594 PART 5^ |^ LIFE
region within which orbiting planets have temperatures permit-
ting the existence of liquid water. Th e sun’s habitable zone
extends from near the orbit of Venus to the orbit of Mars, with
Earth right in the middle. A low-luminosity star has a small and
narrow habitable zone, whereas a high-luminosity star has a large
and wide one.
Stable planets inside the habitable zones of long-lived stars
are the places where life seems most likely, but, given the tenacity
and resilience of Earth’s life forms, there might be other, seem-
ingly inhospitable, places in the universe where life exists. You
should also note that three of the environments considered as
possible havens for life—Europa, Titan, and Enceladus—are in
the outer solar system, far outside the sun’s conventionally
defi ned habitable zone.fossils of ancient Martian microorganisms could possibly be non-
biological mineral formations instead.
In 2009 astronomers observing the Martian atmosphere
announced detection of faint traces of methane, a substance
made abundantly by living things on Earth. Methane would be
destroyed in the Martian environment by solar UV radiation
and chemical reactions, so the methane that is present must
have been produced recently. Th ere are also geological pro-
cesses that can emit methane, but those processes are con-
nected with volcanism. Th e map of Martian atmospheric
methane concentrations in Figure 26-9c shows methane pri-
marily in locations that are not volcanic provinces. Perhaps
organisms living in the Martian soil are making methane right
now. Th is evidence regarding potential life on Mars remains
highly controversial. Conclusive evidence of life on Mars may
have to wait until a geologist from Earth can scramble down
dry Martian streambeds and crack open rocks looking for fos-
sils, or drill into the soil seeking signs of metabolizing
microorganisms.
Th ere is no strong evidence for the existence of life in the
solar system other than on Earth. Now your search will take you
to distant planetary systems.
Life in Other Planetary Systems
Could life exist in other planetary systems? You already know that
there are many diff erent kinds of stars and that many of these stars
have planetary systems. As a fi rst step toward answering this ques-
tion, you can try to identify the kinds of stars that seem most likely
to have stable planetary systems where life could evolve.
If a planet is to be a suitable home for living things, it must
be in a stable orbit around its sun. Th at is easy in a planetary
system like our own, but planet orbits in binary star systems
would be unstable unless the component stars are very close
together or very far apart. Astronomers can calculate that, in
binary systems with stars separated by intermediate distances of
a few AU, the planets should eventually be swallowed up by one
of the stars or ejected from the system. Half the stars in the gal-
axy are members of binary systems, and many of them are
unlikely to support life on planets.
Moreover, just because a star is single does not necessarily
make it a good candidate for sustaining life. Earth required per-
haps as much as 1 billion years to produce the fi rst cells and
4.6 billion years for intelligence to emerge. Massive stars that
shine for only a few million years do not meet this criterion. If the
history of life on Earth is representative, then stars more massive
and luminous than about spectral type F5 last too short a time for
complex life to develop. Main-sequence stars of types G and K,
and possibly some of the M stars, are the best candidates.
Th e temperature of a planet is also important, and that
depends on the type of star it orbits and its distance from the star.
Astronomers have defi ned a habitable zone around a star as a
SCIENTIFIC ARGUMENT
What evidence indicates that life is possible on other worlds?
A good scientifi c argument involves careful analysis of evidence.
Fossils on Earth show that life originated in the oceans at least
3.4 billion years ago, and biologists have outlined likely chemi-
cal processes that, over long time intervals, could have changed
simple organic compounds into reproducing molecules inside mem-
branes, the fi rst simple life forms. Meager fossil evidence indicates
that life developed slowly at fi rst. The pace of evolution quickened
about half a billion years ago, when life took on complex forms.
Later, life emerged onto the land and continued evolving rapidly
into diverse forms. Intelligence is a relatively recent development:
It is only a few million years old.
If this evolutionary process occurred on Earth, it seems reason-
able that it could have occurred on other worlds as well. Earth-
like worlds could be plentiful in the universe. Life may begin and
eventually evolve to intelligence on any world where conditions are
right. Now make a related argument. What are the conditions you
should expect on other worlds that host life?Intelligent Life in the
UniverseCould intelligent life arise on other worlds? To try to answer
this question, you can estimate the chances of any type of life aris-
ing on other worlds, then assess the likelihood of that life develop-
ing intelligence. If other civilizations exist, it is possible humans
eventually may be able to communicate with them. Nature puts
restrictions on the pace of such conversations, but the main prob-
lem lies in the unknown life expectancy of civilizations.Travel Between the Stars
Th e distances between stars are almost beyond comprehension.
Th e fastest human device ever launched, the New Horizons probe
currently on its way to Pluto and the Kuiper belt (see Chapter 24),
will take about 90,000 years to travel the distance to the nearest26-3