Can gas moons exist?

The answer to this is probably. We can’t be completely certain until we find one. There are definitely no gas moons in the Solar System – Titan has a thick atmosphere, but it’s not more a gas moon than the Earth is a gas planet.

At the moment, we’re still not completely sure we have discovered any moons outside the Solar System, known as exomoons. Several candidates have been reported, but their status is still in dispute, so obviously we can’t be certain of their composition.

That said, the first two reported exomoons, Kepler-1625b-i and Kepler-1708b-i are thought to be about the size of Neptune. If that’s true, it’s unlikely either would be solid, or even mostly liquid with a thick atmosphere. Evidence for and against the pair’s existence has been published in the way science is supposed to be done, and usually is when vested interests and ideologies don’t care enough to interfere.

Even if Kepler-1625b-i is an error in the data, there is still no reason astronomers are aware of that gas moons can’t exist, and if there is one thing the universe keeps proving to us, if something can exist it probably does somewhere.

Some moons are small (or in Triton’s case large) asteroids/Kuiper Belt objects captured by their planet. In most cases, however, it is thought they form out of the same protoplanetary disk their planets do. Consequently, there are two paths to a gas moon’s formation – either a smaller gas planet gets captured by a larger one, or a planet’s disk has enough material far from the center to form something that large.

So why none in the Solar System?

Just because a gas moon is possible, does not mean they will form easily. The key thing about gas planets is that they are big. Large in volume of course, because gas is less dense than solids, but also high in mass. The lowest mass gas giant in the Solar System, Uranus, has more than 14 times the mass of Earth.

That makes sense if you think about it – a lightweight patch of gas doesn’t have a lot of gravity to hold it together. Even in deep space it’s likely to drift apart, and at the edge of a planet’s gravity well there’s no chance of surviving long.

Planetary gas giants within our Solar System have substantial solid cores. It’s believed the cores formed in a similar manner to the rocky planets, but in regions where there was more gas for them to grab hold of, a process known as bottom-up formation. There’s an alternative way to make a gas giant, known as top-down, but theoretical models indicate it only works for objects at least three times Jupiter’s mass, so it was never going to happen in our Solar System.

By definition a moon has to have less mass than its planet, and as far as we know that is always quite a lot less. Any object significantly less massive than Uranus or Neptune probably would not have been big enough to hold onto a substantial quantity of light gases, so gas moons were never a possibility for them.

Saturn and Jupiter are at least massive enough to have gas moons, but there’s still the question of where one would come from. Even if all four of Jupiter’s large moons had combined, it’s unlikely they would have been large enough to hold onto the gas needed to make a truly gas moon, although a Titan-like moon is a more intriguing question. We also don’t know there was enough hydrogen and helium at a safe distance from Jupiter for this hypothetical super-moon to collect. 

Alternatively, a gas planet could form independently and then be captured by a larger one. Astronomers think that’s the more likely explanation for Kepler 1625b-i and 1708b-I if they’re real.

However, it’s important to remember that moon-capturing is rare. There are hundreds of thousands of asteroids, comets and Kuiper Belt objects in the Solar System, only a tiny proportion of which have been trapped in orbit around a planet. If Uranus or Neptune had once had orbits that caused them to cross the path of one of the two larger planets they might have been captured, but it would have taken a perfect storm of circumstances. 

What is amazing is not that we don’t have a gas moon we can visit, but that there might be two close enough that we have found them, if they turn out to be real.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.