Because the planets had already swept up most of the available material, there just wasn’t enough left to form a moon massive enough to have enough gravity to hold onto a lot of gas. In fact, only one moon in the solar system even has an atmosphere, and that’s Saturn’s largest moon Titan

There are no gas moons because planets have already used up most of the available material. This means there isn’t enough left to form a moon that’s large enough to have enough gravity to hold onto gas. 

A gas moon would need to be much larger than its rocky counterparts. If it were too small, it wouldn’t have enough gravity to hold onto large amounts of light gases like hydrogen and helium. 

Only one moon in the solar system has an atmosphere, and that’s Saturn’s largest moon Titan.

Gas planets have more moons than terrestrial planets because they are more massive and have stronger gravitational fields. This allows them to attract and hold onto more moons. 

Here are some examples of gas planets and their moons: 

• Jupiter: Has 95 moons, including Ganymede, which is larger than Mercury 
• Saturn: Has 146 moons, including Titan 
• Uranus: Has a moon called Titania, which has a rocky core and an ice mantle 
• Neptune: Has a moon called Triton, which has a deep surface ice 

Some terrestrial planets, like Mercury and Venus, don’t have any moons. Earth has one moon, and Mars has two moons.

Terrestrial planets have few or no moons because they have smaller mass and weaker gravitational force. This means they have a smaller volume of space for their gravitational influence to pull in more material and create a moon. 

The Sun’s gravitational force also plays a role in the number of moons a planet has. For example, Mercury and Venus don’t have moons because they are closer to the Sun than Earth. The Sun’s gravitational pull would make it difficult for any moon to remain in these planets’ orbit. 

Terrestrial planets formed much closer to the Sun with much less material around them. They grew slowly by accreting many small rocky planetesimals and never grew massive enough to capture a disk of gas and dust.

Planets closer to the Sun have less moons because the Sun’s gravitational pull makes it difficult for a moon to remain in orbit. For example, Mercury is so close to the Sun that any moon would likely crash into it or orbit the Sun and eventually be pulled in. 

The outer planets, also known as the giant planets, have more moons than the inner planets because of their greater mass and stronger gravitational fields. These planets can also capture passing objects, called irregular satellites, as moons.

As of July 2021, astronomers have not confirmed the detection of an exomoon. Exomoons are moons that orbit planets around distant stars. They are difficult to detect and confirm using current techniques. 

Some candidates for exomoons include: 

• Kepler-1625b-i: The first moon of the planet Kepler-1625b 
• Kepler-1708b-i: A giant exomoon candidate orbiting the planet Kepler 1708b, which is 5,500 light-years from Earth 

Other candidates include: 

• Kepler-1625b 
• Kepler-1708b 
• An exoplanet about 5,400 light-years from Earth

The gas exomoons

It seems that indeed they can! “It may be the case that there are Neptune-sized moons around giant exoplanets,” said Christiansen.

The two candidate exomoons mentioned at the beginning of this article — Kepler 1625b-iand Kepler 1708b-i — are both gas giants in their own right, but they appear to be satellites to even larger gas giants.

“I will stress that both of these are candidates,” said Christiansen. “We see something in the data that is consistent with a moon, but there are other things that could explain it too

However, despite all this, they are probably not true moons! Instead, they are both probably examples of double planets rather than exomoons. A double planet is when both worlds orbit a common center of mass in the space between them, rather than one orbiting around the other. We have a double planet in our own Solar System, in the guise of Pluto and its largest companion, Charon.

So, there are gas moons, of a sort — but to make them, nature has to cheat!(full article source google)

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