### Can small gas planets exist?

• 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?

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: Just put your \$\LaTeX\$ equation in single or double dollar, like \$e^{-x^2}\$.

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.

9 years ago

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.