Earth’s atmosphere also produces a greenhouse effect, which keeps the planet from getting too cold, but it is much weaker than Venus’s, so water is able to exist in all three states. The hot temperature on Venus means that if water were to exist on its surface, it would exist only as a gas (water vapor or steam). The stronger the greenhouse effect has in an atmosphere, the more the planet is heated. However, at some point Venus’s atmosphere filled with very high amounts of carbon dioxide and other gases, which caused an extreme greenhouse effect, heating the planet’s surface to about 462° C (864° F). Planetary scientists think that Venus may have had surface water, possibly even a shallow ocean, billions of years ago.
Venus and Mars are both at distances from the sun in which water could have been possible on the surface. This is why Earth can maintain a vast ocean of liquid water, which makes Earth a place where life can thrive. In other words, Earth’s orbit is within the sun’s Goldilocks Zone. Earth’s orbit is farther from the sun than Venus but closer than Mars. Based on the idea that liquid water on a planet’s surface makes life possible, the Goldilocks Zone of our solar system extends approximately from the orbit that Venus takes around the sun to the orbit that Mars takes around the sun. Temperatures that allow for liquid water are considered “just right” because life as we know it requires water. This range is known as the Goldilocks Zone. In astrobiology, the Goldilocks Principle applies to the range of distances that a planet can be from its star and maintain surface temperatures that are just right for water to be liquid.
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