How Saturn Got To Keep Its Giant Moon

The moons of a gas giant form from swirling gas. Models suggest moons need companions to prevent their orbits decaying. Now an explanation has been found for how a moon like Titan could survive. Nagoya University

In Roman myth, Saturn ate his children. Based on astronomer models, the planet that bears his name should have done the same to many of its offspring, including the giant moon, Titan. New research credits a safety-zone created by the temperature gradient of gas surrounding a still-forming Saturn for helping them avoid this terrible fate.

Giant planets are surrounded by gas and dust as they come into being, some of which coalesce into moons. Attempts to virtually reconstruct the process lead either to several large moons (like Jupiter's whose gravitational interactions stabilize each other) or to nothing but small ones (like with Uranus). Neptune’s large moon is thought to be a captured Kuiper Belt Object, rather than a product of Neptune’s birth. Titan, however, has been an inexplicable anomaly.

Dr Masahiro Ogihara of the National Astronomical Observatory of Japan believes the answer may lie in the temperature gradient that occurs around a new gas giant, with gas being warmer closer to the planet. In Astronomy and Astrophysics Letters, Ogihara and Dr Yuri Fujii of Nagoya University show how this warmer gas provides an outward pressure on moons orbiting beyond a certain distance. Without it, they would otherwise experience orbital decay from the friction with stray material.

This marks the first explanation of how a moon like Titan can avoid the death spiral into its planet, but Ogihara noted in a statement: "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."

On the other hand, if Saturn-like systems prove to be rare, we may need another explanation for the survival of the one moon known to support a thick atmosphere.

The discovery coincides with work revealing a different sort of Saturn-like system that may be common. Some “cotton candy” planets, whose densities are thought to be far lower than anything in our own Solar System, may instead sport Saturn-like rings. Also known as super-puff planets, this category has been proposed to explain objects that block out a lot of light when passing across the face of their parental star, suggesting they are very large but with a gravitational influence that is quite small.

In the Astronomical Journal, The Carnegie Institute's Dr Anthony Piro has proposed something different. “We started thinking, what if these planets aren’t airy like cotton candy at all,” Piro said in a statement. “What if the super-puffs seem so large because they are actually surrounded by rings?”

Piro and colleagues concluded some, but not all, the super-puff planets Kepler has found might actually have enormous ring systems. However, since all these worlds orbit close to their stars, the rings would have to be made up entirely of grains of rock, whereas Saturn’s are mostly ice.

The search for gas giants around other stars usually focuses on Jupiter or Neptune-like planets, but in many cases, it seems Saturn is our most useful template.

Artist's impression of what might be happening when so-called "cotton candy" planets block out light from their parental star. Robin Dienel and courtesy of the Carnegie Institution for Science

 

 

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