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