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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?

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    $\begingroup$ 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! :( $\endgroup$ – Cheeku Sep 29 '13 at 1:47
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    $\begingroup$ @Cheeku: Just put your $\LaTeX$ equation in single or double dollar, like $e^{-x^2}$. $\endgroup$ – Gerald Jan 30 '14 at 15:43
  • $\begingroup$ 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. $\endgroup$ – LocalFluff Dec 31 '16 at 15:10
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According to the article "Minimum planetary size for forming outer Jovian-type planets - Stability of an isothermal atmosphere surrounding a protoplanet" (Sasaki, 1989), not only is this possible, but is suggested to have been an evolutionary stage for the development of Uranus and Neptune. But there are a few conditions to allow this to happen, primarily due to distance from the sun, hence temperature.

The article's findings suggest that the critical mass for 'atmospheric collapse' is 0.2 of the current mass of the Earth for distances of 5 AU or greater for our solar system (this value of course would vary, depending on the parent star). This critical mass increases the closer you are to the parent star - the article explicitly states that Mercury and Mars are too small for this to occur.

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There are a few major problems with calling something a small gas planet and of course depends on your definition of a 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 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 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 its path out of the protoplanetary disk and went rogue. So you might end up calling such small gas planet a free-floating exoplanet or a 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 or 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 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, 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.

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This may help you

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!

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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.

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    $\begingroup$ What if they have a high density core? For example, an Earth-mass gas planet with Mars-sized rocky core? $\endgroup$ – user259412 Dec 31 '16 at 7:28
  • $\begingroup$ @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) $\endgroup$ – Michael Feb 23 '17 at 18:49
  • $\begingroup$ @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. $\endgroup$ – user259412 Feb 23 '17 at 19:15

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