If we discover that there are planets resembling Earth around
stars somewhat like the Sun, we will find ourselves at the threshold of an
even bigger question: Do they harbor life?
A goal of Terrestrial Planet Finder (TPF) is to search for signs of
the large-scale effects that life would have on a planet's chemistry. By
analyzing the colors of infrared radiation detected by TPF, astronomers can
search for atmospheric gases such as carbon dioxide, water vapor, and ozone.
Together with the temperature and radius of the detected planets, this information
will allow astronomers to determine which planets are habitable, or even
whether they may be inhabited by rudimentary forms of life.
The best candidates for closer study would be located in the habitable zone;
that is, the region around the system's star where we can expect to find
liquid water at the surface. If the planet is too hot, the water becomes
vapor and is lost from the atmosphere. If the planet is too cold, the water
freezes. Either of these conditions would make a planet very inhospitable
for life. The habitable zone for our Sun starts beyond Venus and ends before
Follow the ozone
The existence of large amounts of oxygen in a planet's atmosphere alone
would be a strong indicator of life. In the Earth's atmosphere, oxygen
is a byproduct of photosynthesis, the process by which green plants and certain
other organisms use sunlight to convert carbon dioxide and water into carbohydrates.
Furthermore, oxygen molecules don't remain in the atmosphere, but combine
with other molecular types in a process known as oxidation. Therefore, a
planet with an atmosphere rich in oxygen (like Earth) would imply a source
to keep it replenished (life).
However, we know of non-biological processes that can also result
in an oxygen-rich atmosphere. The runaway greenhouse effect on Venus is
one example. A frozen, Mars-like planet big enough to hold its oxygen would
So, the presence of oxygen alone -- while exciting and significant
-- couldn't be taken as an unambiguous indicator of life. Furthermore,
oxygen doesn't produced spectral lines that can be easily observed in the
infrared. However, ozone, a form of oxygen, does. The detection of ozone
coexisting with a reduced gas such as nitrous oxide or methane could be taken
as convincing evidence not only that a planet is habitable -- but that it
Methane and nitrous oxide are strongly out of equilibrium with oxygen
in Earth's atmosphere. They are present because they, too, are produced
by organisms. Unfortunately, these gases are not very abundant in Earth's
present atmosphere, so they would be impossible to see with a first-generation
TPF instrument. Eventually, however, a much larger version of TPF might be
able to make such a measurement. This would be the best way to determine
if life exists on planets around other stars.
In any case, such large-scale clues won't tell us about the complexity
of the discovered life; it could be either algae or a developed civilization.
It is also possible that planets without oxygen could sustain life. Photosynthesis
might conceivably occur with another element, such as sulfur, playing the
role of oxygen. In the search for life, we must control our preconceived
assumptions of what it means to be living.
Beyond Terrestrial Planet Finder
The findings of Terrestrial Planet Finder would guide a possible subsequent
mission called Life Finder. Like its predecessor, Life Finder would consist
of an array of telescopes flying in formation. The telescopes would combine
infrared light to produce high-resolution spectra of the atmospheres of distant
Scientists would use this information to search more closely for
markers of biological activity, such as seasonal variations in the levels
of methane and other gases, changes in atmospheric chemistry and spectral
variations in the dominant biomass.
Throughout our quest for life, we will have to keep in mind the
history of Earth, so far the only confirmed prototype of a world where life
has emerged. The simplest life forms existed here well before an abundance
of oxygen appeared in the atmosphere, which in turn allowed multicelled
organisms to flourish. NASA's astrobiology research will help expand our
knowledge of "life signs" that would appear at different stages in a planet's
history, as well as signs that would appear given a planetary chemistry that
isn't exactly the same as our own. These insights will give us the best
possible chance of recognizing life if and when we find it somewhere else.