Evolution of a habitable planet
ANNUAL REVIEW OF ASTRONOMY AND ASTROPHYSICS, (2003)Abstract
Giant planets have now been discovered around other stars, and it
is only a matter of time until Earth-sized planets are detected.
Whether any of these planets are suitable for life depends on their
volatile abundances, especially water, and on their climates. Only
planets within the liquid-water habitable zone (HZ) can support life
on their surfaces and, thus, can be analyzed remotely to determine
whether they are inhabited. Fortunately, current models predict that
HZs are relatively wide around main-sequence stars not too different
from our sun. This conclusion is based on studies of how our own
planet has evolved over time. Earth's climate has remained conducive
to life for the past 3.5 billion years or more, despite a large increase
in solar luminosity, probably because of previous higher concentrations
Of CO2 and/or CH4. Both these gases are involved in negative feedback
loops that help to stabilize the climate. In addition to these topics,
we also briefly discuss the rise of atmospheric O-2 and O-3, along
with their possible significance as indicators of life on other planets.
