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Life In The
Universe
Habitable Zones
BY DAVID F. COPPEDGE *
When speculating about life in the universe, scientists need to
be more realistic than Hollywood.
In Star Trek, no matter where the actors
land, they can walk around and breathe the air.
That may be easier on directors, but for a surface to be
habitable, there are physical requirements.
Astrobiologists
limit their searches to regions around stars where liquid water can exist.
Because
liquid water is bounded by its freezing and boiling points, a
"Goldilocks" zone neither too hot nor too cold must be found at
particular distances from a star.
Extending
the inner and outer radii in the orbital plane produces a ring-shaped region
called the Continuously Habitable Zone, or CHZ.
Under-surface
oceans may exist on some planets or moons, but surface life must be in the
zone.
A
paper in Icarus last August added a complication to the CHZ
concept.
Earlier
estimates overlooked the hazards of ultraviolet light. Highly-ionizing UV
radiation rapidly destroys organic molecules.
Many
massive stars output prodigious amounts of UV. This cuts down on the number of
candidate stars.
Of
twenty-one extrasolar planets studied, only five had an overlap between the
UV-safe zone and the CHZ, where life could exist.
Smaller
stars have a different problem: a much narrower CHZ.
The
zone is also closer in, meaning that any planet lucky enough to fit in the zone
would become gravitationally locked to the star, with one hemisphere always
facing the star, overheating, and the other hemisphere facing away, forever freezing.
It
is unlikely life could survive except along a very thin longitude near the
terminator (the boundary between light and darkness).
This
means that life is highly improbable except around sun-like stars, a mere 5% of
all stars.
Further
complications arise when considering a star's host galaxy.
Conditions
too close to the center are exposed to hazardous radiation levels; too far out
lack the heavy elements required for life.
This
means there is also a Galactic Habitable Zone (GHZ) to consider.
Having
a zone defined, of course, does not mean an earth-like planet will be present.
Astronomers
continue to find extra-solar planets at an accelerating clip.
Upcoming
missions like the Terrestrial Planet Finder (TPF), Kepler, and Space Interferometry
Mission (SIM) may one day succeed in finding an earthlike planet around another
sun-like star.
Spectral
analysis may even be able to infer the possible presence of life from certain
"biosignatures" such as gas ratios unexpected from geological or
atmospheric processes alone.
Given
the vastness of space and the number of stars in the "cosmic
lottery," astrobiologists are not discouraged at the prospects for life,
even with few suitable zones.
What
does this mean for Biblical creationists?
The
Bible does not specifically rule out some kind of life on other worlds.
Many
Christian thinkers have speculated about it.
In
the eighteenth century, in fact, the majority thought it foolish to deny it.
Now
that we have the means, searching for data to replace speculation is a good
thing that Christians should welcome.
A
day of evidence is worth a millennium of conjecture.
*David F. Coppedge works in the Cassini program at the Jet
Propulsion Laboratory.
Cite this article: Coppedge, D. 2007. Habitable Zones. Acts
& Facts. 36 (4).
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