<p><a href="http://www.cfa.harvard.edu/news/2014-01" rel="noreferrer">This may help you</a></p>

<p>KOI-314c is small even when compared to Uranus or Neptune.</p>

<p>Also, look for "Mini-Neptune" on Wikipedia.</p>

<p>So, it seems those "dwarf-giant" planets are possible.</p>

<p>Although I do not have the required knowledge, I wonder if it is possible to exist a planet <em>smaller than</em> the earth that still could be considered a gas planet.</p>

<p>For example, just think of something mars-sized or between mars and the earth in size and mass (by size I mean not only the "solid" part, but including the atmosphere outer layers, just as one would see at the eyepiece - Venus, Titan). Now, think of it as a heftier version of Titan with a much extended and denser atmosphere... could that be called a Gas planet? Besides, nature don't have to fit exactly to our classifications/expectations. </p>

<p>I know Hydrogen and Helium are very volatile and have a small atomic mass, but I guess in a very cold environment, for example few AUs around a red/brown dwarf, a small planet with a small mass (that means low escape velocity) could retain all of its original atmosphere for aeons.</p>

<p>I dare say that even the most creative sci-fi authors coudn't predict the variety of planets that are being discovered nowadays!</p>

CC BY-SA 3.0
2014-01-30T13:28:36.403Z
2014-01-30T14:43:59.300Z
2014-01-30T14:43:59.300Z
Bruno Alessi
<p>There are a few major problems with calling something a <em>small gas planet</em> and of course depends on your definition of <em><a href="https://en.wikipedia.org/wiki/Planet">a planet</a></em>. By currently accepted classification (that excludes Pluto), a planet has to be:</p>

<ul>
<li>massive enough to be rounded (i.e. held together) by its own mass, </li>
<li>has to orbit a star, and </li>
<li>has cleared its orbital neighborhood. </li>
</ul>

<p>All of these points are largely argumentative, but two major problems that a <em>small gas planet</em> would have to sustain itself for longer time during and after the formation within the <a href="https://en.wikipedia.org/wiki/Protoplanetary_disk">protoplanetary disk</a> are:</p>

<ul>
<li>It will either be too close to the sun it orbits and the planet's own gravity holding it together wouldn't be sufficient to prevent it being effectively blown away layer by layer by sun's activity, so you might end up calling such <em>small gas planet</em> a <a href="https://en.wikipedia.org/wiki/Protoplanet">protoplanet</a> as it wouldn't persist, or </li>
<li>it would likely be formed large distance from its sun and might not stay in orbit around it after it cleared with <a href="https://en.wikipedia.org/wiki/Accretion_%28astrophysics%29">accretion</a> its path out of the protoplanetary disk and went rogue. So you might end up calling such <em>small gas planet</em> a free-floating <a href="https://en.wikipedia.org/wiki/Extrasolar_planet">exoplanet</a> or a <a href="https://en.wikipedia.org/wiki/Rogue_planet">rogue planet</a>.</li>
</ul>

<p>Another problem that we didn't yet discuss is that we have no observational proof that such <em>small gas planets</em> actually exist, whatever you might end up calling them. Problem of course lies in the fact there simply aren't any smaller gas planets than <a href="https://en.wikipedia.org/wiki/Uranus">Uranus</a> or <a href="https://en.wikipedia.org/wiki/Neptune">Neptune</a> (you pick your candidate for the <em>smallest</em> one; former is least massive, and latter is least voluminous of the four gas giants) in our own Solar system, while <a href="https://en.wikipedia.org/wiki/Extrasolar_planet">extrasolar</a> <em>small gas planets</em> would likely be too small to be detectable over enormous distances to even our nearest stars by any <a href="https://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets">methods of detecting such extrasolar planets</a>, or we wouldn't be observing enough into the system's past (i.e. distance) to see it still forming and it would have already lost grip to its former planets now exoplanets, or erode the smaller protoplanets.</p>

<p>So we have this conundrum between whether they exist (likely they do), are in theory possible (I have no doubt), and how we'd end up calling them, because neither of the two possibilities I can think of for their existence would be considered <em>planets</em> by mentioned current convention of what actually constitutes <em>a planet</em>. Protoplanet wouldn't have cleared its neighborhood yet, and exoplanet wouldn't orbit a star.</p>

CC BY-SA 3.0
2013-09-29T02:01:59.057Z
2013-09-29T02:01:59.057Z
TildalWave
<p>Since gas giants have a low density, they normally weigh less than a rocky planet of the same size would. This also means that gravity is very low and it could be unable to clear it's neighbourhood. Thus it is not defined as a planet.</p>

CC BY-SA 3.0
2016-12-31T07:28:14.030Z
What if they have a high density core? For example, an Earth-mass gas planet with Mars-sized rocky core?
peterh
CC BY-SA 3.0
2017-02-23T18:49:00.803Z
@peterh At what point does the rocky core turn it from a gas dwarf into a rocky planet with a thick atmosphere (c.f. Venus)
Michael
CC BY-SA 3.0
2017-02-23T19:15:22.460Z
@Michael Although the Venus has a thick atmosphere, compared to the Earth, it is absolutely incomparable with, for example, the Uranus. The whole mass of the Uranus is gas, while the Venusian atmosphere, although it is thick, it is nearly nothing compared to the mass of the whole planet. Planet with such a structure, which would have, for example, a Martian-sized rocky core, and an Earth-mass gaseous atmosphere, doesn't exist in the Solar System and the knowledge from the exoplanets is very far to find one. But it seems to me possible. I only asked, what if it exists.
peterh
CC BY-SA 3.0
2016-12-31T07:21:44.463Z
2016-12-31T10:46:36.630Z
2016-12-31T10:46:36.630Z
The Rino
<p>It appears that some small gas planets do exist, there is even a hypothesis (to explain its high iron comp) that Mercury was once one such small gas planet &quot;a hot neptune&quot; that 'fell' to a closer orbit and over the course if the last billion years or so had its atmosphere blasted away. In recent years, two exo planets were discovered that can be classified as small gas planets, however they are still larger than the earth (as this is what our present tech allows us so far) these planets are called 'Gas Dwarfs' or Mini-Neptunes with Kepler 138 being the smallest known so far being estimated at about 1.7-3 earth radii or something like that. It wouldnt surprise me if transitional planets could be as small as Venus, or at the very least for there to be Venus-sized planets that superficially appear to be gas planets but are technically mostly rocky planets with an liquid/gas atmosphere composed of hydrogen, ammonia, helium(the usual stuff) as thick percentile-wise as Europa's undersurface ocean.</p>
<p>As of what's mathematically possible, that should vary from au, temperature, mass, and composition. The further away from a star and thus lower the temperate, the smaller it is possible for a planetary body to be and remain stable, and the more likely it is to be smaller given available material at formation.</p>

CC BY-SA 4.0
2022-10-10T20:07:00.013Z
This is a good answer, but it would be even better with a reference or two, especially in regards to that theory about Mercury.
PM 2Ring
CC BY-SA 4.0
2022-10-10T12:20:51.960Z
2022-10-10T12:20:51.960Z
kaiserg
<p>Most of the known gas planets (Jupiter, Saturn, etc.) are huge. They are even called "gas <em>giants</em>". </p>

<p>is it possible to exist a gas planet around the size of Earth? If yes, why; if no, why?</p>

CC BY-SA 3.0
2016-12-31T15:10:45.217Z
Titan is small and rocky and has a thick atmosphere. It has no magnetic field or enough gravity to keep it in the long run, so I suppose it is replenished by active out-gassing. Some star likely has captured some planet sized comet in close orbit so that it sweats out its heavy volatiles as gasses at a much faster pace. Titan isn't exactly a new kid on the block, so it might count as "stable" in terms of how long a star like the Sun exists.
LocalFluff
CC BY-SA 3.0
2014-01-30T15:43:27.973Z
@Cheeku: Just put your $\LaTeX$ equation in single or double dollar, like $e^{-x^2}$.
Gerald
CC BY-SA 3.0
2013-09-29T01:47:16.123Z
Since it is a beta, I think I can mention stuff in my comment! Such are the questions where I feel the need of LaTeX. It will take me 5 minutes to put equations and answer this questions, but can't figure out how to explain it in words! :(
Cheeku
<planet><gas-giants>
Can small gas planets exist?
Zoltán Schmidt
# Can small gas planets exist?

(Asked by: username_0 on 2013-09-29)

Most of the known gas planets (Jupiter, Saturn, etc.) are huge. They are even called "gas *giants*".

is it possible to exist a gas planet around the size of Earth? If yes, why; if no, why?

**Tags:** planet, gas-giants

> **Comments:**
> **username_3**: Since it is a beta, I think I can mention stuff in my comment! Such are the questions where I feel the need of LaTeX. It will take me 5 minutes to put equations and answer this questions, but can't figure out how to explain it in words! :(  
> **username_2**: @username_3: Just put your $\LaTeX$ equation in single or double dollar, like $e^{-x^2}$.  
> **username_1**: Titan is small and rocky and has a thick atmosphere. It has no magnetic field or enough gravity to keep it in the long run, so I suppose it is replenished by active out-gassing. Some star likely has captured some planet sized comet in close orbit so that it sweats out its heavy volatiles as gasses at a much faster pace. Titan isn't exactly a new kid on the block, so it might count as "stable" in terms of how long a star like the Sun exists.  

---

## 4 answers

### Answer by username_5 on 2013-09-29. Score: 8

There are a few major problems with calling something a *small gas planet* and of course depends on your definition of *[a planet](https://en.wikipedia.org/wiki/Planet)*. By currently accepted classification (that excludes Pluto), a planet has to be:

* massive enough to be rounded (i.e. held together) by its own mass,
* has to orbit a star, and
* has cleared its orbital neighborhood.

All of these points are largely argumentative, but two major problems that a *small gas planet* would have to sustain itself for longer time during and after the formation within the [protoplanetary disk](https://en.wikipedia.org/wiki/Protoplanetary_disk) are:

* It will either be too close to the sun it orbits and the planet's own gravity holding it together wouldn't be sufficient to prevent it being effectively blown away layer by layer by sun's activity, so you might end up calling such *small gas planet* a [protoplanet](https://en.wikipedia.org/wiki/Protoplanet) as it wouldn't persist, or
* it would likely be formed large distance from its sun and might not stay in orbit around it after it cleared with [accretion](https://en.wikipedia.org/wiki/Accretion_%28astrophysics%29) its path out of the protoplanetary disk and went rogue. So you might end up calling such *small gas planet* a free-floating [exoplanet](https://en.wikipedia.org/wiki/Extrasolar_planet) or a [rogue planet](https://en.wikipedia.org/wiki/Rogue_planet).

Another problem that we didn't yet discuss is that we have no observational proof that such *small gas planets* actually exist, whatever you might end up calling them. Problem of course lies in the fact there simply aren't any smaller gas planets than [Uranus](https://en.wikipedia.org/wiki/Uranus) or [Neptune](https://en.wikipedia.org/wiki/Neptune) (you pick your candidate for the *smallest* one; former is least massive, and latter is least voluminous of the four gas giants) in our own Solar system, while [extrasolar](https://en.wikipedia.org/wiki/Extrasolar_planet) *small gas planets* would likely be too small to be detectable over enormous distances to even our nearest stars by any [methods of detecting such extrasolar planets](https://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets), or we wouldn't be observing enough into the system's past (i.e. distance) to see it still forming and it would have already lost grip to its former planets now exoplanets, or erode the smaller protoplanets.

So we have this conundrum between whether they exist (likely they do), are in theory possible (I have no doubt), and how we'd end up calling them, because neither of the two possibilities I can think of for their existence would be considered *planets* by mentioned current convention of what actually constitutes *a planet*. Protoplanet wouldn't have cleared its neighborhood yet, and exoplanet wouldn't orbit a star.


---

### Answer by username_4 on 2014-01-30. Score: 7

[This may help you](http://www.cfa.harvard.edu/news/2014-01)

KOI-314c is small even when compared to Uranus or Neptune.

Also, look for "Mini-Neptune" on Wikipedia.

So, it seems those "dwarf-giant" planets are possible.

Although I do not have the required knowledge, I wonder if it is possible to exist a planet *smaller than* the earth that still could be considered a gas planet.

For example, just think of something mars-sized or between mars and the earth in size and mass (by size I mean not only the "solid" part, but including the atmosphere outer layers, just as one would see at the eyepiece - Venus, Titan). Now, think of it as a heftier version of Titan with a much extended and denser atmosphere... could that be called a Gas planet? Besides, nature don't have to fit exactly to our classifications/expectations.

I know Hydrogen and Helium are very volatile and have a small atomic mass, but I guess in a very cold environment, for example few AUs around a red/brown dwarf, a small planet with a small mass (that means low escape velocity) could retain all of its original atmosphere for aeons.

I dare say that even the most creative sci-fi authors coudn't predict the variety of planets that are being discovered nowadays!


---

### Answer by username_6 on 2016-12-31. Score: 2

Since gas giants have a low density, they normally weigh less than a rocky planet of the same size would. This also means that gravity is very low and it could be unable to clear it's neighbourhood. Thus it is not defined as a planet.

> **Comments:**
> **username_7**: What if they have a high density core? For example, an Earth-mass gas planet with Mars-sized rocky core?  
> **username_8**: @username_7 At what point does the rocky core turn it from a gas dwarf into a rocky planet with a thick atmosphere (c.f. Venus)  
> **username_7**: @username_8 Although the Venus has a thick atmosphere, compared to the Earth, it is absolutely incomparable with, for example, the Uranus. The whole mass of the Uranus is gas, while the Venusian atmosphere, although it is thick, it is nearly nothing compared to the mass of the whole planet. Planet with such a structure, which would have, for example, a Martian-sized rocky core, and an Earth-mass gaseous atmosphere, doesn't exist in the Solar System and the knowledge from the exoplanets is very far to find one. But it seems to me possible. I only asked, what if it exists.  

---

### Answer by username_9 on 2022-10-10. Score: 0

It appears that some small gas planets do exist, there is even a hypothesis (to explain its high iron comp) that Mercury was once one such small gas planet "a hot neptune" that 'fell' to a closer orbit and over the course if the last billion years or so had its atmosphere blasted away. In recent years, two exo planets were discovered that can be classified as small gas planets, however they are still larger than the earth (as this is what our present tech allows us so far) these planets are called 'Gas Dwarfs' or Mini-Neptunes with Kepler 138 being the smallest known so far being estimated at about 1.7-3 earth radii or something like that. It wouldnt surprise me if transitional planets could be as small as Venus, or at the very least for there to be Venus-sized planets that superficially appear to be gas planets but are technically mostly rocky planets with an liquid/gas atmosphere composed of hydrogen, ammonia, helium(the usual stuff) as thick percentile-wise as Europa's undersurface ocean.

As of what's mathematically possible, that should vary from au, temperature, mass, and composition. The further away from a star and thus lower the temperate, the smaller it is possible for a planetary body to be and remain stable, and the more likely it is to be smaller given available material at formation.

> **Comments:**
> **username_10**: This is a good answer, but it would be even better with a reference or two, especially in regards to that theory about Mercury.  

---

