Habitable Zone The Earth is the only planet that formed in the Habitable Zone (HZ) of the Solar System, the region where temperature and pressure is such that liquid water is stable at the planetary surface. Further in from the Earth towards the Sun (e.g., at Venus), temperatures are too hot and water evaporates; further out (e.g., at Mars), temperatures are too cold and water solidifies to ice. Five billion years from now... Although the Sun has evolved and become hotter over its 4.56 billion year lifetime, the Earth has continually occupied the HZ since its initial accretion. This, however, will not always be the case: the Sun is approximately half way through its stellar evolutionary history, and by around 5 billion years in the future will have grown to a red giant star, with a diameter reaching much further out into the Solar System than it does currently. By this stage, the surface temperature of the Earth will be too hot to maintain liquid water, thus the Earth will no longer occupy the HZ. Indeed, the HZ might well have migrated out to as far as Jupiter or Saturn. Liquid water on other planets and moons Recently, the concept of the HZ has been amended to take account of factors other than surface temperature and pressure. For example, although the surface temperature and pressure of Mars is too low to permit liquid water at the martian surface, it is feasible that liquid water exists at the relatively high static pressures that pertain within pore spaces between mineral grains in crystalline rocks just below the surface soil layer. Heat sources other than solar radiation also contribute to maintaining a liquid water layer: for instance, on Jupiter's satellite Europa, an ocean of liquid water has been proposed, trapped below a surface layer of ice. The heat source that keeps the water liquid is Jupiter's gravitational drag. Exoplanets in the Habitable Zone Recently, a number of exoplanets in orbit around other stars have been found in the HZ at those stars. None of these are the Earth-like though, they are all giant planets like Jupiter. Nevertheless, it is an interesting thought that they may possess moons on which liquid water may be present.