Some planets in our solar system are rocky, and some are gaseous. Even gas giants orbiting
our solar system have rocky moons. Currently, there are more than 200 moons in
the solar system. Why aren't some moons in the solar system made of gas? Does the universe
contain gaseous moons? Even though there are no gaseous moons outside
our solar system, it may still be possible under the right conditions.
Several factors are involved, including the mass of the Moon, ambient temperature and
wave forces, as well as the gravitational pull of any nearby object, such as the planet
orbiting the Moon. Let's take the moon as it is a rock and then
create a hydrogen atmosphere around it. We know thermal effects will cause the hydrogen
atmosphere to escape very quickly. The warmth of the sun would cause the hydrogen to evaporate
away. Gases such as hydrogen are much less dense
than rock, so the moon would grow to the size of the Earth. Therefore, it would be more
appropriate to call the gas moon a "binary planet", as it is likely to have the same
size as its parent planet. We can conclude from this that a hydrogen-only
moon will not be stable. But even if our imaginary gas moon were as big as Earth and the temperature
around it was too cold, the host planet would tear it up.
Also, because it is so small, gravity won't be powerful to hold those light elements together.
As with gaseous planets, the gas must be larger than its rock counterpart. This is because
if it were too small, it would not have enough gravity to hold lighter gases like methane.
Our moon is subject to tidal forces from Earth, but it does not decompose because it is combined
with some physical force. Since the hypothetical gaseous moon is gas,
not a solid - even if it's very cold - if it orbited around another planet, then the
moon would be tidally stripped by the tides. So how is it possible to have a gaseous moon?
It would have to be extremely distant and very cold, or very large.
It is not possible to work on a body as large as our moon anywhere in our solar system.
Can it work in interstellar space? Probably not.
Certainly, you can make something as big as a planet around Jupiter, for example, Neptune.
The gravitational forces holding these huge bodies together would likely prevent tidal
forces from destroying the Neptune-like moon. There could be two types of gaseous moons:
one could have a rocky or metallic core that formed on the edge of the protoplanetary disk
and then migrated inward, the other could only be gas without a solid core, but this
is rare. Astronomers have detected what appears to
be the first moon outside our solar system. This is a gaseous world that's about the size
of Neptune and orbits a gas giant much larger than Jupiter.
By solar system standards, it is a big and strange moon, known as an exomoon because
it is outside the solar system. All of the moons in our solar system are rocky or icy.
The newly discovered bodies are both gaseous, an unexpected pairing. About 5400 light-years
away from Earth, they are located in the constellation Cetus.
The Hubble Space Telescope and Kepler Space Telescope provided the first clear evidence
of an exomoon, but more Hubble observations will be needed to confirm the discovery.
The exomoon is exponentially larger than the largest moon of our solar system, Jupiter's
moon, Ganymede, which has a diameter of 3,270 miles.
According to estimates, the exomoon has a diameter of approximately 30,600 miles, about
the same as that of Neptune, the smallest of our solar system's four gas planets.
The exomoon's mass is about 1.5 percent that of its planet. Its size and gaseous composition
challenge current moon formation theories. The main reason we don't have gas moons is
that they are not big enough. Because gas has a high degree of movement, it is very
volatile, making it easy to escape. All bodies in the solar system with gas in
their atmospheres experience this process. However, the rate at which they lose gas tends
to be very slow. A high movement rate means objects have to
reach a certain size before they can hold onto an atmosphere. Almost all moons fall
below this size limit. The moon's gravitational influence usually
cannot rival that of its parent planet since it is so small. As a result, the planet usually
robs moons around it of its gas. Gas moons, similar to gas planets, are not
possible. Closest you could get would be a binary planet system, with roughly similar-sized
gas planets orbiting each other. Gas binary planets may exist, but they are
very rare because planetary formation usually results in two planets merging or separating
from each other. As a result, gas satellites have not been discovered.