Moons come in many forms.
In our solar system we have rocky moons (e.g Soil‘S Moon), ocean moons (e.g Europe And Enceladus) and frozen ice moons (e.g Triton) but there are no gas moons. Are we just unlucky not to have gas moons, or are there physical reasons why they can’t exist?
There are actually gas moons! Even though they are not in our solar system. Although there are more than 5,500 exoplanets Only two may have been discovered so far exomons have been spotted and neither is 100% confirmed yet. The strange thing about these two ‘exomoons’ is that they are gas giants, orbiting even larger ones 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’ formation, the other is ‘top-down’.
Gas worlds form from the bottom up
Bottom up, or ‘core accretion‘, formation is how the gas giant planets of our solar system came into being. If we could travel 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 planets formed. At first they piled up as rocky bodies, growing as they collected dust, pebbles and asteroids. Some only grew as big as Mars or Venusbut others continued to grow, forming gigantic rocky bodies with masses up to ten times that of Jupiter.
Once they reached this mass, they had a gravitational force strong enough to suck in large amounts of gas from the protoplanetary disk. Exactly how much gas they robbed and how big they grew depended on their gravity and how much gas was available.
But in the end, our solar system was left with four gas giant planets – Jupiter And Saturnand the colder ‘ice giants’ Uranus And Neptune. NASAs Juno The mission to Jupiter helped find evidence to support the nuclear accretion model by detecting the gravitational pull of a large, rocky yet diffuse core about ten times the mass of Earth at Jupiter’s center.
Gas worlds form from above
In the top-down model, gas worlds emerge directly from a collapsing clump of gas in a nebula, just as stars do. However, there is a minimum amount of mass this process can produce.
When a large lump 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 warm, it wants to expand, so to keep contracting the gas lump must radiate away its excess heat. That’s why we often see collapsing gas clouds glowing in the light of thermal infrared energy.
However, there is a limiting factor called the “Fragmentation Coverage Limit.”
‘Radiating enough heat for the gas to cool and still collapse depends on the opacity of the dust, its temperature and its density, and that process becomes much less efficient for smaller objects, to the point where at about 3 Jupiter has a mass. cannot radiate enough heat to continue collapsing,” said Sam Pearson of the European Space Agency in an interview.
The smaller the volume, the more concentrated and opaque the dust becomes, and the process of radiating away the excess heat from the gravitational contraction becomes increasingly inefficient. So nothing smaller than 3 Jupiter masses can be formed in the top-down process.
Why the solar system has no gaseous moons
Like their parent planets, most moons in our solar system formed by the bottom-up nuclear accretion process in disks of leftover material that surrounded their parent planets. Since the planets had already swept up most of the available material, there was simply not enough left to form a moon large enough to have enough gravity to hold much gas. In fact, only one moon in the solar system even has an atmosphere, and that is Saturn’s largest moon Titan.
Likewise, a top-down process could not have occurred because there was not enough gas left, and if it had, it would have been the largest world in the Solar System by a fair margin, at a minimum of 3 Jupiter masses.
Strange moons
So we cannot form gas moons through the two most conventional processes for producing gas worlds. However, there are several oddities in the solar system that formed differently.
In the case of Earth, the moon likely formed from material blown off the Earth after a gigantic collision with a Mars size protoplanet. This debris formed a ring that built Earth’s moon through nuclear accretion. Could an impact on a gas giant planet emit enough gas to form a gas moon?
Unfortunately not. “Rocky planets can have such consequences, but remember when comets Shoemaker-Levy 9 Hit Jupiter [in 1994]? It just disappeared,” Jessie Christiansen of Caltech told Space.com in an interview. “Gas giants eat everything.”
Anything that collides with a gas giant is simply absorbed by the gas giant and becomes part of it, instead of ejecting debris into space.
Another oddity is captured manes. Mars’ for example two moons Phobos And Deimos are captured asteroids. Saturn’s outer moon Phoebe is a captured comet, and Neptune’s moon Triton is a captured comet. Kuiper belt object. They did not form around a planet, but instead formed independently in space, and later got too close and were captured by a planet’s gravity.
This begs the question: could a smaller gas planet be occupied by a larger gas planet? After all, gaseous worlds can reach a mass of up to twelve times the mass of Jupiter, so in principle they could easily obtain a gas world with, for example, the mass of Neptune.
The gas exomoons
It looks like they can indeed! “There may be moons the size of Neptune surrounding giant exoplanets,” Christiansen says.
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 they appear to be satellites of even larger gas giants.
“I will emphasize that these are both candidates,” Christiansen said. “We see something in the data that is consistent with a moon, but there are also other things that could explain this.”
Assuming it is a real moon, 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 gaseous planet with 30 times the mass of Earth and a diameter half that of Jupiter.
Kepler 1708b-i is even heavier: It weighs about 37 times the mass of Earth and orbits a giant planet 4.6 times heavier than Jupiter.
“They challenge a lot of theories,” Christiansen said. “It’s hard to think of a way they formed like that, so they must have been captured.”
Because they are captured objects, they would in principle be similar to the captured moons in our solar system. They would have formed as planets by nuclear accretion in a disk and then become trapped due to their migration to their star.
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Migration appears to be a common process in young planetary systems. This is how astronomers explain ‘hot Jupiters’, gas giants that are very close to their star, but which could have formed this close. In the case of the exomoons Kepler 1625b-i and 1708b-i, they were captured during their migration by larger planets ahead of them.
Despite all this, they are probably not real moons! Instead, they are both likely examples of double planets rather than exomoons. A double planet exists 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, under the guise of Pluto and his greatest companion, Charon.
So there are some kind of gas moons, but to create them, nature has to cheat!