Enigmas of the Cosmos: What Accounts for the Absence of Gas Moons?

We observe moons made of rock, moons with oceans, and moons covered in ice, but the absence of gas moons raises a perplexing question.

In our solar system, we encounter moons of various compositions such as rocky moons (e.g., Earth’s moon), ocean moons (e.g., Europa and Enceladus), and frozen-ice moons (e.g., Triton). Surprisingly, there are no gas moons in our immediate cosmic vicinity. Is this a matter of cosmic chance, or are there fundamental reasons preventing the existence of gas moons?

Interestingly, gas moons do exist, but not within our solar system. Despite the identification of over 5,500 exoplanets, only two potential exomoons have been detected, both yet to be fully confirmed. What distinguishes these ‘exomoons’ is their peculiar nature as gas giants orbiting even larger gas giants. However, they stand as exceptions rather than the norm.

To comprehend the absence of gas moons in our solar system, it is essential to delve into the formation process of gas giant planets. Two primary scenarios for gas-giant planet formation are known as ‘bottom-up’ formation and ‘top-down’ formation.

Forming gas worlds from the bottom-up

The formation process known as ‘core accretion’ or ‘bottom-up’ elucidates the origin of the gas giant planets within our solar system. If one were to journey back 4.5 billion years, the scene would unfold with a youthful sun encircled by a disk composed of gas and dust—a protoplanetary disk that birthed all the planets. Initially, these celestial bodies took shape as rocky entities, gradually expanding as they accumulated dust, pebbles, and asteroids. While some grew only to the size of Mars or Venus, others continued to grow, evolving into colossal rocky bodies with masses reaching up to ten times that of Earth.

Upon attaining significant mass, these bodies possessed gravitational forces potent enough to commence the sweeping up of substantial amounts of gas from the protoplanetary disk. The extent to which they plundered gas and their ultimate size depended on factors such as their gravity and the abundance of available gas.

In the end, our solar system retained four gas giant planets—Jupiter and Saturn, along with the frigid ‘ice giants’ Uranus and Neptune. NASA’s Juno mission to Jupiter has contributed to supporting the core accretion model by detecting the presence of a sizable, rocky yet diffuse core at the center of Jupiter, approximately ten times the mass of Earth, as evidenced by gravitational measurements.

Forming gas worlds from the top-down

In the top-down model, gas giants come into existence directly through the collapse of a gas clump within a nebula, akin to the formation of stars. However, this process has a minimum threshold for mass production.

As a sizable gas clump contracts under the influence of its own gravity, it undergoes heating because the gas is compressed into an ever-diminishing, and thus denser, volume. However, warm gas tends to expand, creating a need for the gas clump to radiate away its excess heat to sustain the contraction. Consequently, we frequently observe collapsing gas clouds emitting thermal infrared energy.

Nonetheless, a limiting factor known as the ‘opacity limit for fragmentation’ comes into play. “Radiating away enough heat so that the gas can cool down and still collapse depends on the opacity of the dust, and the temperature and the density, and that process becomes much less efficient with smaller objects to the point where at about 3 Jupiter masses it cannot radiate out enough heat to keep collapsing,” explained Sam Pearson of the European Space Agency in an interview.

As the volume decreases, the dust becomes more concentrated and opaque, causing the process of radiating away excess heat from gravitational contraction to progressively lose efficiency. Therefore, the top-down process cannot yield objects smaller than 3 Jupiter masses.

Why the solar system has no gas moons

Much like their parent planets, the majority of moons within our solar system underwent formation through the ‘bottom-up’ core accretion process. This occurred within disks comprised of residual material encircling their parent planets. However, as the planets had already assimilated most of the available material, the remaining resources were insufficient to generate a moon of substantial mass capable of retaining a significant amount of gas. In fact, among all the moons in the solar system, only Saturn’s largest moon, Titan, possesses an atmosphere.

Conversely, a ‘top-down’ process could not have transpired due to the limited remaining gas. Even if it had occurred, a minimum mass requirement of 3 Jupiter masses would have resulted in the creation of a celestial body surpassing all others in the solar system by a considerable margin.

Odd moons

So, the conventional processes for forming gas worlds do not allow for the creation of gas moons. However, there are anomalies in the solar system that developed through alternative means.

Take Earth, for instance; its moon likely formed from material ejected during a colossal collision with a Mars-sized protoplanet. This debris, forming a ring, led to the creation of Earth’s moon through core accretion. Could a gas moon be formed by impacting a gas giant planet and ejecting enough gas?

Regrettably, this is not the case. While rocky planets can experience impactful events, as demonstrated by the collision of comet Shoemaker–Levy 9 with Jupiter in 1994, gas giants absorb anything that collides with them, rendering the idea of ejecting debris into space implausible.

Another anomaly involves captured moons. For example, Mars’ moons Phobos and Deimos are captured asteroids, Saturn’s outermost moon Phoebe is a captured cometary object, and Neptune’s moon Triton is a captured Kuiper Belt object. These moons did not form around a planet but rather originated independently in space, later approaching and being captured by a planet’s gravitational pull.

This raises the question: could a smaller gaseous planet be captured by a larger gas planet? In principle, given that gaseous worlds can attain masses up to a dozen times that of Jupiter, they could conceivably capture a gas world with a mass equivalent to, for example, Neptune.

The gas exomoons

Indeed, it appears that they can! “It might be the case that there are Neptune-sized moons around giant exoplanets,” stated Christiansen.

The two potential exomoons mentioned earlier—Kepler 1625b-i and Kepler 1708b-i—are both substantial gas giants themselves but seem to function as moons to even more massive gas giants.

“I will stress that both of these are candidates,” emphasized Christiansen. “We observe something in the data that aligns with a moon, but there are alternative explanations.”

Assuming these are genuine moons, Kepler 1625b-i possesses a mass 19 times that of Earth (approximately 6% of Jupiter’s mass), placing it in the same mass range as Neptune. It orbits a gas giant with a mass 30 times that of Earth and a diameter half that of Jupiter.

Kepler 1708b-i is likely less massive than Kepler-1625b-i, boasting a diameter roughly five times that of Earth (about half of Kepler-1625b-i’s size). It revolves around a giant planet that is 4.6 times more massive than Jupiter.

“They pose challenges to numerous theories,” Christiansen remarked. “It’s difficult to conceive of a process by which they formed in that manner, suggesting they likely underwent capture.”

If they were indeed captured objects, in principle, they would share similarities with the captured moons within our solar system. Initially forming as planets through core accretion within a disk, they would later be captured due to their inward migration towards their star.

Migration emerges as a prevalent phenomenon in nascent planetary systems, serving as the explanation for “Hot Jupiters,” gas giants situated remarkably close to their stars, a proximity challenging their formation at such proximity. In the case of the exomoons Kepler 1625b-i and 1708b-i, their migration led to entrapment by more massive planets positioned ahead of them.

However, despite these dynamics, they likely do not qualify as true moons. Instead, both probably represent instances of double planets rather than exomoons. A double planet scenario occurs when both celestial bodies orbit a shared center of mass situated between them, as opposed to one revolving around the other. In our own Solar System, Pluto and its prominent companion, Charon, exemplify a double planet.

Hence, while gas moons exist in a sense, nature employs a form of “cheating” to bring them into existence!

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