In the Solar System there is as apparent relationship between the relative abundance of ice, rock, or gas and planet mass. This can be seen in the image to the left. Given these different compositions, scientists (such as professor Sara Seager) can begin to categorize the different types of atmospheres, and predict what we might expect from an atmosphere on a given type of planet. Although the relationship between mass and composition is quite strong in our solar system, scientists are finding that the composition and mass distribution of planets likely fills the entire range of possibilities shown in the image to the left. Still, exoplanet atmospheres can be roughly grouped in the following categories:
1) Hydrogen and Helium dominated
Throughout the Milky Way galaxy, the average composition of all visible matter is roughly 75% hydrogen and 25% helium (1-3% comes from other elements). Planets which are dominated by hydrogen and helium, similar to the gas giants in our solar system, will likely have these same elemental composition ratios, which would indicate that they are formed primarily by capture from the protoplanetary nebula (or by planet formation from gravitational collapse).
2) Hydrogen-rich Outgassed Atmospheres
Other planets will have atmospheres that are made of up of something other than captured cosmic material; some of these planets will have outgassed material from an icy or rocky surface. If the planet is massive enough, outgassed hydrogen can become trapped in the planet's atmosphere, and the atmosphere itself can be composed of up to 50% hydrogen by weight. These hydrogen rich atmopheres will likely contain Hydrogen gas (H2), naturally occurring water (H2O), and methane (CH4) or carbon monoxide (CO), but not the carbon dioxide (CO2) or nitrogen gas (N2) that is seen in the terrestrial planets in our Solar System. Helium would also not likely be present, since this element is not trapped in rock and therefore would not be outgassed into the atmposphere.
3) Carbon Dioxide-rich Outgassed Atmospheres
These outgassed atmospheres are similar to those above, except that they have lost their hydrogen and helium and are dominated by carbon dioxide (CO2). On Earth, our atmosphere could potentially be in this category except that CO2 became dissolved in our oceans and sequestered in limestone sedimentary rocks, leaving nitrogen (N2)as the dominant atmospheric gas. In these atmospheres, signs of water molecules (H2O) may indicate a liquid water ocean.
4) Hot Super-Earths Lacking Volatiles
This category is reserved for atmospheres that are very hot. When atmospheric temperatures reach 1,500 degrees Kelvin, these hot Super Earths will lose their hydrogen and other volatiles such as carbon, nitrogen, oxygen, and sulphur. Instead, these atmospheres would likely be composed of silicates and other refractory elements such as calcium, aluminum, and titanium.
5) Atmosphereless Planets
The final category includes planets that have been unable to retain any significant atmosphere. Less massive planets, very hot planets, planets without active outgassing, and planets subject to strong solar winds can potentially find themselves in this category.
In the end, the actual planet atmospheric composition depends on the interior composition of the planet, the mass and temperature of the planet, and the planet evolution.