Giant in its formation process or in its youth plays a key role in the development of a large moon comparable to Saturn's largest moon Titan. The planetologists show that the dust of the disk evolving around the young Gas Giant can generate a « safety zone » that prevents the world from being swallowed by the big planet as the system develops. A major goal is to understand or to determine the mechanisms that have led to the current orbital configuration of the numerous moons of Saturn. The system of the Ringed Planet is, to a certain extent, a mini Solar System.

Specialists of the Solar System believe that numerous moons present in the Solar System took shape around their planet during its formation process. Those moons may have emerged in the disk of gas and dust evolving around the planet in its youth or in its formation process. One can imagine a disk of gas, dust, ice, comets, asteroids and planetesimals spinning around the young planet like cars in the Indianapolis Motor Speedway. However, the current configuration of the orbit and distribution of the moons in the Saturn System can't be correctly explained by previous simulations which result in configurations where all large moons are attracted and swallowed by the Gas Giant or in configurations where multiple large moons take shape and become stable around the Gas Giant. Today, the Saturn System which has been largely explored by the Cassini spacecraft and by the Huygens probe between 2004 and 2017 is composed of the second largest moon in the Solar System and is composed of numerous icy moons which are much smaller than Titan. That configuration with a big moon and many small moons is not a potential outcome of previous simulations. That's why new models had to be envisaged.

Yuri Fujii who is a designated assistant professor at Nagoya University and Masahiro Ogihara who is a project assistant professor at the National Astronomical Observatory of Japan (NAOJ) have produced and proposed a new model of circumplanetary disks mobilizing a more realistic temperature distribution determined on the basis of multiple sources of opacities such as dust and ice. Their simulations allowed them to study the orbital migration of moons on the basis of parameters such as the pressure from the disk of gas and the gravity exerted by the other moons evolving around the big planet or Gas Giant. The new simulations reveal that there is a « safety zone » where a world orbiting the young Gas Giant is pushed away from the main planetary body. The phenomenon can be explained by the presence of a warmer gas inside the orbit that tends to push the moon outward eliminating the potential risk of drifting toward the Gas Giant or falling into the big planet. Yuri Fujii pointed out : « We demonstrated for the first time that a system with only one large moon around a giant planet can form. » She added : « This is an important milestone to understand the origin of Titan. »

In the formation process of the big planet which occurs in parallel to the formation process of its moons, the Gas Giant can act as a powerful vacuum cleaner for many worlds evolving at a relatively small distance from that planetary body. Over time, many moons will tend to migrate inward and to crash into the Gas Giant. However, there is an orbital distance that is high enough to allow the moon to migrate outward or to keep a relatively stable orbit. The researchers observe a configuration where only one moon survives from the inward motion. One of the worlds orbiting the Gas Giant moves to a safety zone. At the end of the dissipation process of the circumplanetary disk of gas, dust and ice, the large moon is still there even if the mean distance of its orbit decreases at the end of the process studied by both researchers. Masahiro Ogihara argued : « 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. »

The four known Gas Giants present in the Solar System are surrounded by a multitude of natural satellites. In the system of Jupiter, there are 79 known moons. In the system of Saturn, there are at least 82 moons. In the system of Uranus, there are 27 known moons. In the system of Neptune, there are 14 known moons. Therefore, at first sight, one can assume that the formation process of any Gas Giant implies the development of moons around the main planetary body. The study of the size and of the orbital configuration of the moons around the four Gas Giants allows us to deduce the potential formation processes of the Gas Giants and of their numerous moons. Planetologists are in a position to create new models or simulations in order to better understand the dynamics of the systems of moons around the big planets dominated by hydrogen and helium. Around Jupiter, there are four major moons that can be seen from a typical telescope on Earth. The largest moon in the Solar System, Ganymede, evolves around Jupiter, the largest planet in the Solar System. Io is the closest moon to Jupiter among the four major moons of the Gas Giant. The icy moon Europa evolves farther but closer to Jupiter than Ganymede. And Callisto evolves farther from Jupiter than Ganymede.

The system of moons around Saturn is captivating and surprising. Titan is clearly the « center of gravity » of the moons orbiting the Ringed Planet. Titan is the largest moon as well as the most massive moon of the Saturn System. If we observe the distribution of moons around Saturn, we can notice that the main moons of Saturn which are smaller tend to evolve closer to Saturn. The icy moon Mimas evolves at a relatively small distance from Saturn. The tiny moon Enceladus evolves farther from Saturn and that active moon is bigger than Mimas. Tethys evolves farther from Saturn than Enceladus and is bigger than Enceladus as well. Dione evolves farther from Saturn than Tethys and is also bigger than Tethys. Rhea evolves farther from Saturn than Dione and is bigger than Dione. Beyond Rhea, there is a giant moon which can be regarded as the center of gravity of the system of moons. Much farther from Saturn than Titan, there is another major moon whose diameter is higher than 1000 km. That moon unveiling a strange shape is Iapetus. The physical configuration and the orbital configuration of the system of moons around Saturn are likely to tell us a lot regarding the formation process of the Saturn System.

One can imagine that Titan and the other major moons evolving closer to Saturn than Titan have been generated in the presumed circumplanetary disk of Saturn during the formation process of the Gas Giant and its moons. The composition of the icy moons clearly reveals that the circumplanetary disk was rich in water ice. When did the formation process happen and how long did it last ? Researchers admit that the relatively large moons must have taken shape in the hypothetical circumplanetary disk of the Gas Giants. That's the case for the system of Jupiter dominated by Io, Europa, Ganymede and Callisto. That's also the case for the system of Saturn dominated by Titan. However, in the system of Neptune, Triton, the largest moon of the system, may represent a captured world due to its orbital characteristics. Planetologists have shown that producing a single-moon system is more difficult than producing systems containing multiple moons or systems without any natural satellite. One can clearly say that the systems of the four Gas Giants of our Solar System are rich in moons.

The formation process of the moons will be intimately linked to the properties of the circumplanetary disk. The moons will change their direction, their speed or migrate inward or outward depending on the characteristics of the circumplanetary disk. The temperature structure of the circumplanetary disk plays a key role in its dynamics or evolution. In the new study, the researchers were interested in the orbital evolution of a moon whose size is comparable to the size of Titan. What was the structure of the presumed circumplanetary disk of Jupiter or Saturn ? The planetologists wanted to determine the potential evolution of a circumplanetary disk containing multiple moons. Can it engender a stable system with only one large moon ? Many moons tend to go inward or to crash into the Gas Giant due to the temperature structure of the disk, to the dust opacity or to the dust density. In the outer orbits, the moons can find a safety zone which allows them to move outward or to become relatively stable in their orbit. The Saturn System is clearly a natural laboratory to study the dynamics of any system of moons around a Gas Giant.

The image above reveals the disk of Titan as well as a portion of the disk of the icy moon Tethys where a giant crater can be clearly seen. The view was taken from the ISS Narrow-Angle Camera of the Cassini spacecraft on November 26, 2009. Tethys whose diameter is over 1000 km evolves much closer to Saturn than the Opaque Moon. Image credit: NASA/JPL-Caltech/Space Science Institute.

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