Extrasolar Planets & SETI, Page 13: [Previous Page] [Next Page] [Syllabus] Textbook pages 260 - 268

Habitable Zones around a Star

The habitable zone (HZ) of a star is defined to be that distance where a planet could reside and maintain a surface temperature that allowed liquid water to exist. It is time dependent, always moving outward as the star ages.

Continuously Habitable Zone (CHZ) is the region around a star that is located within the habitable zone long enough for life to develop and exist. For the Sun, its Habitable Zone is, conservatively from 0.95 to 1.4 AU, but possibly as wide as from 0.84 to 1.7 AU. However, its CHZ, is much more restricted:from 0.95 to 1.2 AU.

Earth.
Is inside the HZ and the CHZ, mostly because of its greenhouse gases. In less than a billion years, however, we will move outside the Sun's HZ.

Venus
Is just outside the Sun's HZ. Venus boiled all its water to the atmosphere, leaving all the CO₂ in the atmosphere, creating an extreme greenhouse. Early Venus may have been further inside the HZ when the Sun was younger.

Mars
Is now outside the HZ, though it may have been earlier. This is puzzling since the Sun was less luminous then than now. It is clear that distance is not the only issue.