How do greenhouse gases affect the inner and outer boundaries of a habitable zone?

In the study of habitable zones, greenhouse gases are critical factors that define the boundaries within which a planet can maintain liquid water on its surface. Their influence on these boundaries is summarized as follows:

• Inner Boundary: This limit is defined by the runaway greenhouse effect. As a planet moves closer to its star, the accumulation of greenhouse gases in its atmosphere traps increasing amounts of infrared radiation. This process causes the surface temperature to rise until it becomes hot enough for the planet's water to boil away entirely, rendering the surface uninhabitable.
• Outer Boundary: This limit is defined by the point at which greenhouse warming is insufficient to maintain surface temperatures above freezing. Beyond this distance, even with the presence of greenhouse gases, the incoming stellar energy is too low to prevent the planet from becoming a frozen world.

Ultimately, the presence and concentration of greenhouse gases—such as carbon dioxide, methane, and water vapor—determine a planet's ability to retain heat, effectively shifting these boundaries based on the specific atmospheric composition of the world in question.1