There are a few factors that determine whether a planet will have an atmosphere or not. These include:

1. How a planet forms.
2. The planet’s mass and how long it takes to "cool off".
3. The planet’s composition.

Let’s briefly review this process so that you have a basic idea of how it all works:

In the development of a planet, there are four different processes that occur. Each planet varies in the amount of time it takes to go through each phase and the extent of change the planet undergoes within each stage. Although all four phases are important, its the first phase which determine most what the atmosphere will be like. (NOTE: This doesn’t mean that the atmosphere will not change over time; it just means that it is the most important phase for what the atmosphere of a planet will be like in the beginning.)

In this first phase, the material making up the planet separates according to density. The more dense material will clump together in the center of the planet, and the least dense material will "float" to the top of the planet, forming the crust or surface. The amount of differentiation is greater if the planet is molten (made of melted material) when it formed. There are two heat sources which can generate the heat required to melt the metals which make up the inner planets: in-falling material (like meteors) will transfer their energy in the form of heat when they hit the planet; in addition, the decay of radioactive elements will generate heat. It is this decay of radioactive elements that are concentrated in the Earth’s crust that warm and soften the rock layers, allowing for movement and change over time.

The creation of a planetary atmosphere from the planet’s interior is called outgassing. What is outgassing? Well, gases from the solar nebula get trapped in the interior of a planet. These gases were mostly hydrogen and helium. These gases were light, and the heat from the planet basically "blew off" these gases. As the rock material baked and cooled, new gases were released from the rocks, forming an atmosphere. Scientists think that planets formed relatively rapidly, and the heat generated from the forming planet could not escape into space. This heat melted the planets as they formed, and in their molten state (liquid rock) gases were allowed to escape. The gases that did not get "blown off" into space were heavier, and that’s why the inner planets have atmospheres with ‘heavier" gases – gravity could hold onto those gas particles. The lighter ones escaped.

Scientists are pretty sure that atmospheres are not formed entirely by outgassing. Some of the gas components of an atmosphere are thought to have been "added" later on by comet impacts and late accretion of matter in the formation of planets.

This is the most violent phase of planet formation. The forming planets are bombarded by material from the solar nebula, and they are hit by meteors of all shapes and sizes. As the meteors impact the forming planet, they form craters. Erosional processes have erased the evidence of many craters on Earth, but if you look at the surface of Earth’s moon on a clear night, you can see the craters easily from Earth.

Flooding actually begins before the cratering phase is complete. The broken crust and the radioactive elements that decay will melt the rock material that forms the planet. This molten rock will gradually seep up through cracks in the crust, and the lava that reaches the surface of the planet will flood the low-lying areas of the planet.

You may be wondering WHY Earth has so little carbon dioxide in the atmosphere when Venus and Mars have so much. Well, it’s like this: Outgassing from a forming planet such as Earth would release mostly carbon dioxide and water vapor. Carbon dioxide is very soluble in water, and the earth’s surface temperature is just right for it to be covered with water. Much of the carbon dioxide released in outgassing dissolves in oceans and combines with minerals in seawater to form deposits of silicon dioxide, limestone, etc. So the carbon dioxide has been removed from Earth’s atmosphere and buried in the Earth’s crust (rocks).