  A habitable zone of a star is defined as a range of orbits within which a
rocky planet can support liquid water on its surface. The most intriguing
question driving the search for habitable planets is whether they host life.
But is the age of the planet important for its habitability? If we define
habitability as the ability of a planet to beget life, then probably not. After
all, life on Earth has developed within only about 800 Myr after its formation.
If, however, we define habitability as our ability to detect life on the
surface of exoplanets, then age becomes a crucial parameter. Only after life
had evolved sufficiently complex to change its environment on a planetary
scale, can we detect it remotely through its imprint on the atmosphere - the
biosignatures, out of which the photosynthetic oxygen is the most prominent
indicator of developed life as we know it. But the onset of photosynthesis on
planets in habitable zones may take much longer time than the planetary age.
The knowledge of the age of a planet is necessary for developing a strategy to
search for exoplanets carrying complex (developed) life - many confirmed
potentially habitable planets are too young (orbiting Population I stars) and
may not have had enough time to develop and/or sustain detectable life. In the
last decade, many planets orbiting old (9-13 Gyr) metal-poor Population II
stars have been discovered. Such planets had had enough time to develop
necessary chains of chemical reactions and may carry detectable life if located
in a habitable zone. These old planets should be primary targets in search for
the extraterrestrial life.
