Why can liquid water exist on Earth’s surface?

Liquid water can exist on Earth’s surface primarily because the planet is located within the habitable zone of the Sun. This orbital region provides the appropriate temperature range to maintain water in a liquid state, preventing it from either boiling away or remaining permanently frozen.1

Several critical factors contribute to this stability:

• Atmospheric Composition: Earth’s atmosphere contains greenhouse gases, such as carbon dioxide, which help regulate surface temperatures and prevent the planet from freezing, even during periods when solar luminosity was lower in the past.12
• Planetary Characteristics: Earth’s sufficient mass and gravitational pull helped retain water vapor that was released from the planet's interior through degassing early in its history.3
• Radiation Balance: The planet’s distance from the Sun and its atmospheric properties create a radiation balance that allows water to cycle through the hydrosphere via the hydrologic cycle, rather than being lost to space.32

These conditions distinguish Earth from other planets in our solar system, such as Venus or Mars, where surface water is either absent or limited by different climatic and atmospheric histories.2