The moon has various shapes.
among us solar system There are also rocky moons (e.g. earth‘s Month), marine satellites (e.g. europa and Enceladus) and frozen icy moons (e.g. triton) But there is no gas month. Is it just bad luck that a gas moon doesn’t exist, or is there a physical reason why a gas moon can’t exist?
There is actually a gas month! Although they are not in our solar system. Although there are over 5,500, exoplanet Found so far, but only two possible exomoon Both have been discovered, but neither is 100% confirmed yet. The strange thing about these two “exomoons” is that they are even larger gas planets, orbiting even larger orbits. gas giant! However, as we will see later, these are the exceptions that prove the rule.
Related: 10 of the strangest moons in the solar system
To understand why there are no gas satellites, at least not 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 the “bottom-up” formation and the other is called the “top-down” formation.
Shaping the gas world from the bottom up
bottom-up, or ‘core attachment‘, Formation refers to how the gas giant planets of our solar system formed. 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 in which all the planets formed. First, they accreted as rocks and grew by collecting dust, pebbles, and asteroids.Some things grew as follows Mars or VenusBut others continued to grow, forming huge rocky bodies with up to 10 times the mass of Earth.
Once it became this large, its gravity was strong enough to begin scavenging gas from large areas of the protoplanetary disk. Exactly how much gas they take up and how big they get depends on 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 colder “Ice Giant” Uranus and Neptune. NASA’s Juno The mission to Jupiter helped find evidence supporting the nuclear accretion model by detecting the gravitational pull of a large, rocky but diffuse core at the planet’s center, about 10 times the mass of Earth.
Shaping the gas world from the top down
In top-down models, gas worlds form directly from collapsing clumps of gas in nebulae, similar to stars. However, there is a minimum mass that can be produced by this process.
When a large lump of gas contracts under its own gravity, it heats up because the gas is packed into a smaller volume and therefore becomes denser. However, when gas is warm, it tends to expand, so the mass of gas must radiate excess heat in order to continue contracting. As a result, we often see collapsing gas clouds glowing in the light of thermal infrared energy.
However, there is a limiting factor called the “fragmentation opacity limit.”
“It depends on the opacity, temperature, and density of the dust that the gas radiates enough heat to collapse even as it cools, and the process is much less efficient for objects as small as Jupiter’s mass of about 3. “It can’t radiate enough heat to continue collapsing,” Sam Pearson of the European Space Agency said in an interview.
As the volume decreases, the dust becomes more concentrated and opaque, and the process of radiating excess heat through gravitational contraction becomes increasingly inefficient. Therefore, a top-down process would not form anything smaller than three Jupiter masses.
Why is there no gas satellite in the solar system?
Like their parent planets, most of our solar system’s moons formed through a bottom-up core accretion process within a disk of material left around the parent planet. The planet had already swallowed most of the available material, so there wasn’t enough material left to form a moon massive enough to have enough gravity to hold so much gas. In fact, only one moon in the solar system has an atmosphere, and that is Saturn’s largest moon. Giant.
Similarly, top-down processes could not occur because there was not enough gas left. If that had happened, it would have been at least three times as massive as Jupiter, making it the largest world in the solar system by a considerable margin.
odd number of satellites
Therefore, the two most common processes that generate gas worlds cannot form gas satellites. But there are some oddities in our solar system that formed differently.
In Earth’s case, the moon was probably formed from material blown away from Earth. huge collision Mars-sized protoplanet. This debris formed the ring that built Earth’s moon by nuclear accretion. Could an impact with a gas giant planet eject enough gas to form a gas moon?
Unfortunately not. “Rocky planets can have such effects, but remember what happened with comets. Shoe store – Levi 9 collide with jupiter [in 1994]? Jesse Christiansen of the California Institute of Technology told Space.com in an interview. “Gas giants eat anything.”
Anything that collides with a gas giant will simply be absorbed into the gas giant and become part of it, rather than ejecting debris into space.
Another oddity is the captured satellite.For example, on Mars two moons Phobos and Deimos be captured asteroid. Saturn’s outermost moon Phoebe is a captured cometary object, and Neptune’s moon Triton is a captured cometary object. kuiper belt object. They did not form around planets, but formed on their own in space, and later got too close to the planet and got caught in the planet’s gravity.
A question arises here. Is it possible for a small gas planet to be captured by a large gas planet? After all, a gas world can be up to ten times the mass of Jupiter, so in principle it could be as large as, say, Neptune’s mass. You can easily capture the gas world with.
gas examoon
Looks like it really can be done! “There could be Neptune-sized moons surrounding giant exoplanets,” Christiansen said.
The two exomoon candidates mentioned at the beginning of this article — Kepler 1625b-i and Kepler 1708b-i — Both are gas giants in their own right, but they appear to be moons of an even larger gas planet.
“I want to emphasize 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.”
Assuming it is a real moon, Kepler-1625b-i would have 19 times the mass of Earth (about 6% of Jupiter’s mass), making it comparable in mass to Neptune, and 30 times the mass and diameter of Earth. It is accompanied by a gas planet with Half of Jupiter.
Kepler-1708b-i is probably less massive than Kepler-1625b-i, about five times the diameter of Earth (about half that of Kepler-1625b-i), and orbiting a giant planet 4.6 times the mass of Jupiter .
“They challenge a lot of theories,” Christiansen said. “It’s hard to think of how they formed that way, so they must have been captured.”
The captured object will in principle be similar to a captured satellite in the solar system. They are thought to have formed like planets through the accretion of a disk’s core, which was then captured as it migrated toward the star.
Migration appears to be a common process in young planetary systems. This is how astronomers describe “hot Jupiter.” Hot Jupiters are gas giant planets that are very close to their stars, but could have formed that close. In the case of the exomoons Kepler 1625b-i and 1708b-i, they were captured by a larger planet in front of them as they traveled to Earth.
But despite all this, they are probably not true satellites. Rather, they are both probably examples of double planets rather than exomoons. A double planet is a condition in which, rather than one world orbiting the other, both worlds orbit a common center of mass in the space between them. There are two planets in our solar system, Pluto and his greatest companion, Karon.
So there is some kind of gas moon. But to make it, nature has to cheat.