The Goldilocks Principle can be summed up neatly as "Venus is too hot, Mars is too cold, and Earth is just right." The fact that Earth has an average surface temperature comfortably between the boiling point and freezing point of water, and thus is suitable for our sort of life, cannot be explained by simply suggesting that our planet orbits at just the right distance from the sun to absorb just the right amount of solar radiation. Our moderate temperatures are also the result of having just the right kind of atmosphere. A Venus-type atmosphere would produce extremely hot conditions on our planet; a Mars atmosphere would leave us shivering in a Martian-type deep freeze.


Instead, parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a pleasant range. The Martian blanket is too thin, and the Venusian blanket is way too thick! The 'blanket' here is a collection of atmospheric gases called 'greenhouse gases' based on the idea that the gases also 'trap' heat like the glass walls of a greenhouse do.


These gases, mainly water vapor H₂0, carbon dioxide CO₂, methane CH₄, and nitrous oxide N₂0, all act as effective global insulators. To understand why, it's important to understand a few basic facts about solar radiation and the structure of atmospheric gases.


Sources
http://www.ucar.edu/learn/1_3_1.htm (Copyrighted, educational purposes permitted.)