There are more than 200 moons in our solar system. Most of the major planets, with the exception of Mercury and Venus, have moons. Pluto and some other dwarf planets, as well as many asteroids, also have small moons. Saturn and Jupiter have the largest number of moons, with dozens orbiting each of the two giant planets. While only one moon revolves around our planet.
Planets in our solar system come in two forms, some are rocky and some are gaseous. But all moons in our solar system are rocky, even those orbiting gas giants. Why aren’t some moons in the solar system made of gas? Are there gaseous moons anywhere in the universe?
There are some good reasons why there are no gaseous moons nearby, Jonathan Lunin, chair of the astronomy department at Cornell University, tells Live Science.
But while we haven’t found any gaseous moons outside our solar system, it may be possible under the right conditions. Specifically, it will depend on the moon’s mass, ambient temperature, and the effect of tidal forces, the gravitational force of a nearby object, such as the host planet the moon orbits.
To illustrate how these conditions can affect a gaseous moon, imagine that our moon’s rocky makeup has been replaced by pure hydrogen. Hydrogen gas is much less dense than rock, so immediately, the Moon will grow to about the size of Earth. This means that the “gas moon” is likely to be a similar size to its parent planet, so it would be more appropriate to call it a “binary planet”.
Gas binary planets may be possible, but they are likely to be very rare because planet formation will usually result in the two planets merging together, or deviating from one another. Therefore, no gaseous satellite has been observed so far.
Huge volume and temperature
In fact, the sheer size of gas giants like Jupiter is one reason for their existence. Because if it were too small, the gravitational force wouldn’t be strong enough to hold those light elements together.
Just like the gaseous planets, a gas moon would have to be much larger than its rocky counterparts, because if it were too small it wouldn’t have enough gravity to hold large amounts of lighter gases like hydrogen and helium.
But size isn’t the only factor influencing, there’s also temperature. “Let’s take the moon as it is, as a rock,” Lunin says. “Then let’s put an atmosphere of hydrogen around it. We know that the hydrogen atmosphere will escape very quickly just because of thermal effects.” The warmth of the sun could cause the hydrogen to evaporate away.
What this tells us is that if our moon was made entirely of hydrogen, it wouldn’t be stable. But even if our imaginary gaseous moon were the size of Earth and the surrounding temperature was extremely cold, the host planet would likely tear it apart.
“The Earth’s moon is subject to tidal forces from the Earth, but it doesn’t rupture because it has some physical force attached to it,” Lunin says. “But that’s not the case with the hypothetical gaseous moon. Because it’s made of gas rather than a solid, even if it’s very cold , and it was about something else, it would be stripped and torn apart by the tides.
How could a gaseous moon be possible?
The moon’s host planet would have to be either very far and cold or very large for a gaseous moon to be possible.
And if a gaseous moon was the same size as ours, and was located anywhere in our solar system, it would not operate stably. But if the gaseous moon is roughly the size of Neptune and orbits around Jupiter for example, then the gravitational forces holding these massive objects together would likely prevent tidal forces from destroying a Neptune-sized moon. It can be quite stable.