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Why might Venus have had a milder climate billions of years ago?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

In the context of astrobiology and the habitable zone, it is hypothesized that Venus may have possessed a more clement, Earth-like climate billions of years ago.

This possibility arises because a star’s luminosity increases over time, causing the boundaries of its habitable zone to shift outward. Consequently, a planet that resides within the habitable zone during its star's youth may eventually become too hot as the star brightens. Venus may have been such a planet, potentially supporting liquid water on its surface early in its history before undergoing a runaway greenhouse effect that caused its water to boil away.

However, confirming this hypothesis is difficult. Because Venus is geologically active, its current surface is too young to retain geological evidence of a more temperate past. While the planet is now an inhospitable wasteland with extreme surface temperatures, its early history remains a subject of significant interest in understanding the divergent evolutionary paths of the rocky inner planets.1