Space
Why there is no gaseous moon in solar system?
Why there is no gaseous moon in solar system?
In the solar system, we have rocky moons (such as Earth’s moon ), oceanic moons (such as Europa and Enceladus ), and frozen icy moons (such as Triton), but there are no gaseous moons. Is it because of bad luck that we don’t have gas moons or is there a physical reason for their absence?
Indeed, gaseous moons exist! Although they are not in the solar system. Although more than 5,500 extrasolar planets have been discovered so far, only two possible extrasolar moons have been identified and the existence of none of them has been definitively confirmed yet. The strange thing about these two exosolar moons is that they are gas giants orbiting larger gas giants. Of course, as we shall see, they are the exceptions that prove the rule.
To understand why gas moons do not exist, at least in the solar system, it is better to learn how gas giant planets form.
Artist’s impression of Kepler 1625b-i, the possible moon of the exoplanet Kepler 1625b and the star of the system.
There are two scenarios for the formation of a gas giant planet: the bottom-up scenario and the top-down scenario.
The formation of gaseous worlds according to the bottom-up scenario
The bottom-up, or “core accretion” scenario, explains how the gas giant planets of the Solar System formed.
If we could go back 4.5 billion years, we would see a young Sun surrounded by a disk of gas and dust. All planets are formed from this protoplanetary disk. They first formed as rocky bodies and grew larger by collecting dust, pebbles, and surrounding asteroids. Some of them only grew to the size of Mars or Venus, but others continued to grow and became giant rocky bodies with about 10 times the mass of Earth.
When planets grow to such large sizes, their gravity is strong enough to pull huge amounts of gas from the protoplanetary disk. How much gas they stole and how big they grew depended 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 Saturn, and the ice giants Uranus and Neptune.
NASA’s Juno mission to Jupiter has detected gravity from a large, rocky, yet diffuse core about ten times the mass of Earth at the center of Jupiter, helping to find evidence for the core accretion model.
The planets of the solar system were formed during the bottom-up process or core accretion in the protoplanetary disk.
The formation of gaseous worlds according to the top-down scenario
In the bottom-up model, gaseous worlds, just like stars, form directly from the disintegrating mass of gas in the nebula. However, there is a minimum amount of mass that can produce this process.
When a large mass of gas contracts under its own gravity, it heats up because the gas is compressed into a smaller and therefore denser volume. But when the gas is hot, it tends to expand, so to maintain the contraction, the mass of gas must remove excess heat. As a result, we often see the collapse of gas clouds that glow as thermal infrared energy.
The radiation of enough heat so that the gas can cool and still decay depends on the dust’s opacity, temperature, and density, and the process becomes more inefficient in smaller objects; So, at a mass about three times that of Jupiter, it cannot generate enough heat to continue disintegrating. The smaller the volume, the cloudier and denser the dust becomes, and the process of radiating the excess heat due to gravitational contraction becomes increasingly inefficient. Therefore, an object smaller than three times the mass of Jupiter cannot form during the top-down process.
Why does the solar system not have a gaseous moon?
Most of the moons of our solar system, like their parent planets, were formed by the process of core accretion from the bottom up in disks of residual material that surrounded their parent planets. Since the planets had already collected most of the available material, there was not enough material left to form a moon with enough mass to have enough gravity to hold a large amount of gas. In fact, only one moon in the solar system even has an atmosphere, and that is Saturn’s largest moon, Titan. Similarly, the top-down process could not occur because there was not enough gas left.
Titan, Saturn’s moon, is the only moon in the solar system that has an atmosphere.
Strange moons
According to the explanations given, in the solar system, gaseous moons cannot be formed through the two conventional processes of producing gaseous universes. However, there are wonders in our cosmic neighborhood that are formed in a different way.
In the case of Earth, the Moon is likely formed from material blasted from Earth following a massive collision with a Mars-sized protoplanet. These remnants formed a ring that created the moon’s core through accretion. But could the impact of a gas giant planet eject enough gas to form a gas moon?
Unfortunately no. Rocky planets can experience such collisions, but remember when Comet Shoemaker-Levy 9 hit Jupiter in 1994 and disappeared, Jesse Christiansen of the California Institute of Technology told Space.com. Gas giants devour everything. Anything that hits a gas giant just becomes part of the gas giant instead of throwing debris into space.
Another strange case is the trapped moons. For example, the moons of Mars, Phobos, and Deimos, are trapped by the red planet’s gravity. Saturn’s outermost moon Phoebe is a captured comet mass, and Neptune’s moon Triton is a Kuiper Belt mass that was trapped by Neptune’s gravity millions of years ago. They did not form around the planet, but formed on their own in space and then drifted until they were finally trapped by a planet’s gravity.
Now, the question arises, can a smaller gas planet be captured by a larger gas planet? After all, gaseous worlds can reach up to 12 times the size of Jupiter, so in principle, they could trap a gaseous world the size of Neptune.
Gaseous extrasolar moons
It seems possible for smaller gaseous bodies to be captured by larger gaseous planets. “It’s possible that there are (gas) moons around the size of Neptune around giant exoplanets,” Christiansen said.
The two possible exomoons mentioned at the beginning of the article (Kepler 1625b-i and 1708b-i) are both gas giants in their own right but appear to be originally moons of larger gas giants. “Both of these are candidates,” Christiansen says. “We see something in the data that is consistent with the moon, but other phenomena could also explain it.”
Assuming that Kepler 1625b-i is a real moon, it has a mass 19 times that of Earth (about 6% of the mass of Jupiter), is similar in mass to Neptune, and accompanies a gas planet with a mass 30 times the mass of Earth and a diameter equal to half that of Jupiter. Kepler 1708b-i is probably less massive than Kepler 1625b-i, has a diameter about five times that of Earth (about half the diameter of Kepler 1625b-i), and orbits a gas planet 4.6 times the size of Jupiter.
Artist’s impression of an exomoon orbiting the exoplanet Kepler 1708b.
“They challenge a lot of theories,” says Christiansen. “It’s hard to find a way for moons to form, so they must be trapped.” Being trapped makes them virtually like captured moons in the solar system. Like planets, they form from accretion cores in the disk and are then captured as they migrate towards their star.
Migration appears to be a common process in young planetary systems. Migration is the astronomers’ explanation for objects known as “hot Jupiters,” which are gas giants very close to their star but may have originally formed further away.
The extrasolar moons Kepler 1625b-i and Kepler 1708b-i were captured by larger planets as they migrated in front of them. However, they are probably not true moons, but rather examples of binary planets rather than extrasolar moons.
A binary planet exists when both worlds orbit the center of mass between them, rather than one orbiting the other. In our solar system, we have a double planet in the form of Pluto and its largest companion, Charon.
So, gas moons exist somehow, but nature has to cheat to make them!
Why is Jupiter not a star due to its large size?
The smallest known main-sequence star in the Milky Way is a red dwarf called EBLM J0555-57Ab, located 600 light-years from Earth. With an average radius of nearly 59,000 kilometers, this star is only slightly larger than the planet Saturn. Therefore, this red dwarf is the smallest known star that has hydrogen fusion in its core; The process that provides the star’s energy to burn until the end of its life.
In the solar system, there are two objects bigger than the mentioned star. One of them is the sun; But the other is the planet Jupiter , whose radius reaches 69,911 kilometers; But why is Jupiter a planet and not a star according to these dimensions?
The answer to the above question is simple: Jupiter does not have enough mass to support the hydrogen-to-helium fusion process. The star EBLM J0555-57Ab is nearly 85 times more massive than Jupiter. If this star was a little lighter, it would not be able to perform the hydrogen fusion process; But if the solar system had a different structure, would it be possible for the planet Jupiter to shine as a star?
Jupiter and the Sun are more similar than you might think
Jupiter may not be a star, but it has a huge influence on the solar system. The mass of this gas giant is 2.5 times the total mass of other planets in the solar system. On the other hand, Jupiter has a low density of 1.33 grams per cubic centimeter. While the density of the Earth is close to 5.51 grams per cubic centimeter, which is four times more than the density of Jupiter.
But it is interesting to point out the similarities between Jupiter and the Sun. The density of the sun is 1.41 grams per cubic centimeter. These two crimes are also very similar in composition. In terms of mass, nearly 71% of the sun is made up of hydrogen and 21% of it is made up of helium, and traces of other elements can be seen in it. On the other hand, 73% of Jupiter is made of hydrogen and 24% of it is made of helium.
Illustration of the planet Jupiter and its moon Io
For the above reasons, Jupiter is sometimes called a failed star; But again, Jupiter is unlikely to even come close to being a star. Stars and planets form in two completely different mechanisms. Stars form when a dense knot of matter in an interstellar molecular cloud collapses under its own gravity. This material begins to rotate in a process called cloud collapse. As rotation continues, more material from the surrounding cloud enters the stellar accretion disk.
With the increase in mass and as a result of gravity, the core of the baby star becomes more and more compact, which causes the temperature to increase and make it hotter. Finally, this mass becomes so compressed and hot that its core ignites and the process of thermonuclear fusion begins.
Based on our understanding of the star formation process, when a star runs out of accretion material, a full portion of its accretion disk remains. Planets form from this residue. According to astronomers, for gas giants like Jupiter, this process, called accretion, begins with small clumps of icy rocks and dust in the disk. With the rotation of these materials around the baby star, their density starts gradually and they stick to each other with the force of static electricity. Finally, these growing masses reach the size of nearly 10 times the mass of the earth; So that they can gravitationally absorb more gases from the surrounding disk.
From this stage, the gradual growth of the customer and its current mass began. The current mass of Jupiter is 318 times the mass of the Earth and 0.001 times the mass of the Sun. When a gas giant absorbs all its available matter, its growth stops. As a result, Jupiter has never even approached the mass of a star. The reason why Jupiter’s composition is similar to the Sun is not that it is a failed star; Rather, the reason for being born in the molecular gas cloud is the same as the sun.
Real failed stars
There are different groups of objects that can be classified as failed stars. These objects are called brown dwarfs and can fill the gap between gas giants and stars. The mass of brown dwarfs starts at 13 times the mass of Jupiter. These objects are heavy enough to support nuclear fusion, but this fusion is not of ordinary hydrogen but of deuterium or heavy hydrogen. Deuterium is an isotope of hydrogen that has one proton and one neutron in its nucleus instead of just one proton. The temperature and pressure of deuterium fusion is lower than the temperature and pressure of hydrogen fusion.
Since deuterium fusion occurs at lower mass, temperature and pressure, it is one of the steps to reach hydrogen fusion for stars whose accretion process continues and absorb the surrounding mass; But some objects never reach the required mass for hydrogen fusion.
Shortly after the discovery of brown dwarfs in 1995, these objects were called failed stars or ambitious planets, but numerous studies show that the formation of these objects like stars was from cloud collapse, not core accumulation; Some brown dwarfs do not even have enough mass to fuse deuterium, making them difficult to distinguish from planets.
Jupiter has exactly the lower mass limit for cloud collapse; The minimum mass required for cloud collapse is approximately equal to the mass of the planet Jupiter. As a result, if the planet Jupiter was formed from the collapse of a cloud, we could place it in the group of failed stars; But data from NASA’s Juno probe suggests that Jupiter at least once had a solid core, which is more consistent with the theory of core formation.
Modeling shows that the upper limit of planetary mass and formation by core accretion method is less than 10 times the mass of Jupiter. As a result, the planet Jupiter is not included in the group of failed stars; But by thinking about the cause of this issue, we can get a better understanding of how the universe works. In addition, the planet Jupiter has a stormy, striped and twisted appearance, and the existence of humans would probably not be possible without this gas giant.
Why do none of the moons of the solar system have rings?
We have many strange moons in our solar system. hot and cold moons; Moons with liquids and dusty moons. One lunar planet is walnut-shaped and another is potato-shaped; But among almost 300 moons that have been discovered so far, not even one of them has rings. This is really strange.
Of the eight planets in the solar system, half have rings of dust and ice that orbit their equator. It is thought that Mars once had a ring, and according to new research, even our blue planet probably had a ring similar to Saturn’s ring about 500 million years ago, which lasted for tens of millions of years.
In addition, some dwarf planets also have rings; Although astronomers have not yet been able to understand how these rings are formed. Even some asteroids have their own rings.
While investigating the concept of ringed moons outside our solar system, Mario Socercchia, an astrophysicist at the Universidad Adolfo Ibánez in Chile, and his colleagues became involved in the question of why moons in our own cosmic neighborhood lack rings. In an interview with Science Alert, he explains:
If the giant planets of the solar system have rings, and if the asteroids beyond the orbit of Jupiter and the non-Neptunian bodies also have rings, why don’t the moons of the solar system have rings? This absence is illogical considering the presence of rings in other places. As a result, it is better to investigate whether there are underlying dynamical reasons that prevent the formation of rings or their long-term stability around moons.
James Webb Space Telescope image of the rings of the planet Uranus.
We have yet to definitively discover an extrasolar moon, but in 2021 Soserkia and his colleagues hypothesized that if a moon had a large ring system, it could engineer its existence by blocking enough starlight. But the group later realized that we have yet to see any ringed moons, so the likelihood of their existence is very low.
When you’re an astronomer with a question in mind and a simulation tool at hand, there’s only one thing you can do: build models of cosmic systems and see what happens when you set them in motion.
There are many raw materials from which rings can form around the moons of the solar system. Some of these materials are dust resulting from the formation of impact craters. Some other moons emit steam or gas of their own, so there seems to be no problem with ring formation.
Considering the gravitational influence of the moon, host planet and other moons, researchers designed and tested physical N simulations and realized that due to these variables, ring formation around moons is difficult.
For example, Saturn’s moon Enceladus releases water vapor, ice particles, and gases from its glaciers in the Antarctic region with its remarkable surface activity. However, instead of forming a ring around this moon, these materials are transported into Saturn’s orbit by strong interactions with neighboring moons, feeding Saturn’s E ring.
In other words, even though the moons produce part of the raw materials necessary for the ring, their surrounding environment makes a large part of these materials available to the host planet and prevents the formation of the ring around the moons themselves.
So far, NASA has discovered 293 moons in the orbit of the planets of the solar system, most of which belong to the planets Jupiter and Saturn. Also, moons around dwarf planets and even asteroids have been discovered.
Soserkia and his team simulated the moons of a variety of solar system objects, from the Earth’s moon to the larger moons of Jupiter and Saturn, over millions of years of evolution. They sought to investigate the stability of these objects, their gravitational environment, possible ring systems, and their changes over time. The results of the investigation were contrary to the expectations of the researchers. Susarkia explains about this:
At first I expected rings to be completely unstable, which directly answered our question. However, contrary to expectation, we found that these structures have maintained their stability in many conditions. Indeed, in a previous paper we showed that isolated moons can have stable rings, but we did not predict that moons would remain stable in harsh gravitational environments despite the large number of other moons and planets that have distributed their rings. Another surprise came when we realized that these harsh environments, instead of destroying the rings, beautified them by creating structures like cracks and waves, which were just like what we see in Saturn’s rings.
Saturn’s moon Iapetus with its prominent equatorial ridge.
Some features of the moons of the solar system are signs of the past of the rings. The simulations suggest that the pebbles found orbiting Saturn’s moon Rhea could be the last remnants of a complete ring system. Also, Saturn’s moon Iaptus has a equatorial groove, which could be the remnant of a ring that fell on this moon, and in this sense, it is just like Saturn’s rings that slowly fall on this gas giant.
The findings show that the reason we do not see rings in the solar system today is that we are not in the right time and place. Solar radiation pressure, magnetic fields, internal heating, and magnetospheric plasma all contribute to the loss of once-existing lunar rings. According to Susarkia:
I believe we are unlucky to some extent; Because we started observing the universe during a period when these structures no longer exist. After doing this research, I was convinced that these rings probably existed in the past.
On the other hand, the only reason we see Saturn’s rings is because we are in the right place and time. For this reason, we see solar and lunar eclipses; Because the moon is gradually moving away from the earth and at some point it will be so far that it can no longer completely cover the sun.
The glory of Saturn’s rings.
The researchers believe that further simulations that take into account more parameters, such as beam pressure and magnetic fields, can help us understand the absence of lunar rings in more detail. We should also look more closely at the moons and look for evidence of the past, such as the craters on Iaptus.
At the same time, Suserkia and his colleagues are looking to expand their search and look for moons of rings around alien extrasolar worlds. He explains:
I wonder what mythical and epic stories we will hear from the inhabitants of other worlds about ringed moons. I mean, how will their stories and culture about the moons of the rings be different from our stories? There are infinite possibilities.
The scientists’ research has been accepted for publication in the Journal of Astronomy and Astrophysics and is available in the archive database.
Why is Jupiter not a star due to its large size?
Why doesn’t Jupiter have big and bright rings like Saturn?
Why do none of the moons of the solar system have rings?
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