1. Low Temperature: The outer planets are much farther from the Sun, resulting in extremely cold temperatures. These frigid temperatures cause gas molecules to move very slowly, reducing their kinetic energy. This makes it far less likely for gas molecules to escape the planet's gravitational pull.

2. Strong Gravity: The outer planets, like Jupiter and Saturn, are massive gas giants with immense gravitational fields. Their strong gravity effectively traps the gas molecules, preventing them from escaping into space.

3. Solar Wind: The Sun emits a constant stream of charged particles called the solar wind. While the solar wind can exert a force on the outer planets' atmospheres, its effects are much weaker at those distances.

4. Atmospheric Escape: While some gas molecules do escape, the rate of escape is very slow. The escape rate depends on factors like the planet's temperature, gravity, and the composition of its atmosphere. For the outer planets, the escape rate is far slower than the rate at which they retain their gas.

5. Magnetic Field: The outer planets have strong magnetic fields, which also play a role in shielding their atmospheres from the solar wind and other external forces.

In summary, the combination of low temperatures, strong gravity, and other factors like the solar wind and magnetic fields create an environment where outer planets are able to retain their gaseous atmospheres over extremely long periods.

It's also worth noting that while we call them gas giants, these planets are not entirely made up of gas. They have solid cores, but these cores are relatively small compared to their massive gaseous envelopes.