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In page Water:

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The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of life on Earth as we know it. The Earth is located in the habitable zone of the Solar System; if it were slightly closer to or farther from the Sun (about 5%, or about 8 million kilometers), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.[1][2] Earth's size also plays a role: its gravity allows it to hold an atmosphere, including air moisture. Smaller planets like Mars have extremely thin or no atmospheres.[3] Water vapor and carbon dioxide in the atmosphere provide a temperature buffer (greenhouse effect) which helps maintain a relatively steady surface temperature.[citation needed]