You can define a continuously habitable zone (or CHZ) as the region in which liquid water can exist over the entire Main Sequence lifetime of a star. As its luminosity changes, the location of its habitable zone will change, too. As the star ages, though, it will, in general, cool off a bit and become more luminous. When the star begins stable hydrogen fusion on the Main Sequence, it will lie in one particular location in the HR diagram, known as the Zero Age Main Sequence, or ZAMS. When we studied stellar evolution, you saw the evolutionary tracks for stars in the HR diagram: stars do not maintain the same color and luminosity over their entire lifetimes. Our colleague Ravi Kopparapu maintains an up to date visualization of the habitable zone that includes all of the known exoplanets that lie inside their parent's HZ. In the case of the Solar System, the Earth is inside of this revised HZ near its inner edge, and Mars is just outside of the outer edge. Work on this particular process by Penn State scientists, including Professor James Kasting, has shown that the habitable zone extends farther from a star than originally assumed. However, modern models for the range of the habitable zone take into account more subtle effects, such as the effect of the carbonate-silicate cycle in regulating carbon dioxide in a planet's atmosphere. The size of the habitable zone clearly depends on the luminosity of the star, which determines the equilibrium temperature of the planet. In this particular illustration, the Earth appears to be precisely in the middle of the habitable zone for the Sun. Notice, as expected, that for low-mass, cool stars the region is closer to the star, and for higher mass, hotter stars, the region is more distant from the star. In the image below, the blue band represents the location of the habitable zone. For the lower mass stars with longer lifetimes, astronomers define the habitable zone (or HZ) as the region surrounding the star in which water can remain in its liquid state. O, B, and most A stars live such short lifetimes that we expect that their planets will not be able to develop complex life forms.
Is there any other evidence beyond the atmosphere that might indicate a planet is capable of supporting life? Additional requirements that we can place on a star that hosts a planet are:
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