The ability of a planet to hold onto an atmosphere depends mainly on two factors: Temperature and Gravity. The temperature of a planet is important because it is really just a measure of how fast, on average, the molecules of gas in the atmosphere are moving around. The higher the temperature, the faster the molecules are moving. The gravity of a planet is important because it determines the escape velocity of a planet. Any object with a velocity greater than the escape velocity will escape the gravitational pull of the planet.
The gravity of a planet is determined by its mass and radius. A planet with a stronger gravitational pull will have a higher escape velocity. The table to the right lists the escape velocities and distances for a few worlds in our solar system.
The temperature of a planet is determined mainly by its distance from the Sun. The table below shows the temperature a planet would have at various distances from the Sun.
Finally, the speed of a molecule of gas in an atmosphere depends on its temperature and on its mass. A heavier molecule moves slower than a light molecule at the same temperature. The velocity of a molecule of gas can be determined from the equation:
A table of a few molecules and their masses are given in the table to the right.
A "rule of thumb" in planetary science is that a planet can hold onto a gas for the age of the solar system if the velocity of the gas is less than one sixth the escape velocity of the planet. In equation form this is:
