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. This range is known as the Goldilocks Zone. Temperatures that allow for liquid water are considered “just right” because life as we know it requires water.

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. Earth’s orbit is farther from the sun than Venus but closer than Mars. In other words, Earth’s orbit is within the sun’s Goldilocks Zone. This is why Earth can maintain a vast ocean of liquid water, which makes Earth a place where life can thrive.

Venus and Mars are both at distances from the sun in which water could have been possible on the surface. Planetary scientists think that Venus may have had surface water, possibly even a shallow ocean, billions of years ago. 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). The stronger the greenhouse effect has in an atmosphere, the more the planet is heated. 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). 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. Additionally, Venus’s atmosphere is so thick that the air pressure on the surface of Venus is about 92 times the air pressure on the surface of Earth. Mars also went through changes long ago and lost liquid water from its surface. Solar wind stripped away the ancient Mars atmosphere, leaving the air pressure too low for water to exist as a liquid for extended periods.

Despite the prior belief that liquid water could only exist within the sun’s Goldilocks Zone, there is now evidence that celestial bodies outside of the Goldilocks Zone can have liquid water as well. These include a moon of Saturn called Enceladus and a moon of Jupiter called Europa. These two worlds may hold an ocean of water underneath a surface of ice. This is possible because as the moons move around their massive planets, their insides are heated, a phenomenon known as tidal heating.

Astrobiologists think that the oceans of Europa and Enceladus are the best places to look for life away from Earth. This means that the “Goldilocks Zone” extends beyond the distance from a star where water can be liquid on a planet’s surface and into areas that are much colder.