by Moons come in many forms.
In our Solar system we have rocky moons (e.g. Land‘s moon), oceanic moons (e.g. Europe and Enceladus) and frozen ice moons (e.g. Triton) but there are no gas windows. Are we just unlucky not to have gas windows or are there physical reasons why they can’t exist?
In fact, there are gas windows! Although they are not in our solar system. Although more than 5,500 exoplanets So far only two possible exomoons They have been detected and neither is 100% confirmed yet. The strange thing about these two ‘exomoons’ is that they are gas giants that orbit even larger gas giants! However, as we will see, they are the exception that proves the rule.
Related: The 10 strangest moons in the solar system
To understand why there are no gas moons, at least in our solar system, it is best to first understand how gas giant planets form.
There are two scenarios for the formation of gas giant planets. One is called “bottom-up” training, the other is “top-down.”
Form gaseous worlds from the bottom up
From bottom to top, or ‘core accretion‘, formation is how the gas giant planets in our solar system formed. If we could go back in time 4.5 billion years, we would witness a young sun surrounded by a disk of gas and dust. This is the protoplanetary disk from which all the planets formed. First, they accreted as rocky bodies and grew as they accumulated dust, pebbles, and asteroids. Some only grew until Mars either Venusbut others continued to grow, forming giant rocky bodies with up to 10 times the mass of Jupiter.
Once they reached this mass, they had strong enough gravity to begin sweeping large swaths of gas from the protoplanetary disk. The exact amount of gas they stole and the size they grew depended on their severity and the amount of gas available.
But in the end, our solar system was left with four gas giant planets… Jupiter and Saturnand the coldest ‘ice giants’ Uranus and Neptune. from NASA Juno The mission to Jupiter has helped find evidence supporting the core accretion model by detecting the gravity of a large, rocky but diffuse core, about ten times the mass of Earth at the center of Jupiter.
Forming gas worlds from the top down
In the top-down model, gaseous worlds form directly from a mass of gas that collapses into a nebula, just as stars do. However, there is a minimum amount of mass that this process can produce.
As a large mass of gas contracts under the force of its own gravity, it heats up because the gas is packed into an increasingly smaller and therefore denser volume. But when gas is hot it wants to expand, so to continue contracting, the mass of gas must radiate its excess heat. Consequently, we often see collapsing gas clouds glowing in the light of infrared thermal energy.
However, there is a limiting factor called the “fragmentation opacity limit.”
“Radiating enough heat so that the gas can cool and still collapse depends on the opacity of the dust, and the temperature and density, and that process becomes much less efficient with smaller objects to the point where, at about 3 Sometimes Jupiter has a mass of 3”. “We can’t radiate enough heat to keep collapsing,” Sam Pearson of the European Space Agency said in an interview.
The smaller the volume, the more concentrated and opaque the dust becomes, and the process of radiating excess heat from gravitational contraction becomes increasingly ineffective. Therefore, in the top-down process nothing less than 3 Jupiter masses can be formed.
Why the solar system doesn’t have gas moons
Like their parent planets, most of the moons in our solar system formed through the process of bottom-up accretion of nuclei into disks of leftover material surrounding their parent planets. Because the planets had already absorbed most of the available material, there simply wasn’t enough left to form a moon massive enough to have enough gravity to hold a large amount of gas. In fact, only one moon in the solar system has an atmosphere, and that is Saturn’s largest moon. Titan.
Similarly, a top-down process could not have occurred because there was not enough gas left, and if it had happened, with a minimum of 3 Jupiter masses it would have been the largest world in the solar system by quite some margin.
odd moons
Therefore, we cannot form gas moons by the two most conventional processes of producing gas worlds. However, there are several oddities in the solar system that formed differently.
In the case of the Earth, the Moon probably formed from material broken off from the Earth after a giant collision with the size of Mars protoplanet. This debris formed a ring that formed Earth’s moon through core accretion. Could an impact on a gas giant planet expel enough gas to form a gas moon?
Unfortunately not. “Rocky planets can have impacts like that, but remember when the comet Zapatero-Levy 9 hit Jupiter [in 1994]? It just disappeared,” Caltech’s Jessie Christiansen told Space.com in an interview. “Gas giants eat anything.”
Anything that collides with a gas giant is simply absorbed by the gas giant and becomes part of it, rather than expelling debris into space.
Another rarity is the captured moons. For example, Mars two moons Phobos and Deimos they are captured asteroids. Saturn’s outermost moon Phoebe is a captured cometary object, and Neptune’s moon Triton is a captured cometary object. Kuiper belt object. They didn’t form around a planet, but formed on their own in space and then got too close and were trapped by a planet’s gravity.
This raises a question: could a smaller gaseous planet be captured by a larger gaseous planet? After all, gaseous worlds can have a mass as large as a dozen times the mass of Jupiter, so in principle they could easily trap a gaseous world with, say, the mass of Neptune.
gas exomoons
It seems that they indeed can! “It may be the case that there are Neptune-sized moons around giant exoplanets,” Christiansen said.
The two candidate exomoons mentioned at the beginning of this article: Kepler 1625b-i and Kepler 1708b-i – are both gas giants in their own right, but appear to be satellites of even larger gas giants.
“I will highlight that they are both candidates,” Christiansen said. “We see something in the data that is consistent with a moon, but there are other things that could also explain it.”
Assuming it is a real moon, then Kepler 1625b-i has a mass 19 times that of Earth (about 6% of Jupiter’s mass), making it similar in mass to Neptune, and it accompanies a planet gaseous with 30 times the mass of the Earth and a diameter. half that of Jupiter.
Kepler 1708b-i is even heavier, weighing about 37 times the mass of Earth and orbiting a giant planet 4.6 times more massive than Jupiter.
“They challenge a lot of theories,” Christiansen said. “It’s hard to find a way they formed like this, so they must have been captured.”
Being captured objects would make them similar, in principle, to the captured moons of our solar system. They would have formed as planets from the accretion of the core into a disk and would then be captured due to their migration towards their star.
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Migration appears to be a common process in young planetary systems. This is how astronomers explain “hot Jupiters,” which are gas giants very close to their star but could not have formed so close. In the case of the exomoons Kepler 1625b-i and 1708b-i, when they migrated they were trapped by larger planets that were in front of them.
However, despite all this, they are probably not true moons! Instead, both are 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 the other. We have a double planet in our own Solar System, in the shape of Pluto and his biggest companion, Charon.
So, there are a kind of gas moons, but to create them, nature has to cheat!
