Are there gas moons?

Are there gas moons?

The planets of our solar system come in two forms: some are rocky and some are gaseous. While all the moons in our solar system are rocky. There are some very good reasons why no nearby moon is gaseous. And while we haven't found a gas moon beyond our solar system, it might be possible under the right conditions, said Jonathan Lunine, president of Cornell University's Department of Astronomy.

Specifically, it would depend on the mass of the moon, its surrounding temperature and the influence of tidal forces, i.e. the gravitational pull of a nearby body, such as its host planet. To illustrate how these conditions could affect a gas moon, he imagines that our moon's rocky composition has been replaced with pure hydrogen. Hydrogen gas is much less dense than rock, so immediately the moon would grow to about the size of Earth, Lunine explained. Indeed, the sheer size of gas giants like Jupiter is one of the reasons so they can exist. If they were too small, gravity wouldn't be powerful enough to hold those light elements together. But size is not the only factor at play; there is also the temperature. "Let's take the moon as it is, like a rock," Lunine told Live Science. "And then we put an atmosphere of hydrogen around it. We know that the hydrogen atmosphere will escape very quickly only due to thermal effects ". In other words, the sun's heat would evaporate the hydrogen.

“Earth's moon is subject to Earth's tidal forces ”Explained Lunine. “So it's not really a sphere. It is attracted a little, but it is not destroyed because it has a certain material strength associated with it ". This is not the case with the hypothetical gas moon. "Since it is a gas and not a solid, even if it is very cold, if it orbits something else, it will be stripped and torn apart by the tides," Lunine said. So how could a gas moon ever be possible? The moon-planet system should be very distant and cold, or very large.

"If it's the size of our moon, anywhere in our solar system, it won't work well," Lunine said. Assuming instead something very large, such as a Neptune around a Jupiter, then of course, it could exist as a gas moon. If so, the gravitational forces holding these huge bodies together would likely prevent the tidal forces from destroying the Neptune-sized moon. “It could be perfectly stable,” Lunine explained.

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