<p>To answer this, we have to consider the definition of an atmosphere. A popular way of looking at it is to think of an atmosphere as a layer of gases surrounding a body. by that definition, we can say that gas giants are really just planets that are massive enough to accrete substantially large atmospheres (because deep down, they have a rocky core - although not a core similar to the "rocky" inner planets in our solar system). So now we can reduce the question to "Why do most natural satellites not have extremely large atmospheres?"</p>

<p>This comes down to mass. The more massive an object is, the more gravitational pull it has on objects around it. This means that a very massive object traveling through a region of gas would attract the gas more than a less massive object would. In the early Solar System, the more massive bodies far enough out in the protoplanetary disk managed to gain a substantial amount of matter via <em>core accretion</em>, as gas and dust were collected by a rocky core. Moons are unable to do this because they are very low-mass; therefore, they cannot become gas giants (in fact, many moons around giant planets may be the remains of the material the planets never accreted).</p>

<p><a href="https://en.wikipedia.org/wiki/Gas_dwarf" rel="nofollow noreferrer">Gas dwarfs</a>, low-mass gas planets with small envelopes, are certainly possible. However, they would have to have radii at least 1.5-2 times the radius of the Earth, and masses scaled appropriately. Moons are nowhere near that size.</p>

CC BY-SA 3.0
2016-11-27T23:40:49.263Z
The rocky Sun core, if existed, wouldn't be just not molten, it would be plasma, and probably mixed with all the hidrogen and helium at the center of the Sun. We can be sure that the Sun has more metalls than the Earth, but they don't  form a rocky core, at least nowadays. However, an interesting question could be if the Sun could have accreted around a rocky core.
Pere
CC BY-SA 3.0
2016-11-27T23:48:15.363Z
@Pere That has to do with formation. The reason giant planets are able to form is that there are enough [volatiles](https://en.wikipedia.org/wiki/Volatiles) ("ices") where they form (beyond the [frost line](https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29)). Nothing inside that is cold enough for volatiles to form; you couldn't have any significantly large core. Besides, stars form differently than planets; they come directly from the collapse of a protostellar cloud, rather than a protoplanetary disk.
HDE 226868
CC BY-SA 3.0
2014-09-21T17:57:56.923Z
So does cores of Gas Giants in Solar System have more Mass than the mass mass of Earth? I mean to hold all the gases in atmosphere will require stonger gravitational pull and hence more mass, as the atmosperic pressure of gas giants is greater than that of Earth.
CrownedEagle
CC BY-SA 3.0
2014-09-21T18:02:04.030Z
We don't know much about the cores of the gas giants (estimates for Jupiter's range from 12 to 45 Earth masses), but the masses of the core and mantle for all four gas giants are each at least about 9 times the mass of Earth, with some being higher.
HDE 226868
CC BY-SA 3.0
2014-09-21T20:56:14.630Z
Does the Sun also have a rocky core?
Scottie
CC BY-SA 3.0
2014-09-21T20:57:32.103Z
@Scottie No. Also, if it did, it would be completely molten.
HDE 226868
CC BY-SA 3.0
2014-09-21T21:00:10.150Z
A molten rocky core is still a rocky core, right?  The Earths core is molten, and we consider it rocky, don't we?
Scottie
CC BY-SA 3.0
2014-09-21T21:02:13.100Z
@Scottie Oh, you didn't mean a solid core. Well, yes, a molten core can still be a rocky core. I don't believe anyone knows for sure about the Sun's core. However, there aren't any plausible explanations for how one could come to exist there in the first place.
HDE 226868
CC BY-SA 3.0
2014-09-21T21:03:17.457Z
@HDE226868 Fair enough!  :)
Scottie
CC BY-SA 3.0
2014-09-21T17:52:02.030Z
2016-11-27T19:01:43.960Z
2016-11-27T19:01:43.960Z
HDE 226868
<p>Why are the natural satelites (moons) of all planets - including the moons of the gas giants - solid or rocky, and not gaseous?</p>

CC BY-SA 3.0
2014-10-02T04:07:43.173Z
What would you consider Saturn's rings? A moon that never formed, a billion tiny moons? A solid? A billion tiny solids? Composed mainly of water ice, are they solid or frozen gas?
Marc
<solar-system><natural-satellites><gas-giants>
Why are the natural satellites (moons) of all planets solid?
CrownedEagle
# Why are the natural satellites (moons) of all planets solid?

(Asked by: username_0 on 2014-09-21)

Why are the natural satelites (moons) of all planets - including the moons of the gas giants - solid or rocky, and not gaseous?

**Tags:** solar-system, natural-satellites, gas-giants

> **Comments:**
> **username_1**: What would you consider Saturn's rings? A moon that never formed, a billion tiny moons? A solid? A billion tiny solids? Composed mainly of water ice, are they solid or frozen gas?  

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## 1 answers

### Answer by username_2 on 2014-09-21. Score: 6

To answer this, we have to consider the definition of an atmosphere. A popular way of looking at it is to think of an atmosphere as a layer of gases surrounding a body. by that definition, we can say that gas giants are really just planets that are massive enough to accrete substantially large atmospheres (because deep down, they have a rocky core - although not a core similar to the "rocky" inner planets in our solar system). So now we can reduce the question to "Why do most natural satellites not have extremely large atmospheres?"

This comes down to mass. The more massive an object is, the more gravitational pull it has on objects around it. This means that a very massive object traveling through a region of gas would attract the gas more than a less massive object would. In the early Solar System, the more massive bodies far enough out in the protoplanetary disk managed to gain a substantial amount of matter via *core accretion*, as gas and dust were collected by a rocky core. Moons are unable to do this because they are very low-mass; therefore, they cannot become gas giants (in fact, many moons around giant planets may be the remains of the material the planets never accreted).

[Gas dwarfs](https://en.wikipedia.org/wiki/Gas_dwarf), low-mass gas planets with small envelopes, are certainly possible. However, they would have to have radii at least 1.5-2 times the radius of the Earth, and masses scaled appropriately. Moons are nowhere near that size.

> **Comments:**
> **username_0**: So does cores of Gas Giants in Solar System have more Mass than the mass mass of Earth? I mean to hold all the gases in atmosphere will require stonger gravitational pull and hence more mass, as the atmosperic pressure of gas giants is greater than that of Earth.  
> **username_2**: We don't know much about the cores of the gas giants (estimates for Jupiter's range from 12 to 45 Earth masses), but the masses of the core and mantle for all four gas giants are each at least about 9 times the mass of Earth, with some being higher.  
> **username_4**: Does the Sun also have a rocky core?  
> **username_2**: @username_4 No. Also, if it did, it would be completely molten.  
> **username_4**: A molten rocky core is still a rocky core, right?  The Earths core is molten, and we consider it rocky, don't we?  
> **username_2**: @username_4 Oh, you didn't mean a solid core. Well, yes, a molten core can still be a rocky core. I don't believe anyone knows for sure about the Sun's core. However, there aren't any plausible explanations for how one could come to exist there in the first place.  
> **username_4**: @HDE226868 Fair enough!  :)  
> **username_3**: The rocky Sun core, if existed, wouldn't be just not molten, it would be plasma, and probably mixed with all the hidrogen and helium at the center of the Sun. We can be sure that the Sun has more metalls than the Earth, but they don't  form a rocky core, at least nowadays. However, an interesting question could be if the Sun could have accreted around a rocky core.  
> **username_2**: @username_3 That has to do with formation. The reason giant planets are able to form is that there are enough [volatiles](https://en.wikipedia.org/wiki/Volatiles) ("ices") where they form (beyond the [frost line](https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29)). Nothing inside that is cold enough for volatiles to form; you couldn't have any significantly large core. Besides, stars form differently than planets; they come directly from the collapse of a protostellar cloud, rather than a protoplanetary disk.  

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