Moons come in many forms.
In our Solar System We have rocky moons (eg Land‘s moon), ocean moons (eg Europe And Enceladus) and frozen ice moons (eg Triton) But there are no gas moons. Are we unlucky not to have any gas moons, or are there physical reasons why they don’t exist?
In fact, there are gas moons! Even though they are not in our solar system. Although more than 5500 Exoplanets So far, only two have been discovered possible com. exomoons They have been observed, and none of them are 100% confirmed yet. The strange thing about these two “exomoons” is that they are gas giants, and they orbit much larger Gas giants! But, 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 the gas giant planet. One is referred to as “bottom-up” formation, the other as “top-down.”
Formation of gas worlds from bottom to top
from bottom to top, or “Basic accumulation“Formation is how the giant gas planets in our solar system formed. If we could go back in time 4.5 billion years, we would see a young sun surrounded by a disk of gas and dust. This is the protoplanetary disk from which all the planets formed. First, it accreted into rocky bodies And they grow as dust, gravel, and asteroids accumulate. Some have only grown in size Mars or VenusBut others continued to grow, forming giant rocky bodies up to 10 times the mass of Earth.
Once they had this massive mass, they had gravity strong enough to start sweeping large swaths of gas out of the protoplanetary disk. Exactly how much gas they steal and how much they grow depends on their gravity and the amount of gas available.
But in the end, our solar system was left with four gas giant planets – Jupiter And SaturnAnd the cooler “ice giants”. Uranus And Neptune. NASA Juno The mission to Jupiter helped find evidence supporting the core accretion model by detecting the gravity of a large but diffuse rocky core with a mass of about ten times that of Earth at Jupiter’s center.
Formation of gas worlds from top to bottom
In the top-down model, gas worlds form directly from a mass of gas collapsed into a nebula, just as stars do. However, there is a minimum mass that this process can produce.
When a large mass of gas contracts under the influence of its own gravity, it heats up because the gas is packed into a smaller, and therefore denser, volume. But when gas is warm, it wants to expand, so, in order to continue contracting, the mass of gas must radiate its excess heat. As a result, we often see collapsing gas clouds glowing in infrared thermal energy.
However, there is a limiting factor called “segmentation opacity limit”.
“Radiating enough heat so that the gas can cool and continue to collapse depends on the dust’s opacity, temperature and density, and this process becomes much less efficient with smaller objects to the point where it is about 3 Jupiter masses.” “It can’t radiate enough heat to continue 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 resulting from gravitational contraction becomes increasingly inefficient. So nothing smaller than 3 blocks of Jupiter can be formed in the top-down process.
Why doesn’t the solar system have gas moons?
Like their parent planets, most moons in our solar system formed through a bottom-up process of accretion of cores into disks of leftover material surrounding their parent planets. Since the planets had already accumulated most of the available material, there was not enough to form a moon massive enough to have enough gravity to hold much gas. In fact, there is only one moon in the solar system that 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, and if it had occurred, it would have been the largest world in the solar system by a wide margin.
Strange moons
Therefore, we cannot form gaseous moons via the two more conventional processes of producing gaseous worlds. However, there are many anomalies in the solar system that formed in a different way.
In the case of Earth, the Moon likely formed from material released from Earth after A Giant collision The size of Mars Protoplanet. This debris formed a ring that formed Earth’s moon through core accretion. Could a collision with a gas giant planet spew out enough gas to form a gas moon?
Unfortunately no. “Rocky planets can have effects like this, but remember when it’s a comet Shoemaker – Levi 9 Hit by Jupiter [in 1994]? “It’s completely gone. Gas giants eat anything,” Jesse Christiansen of the California Institute of Technology told Space.com in an interview.
Anything that collides with a gas giant becomes embedded in the gas giant and becomes part of it, rather than spewing debris out into space.
Another oddity is the moons that were captured. For example Mars Two moons Phobos And Deimos They were arrested Asteroids. Saturn’s outer moon Phoebe is a captured cometary body, and Neptune’s moon Triton is a captured cometary body Kuiper belt Goal. They didn’t form around a planet, they formed on their own in space, and then got too close and were caught by the planet’s gravity.
This raises the question: Can a smaller gas planet be captured by a larger gas planet? After all, gaseous worlds can have up to twelve times the mass of Jupiter, so in principle they could easily collide with, say, a gaseous world with the mass of Neptune.
Exogenous gaseous moons
And it looks like they actually can! “There may be Neptune-sized moons around giant exoplanets,” Christiansen said.
Candidate exomoons were mentioned at the beginning of this article – Kepler 1625b-i And Kepler 1708b-i -Both are gas giants in their own right, but appear to be subordinate to larger gas giants.
“I will confirm that both are candidates,” Christiansen said. “We see something in the data that is consistent with the moon, but there are other things that could explain it as well.”
Assuming it is a true moon, Kepler 1625b-i has a mass of 19 times Earth’s mass (about 6% of Jupiter’s mass), making it similar in mass to Neptune, and a companion gas planet 30 times Earth’s mass and diameter. Half that of Jupiter.
Kepler-1708b-i is likely less massive than Kepler-1625b-i, with a diameter about five times the size of Earth (about half the diameter of Kepler-1625b-i), and orbiting a giant planet 4.6 times larger than Jupiter.
“They challenge a lot of theories,” Christiansen said. “It’s hard to figure out how they formed like that, so they must have been caught.”
The nature of the objects captured would make them similar, in principle, to the moons captured in our solar system. They could have formed like planets from accreting cores in the disk and then been captured as they migrated toward their star.
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 likely formed that close. In the case of exomoons Kepler 1625b-i and 1708b-i, as they migrated, they were caught by larger planets located in front of them.
However, despite all this, they are probably not real moons! Instead, they are both likely examples of double planets rather than exomoons. A double planet is when both worlds orbit around a common center of mass in the space between them, instead of one orbiting the other. We have a double planet in our solar system, in the shape of a planet Pluto And his biggest friend Sharon.
So, there are some kind of gas moons, but to make them, nature has to cheat!
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