Cosmic mystery: why can't astronomers find gas moons anywhere?

In the Solar System, there are four planets that are composed of gas, yet there are no moons with a similar composition, according to IFLScience.

Scientists have long wondered whether gas moons could exist in space, or if such a phenomenon contradicts the laws of nature. It is impossible to say for certain until one of these moons is discovered in space.

There are several candidates that astronomers refer to as exomoons. However, the status of the discovered exomoons remains disputed, as scientists are still uncertain about their composition. This concerns the first two registered exomoons, Kepler-1625b-i and Kepler-1708b-i. Researchers believe they are Neptune-sized, suggesting that it is unlikely these exomoons are primarily solid or liquid with a dense atmosphere.

Some moons are variously sized asteroids from the Kuiper Belt that have been captured by planets. However, in most cases, it is believed that moons form from the same protoplanetary disk as their planets. Therefore, there are two pathways to the formation of a gas moon—either a smaller gas planet is captured by a larger one, or the planet's disk contains enough material far from the center to form something of that size.

That said, the formation of a gas moon appears to be a very complex process. The main characteristic of gas planets is their size. Uranus is the lightest gas giant in the Solar System, with a mass nearly 14 times that of Earth.

The issue is that a light gas clump would not have sufficient gravity to hold everything together.

Planetary gas giants in our Solar System have enormous solid cores. It is believed that these cores form similarly to rocky planets, but in regions where there was more gas for them to latch onto, a process known as bottom-up formation. There is an alternative method of creating a gas giant known as top-down, but theoretical models indicate that it only works for objects with a mass at least three times that of Jupiter, which means this will never happen in our Solar System.

By definition, a moon must have less mass than its planet. Any object significantly smaller than Uranus or Neptune would likely be unable to retain gas in one place. Therefore, Uranus and Neptune would never acquire gas moons.

Saturn and Jupiter are massive enough to have gas moons, but the question of where they could have originated remains open. Even if all four of Jupiter's large moons were to combine, it is unlikely they would be large enough to hold the gas needed to create a true gas moon. It is also unknown whether there was enough hydrogen and helium at a safe distance from Jupiter to gather this hypothetical supermoon.

Alternatively, a gas planet could have formed independently and then been captured by a larger planet. Astronomers believe this is a more likely explanation for Kepler 1625b-i and 1708b-i, assuming they are not the result of a computational error.

However, it is important to remember that the capture of moons is a rare event. There are hundreds of thousands of asteroids, comets, and Kuiper Belt objects in the Solar System, and only a small fraction of them have ended up locked in orbit around a planet. If Uranus or Neptune ever had orbits that caused them to cross paths with one of the two larger planets, they could have been captured, but this would require a perfect set of circumstances.

Recall that Jupiter-like planets have a destructive influence on other worlds. Giant planets can hinder Earth-like worlds from becoming habitable.