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The faint young Sun paradox is an unsolved problem regarding the Sun and Earth: the Sun is presumed to have had 70% of its current luminosity at a time in its early history, enough to freeze all Earth's water, but there are signs that Earth had liquid water at that time. Theories have been developed regarding what might have warmed the Earth, including an instance of the greenhouse effect, tidal heating, a high solar wind, and the result of impacts.