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Many of these clouds have a little bit of rotation. As they
collapse, the cloud spins faster and faster, like an ice skater pulling
in her arms. This causes the cloud to form a flattened disk of gas. At the
center, the main star forms. Out further, smaller eddies can form planets.
Until recently, we did not have any evidence of planets outside our solar
system. In the last couple of years, several teams of astronomers have announced
the discovery of planets surrounding nearby stars (see interview with
Geoff
Marcy and Didier Queloz
). This exciting discovery increases the likelihood
of other planets around many stars. Let's estimate conservatively that one-half
of the stars form planetary systems; the other half form binary star systems,
so f(p) = 0.5.
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Geoffrey Marcy is a professor at San Francisco State University.
There's more on Marcy and his search for extraterrestrial planets from
"What's New" December 1996.
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The n(e) factor is a little tricky. Small stars are cool and red. Planets
would have to orbit very close to be in the ecosphere. Also, this ecosphere
would be very narrow; like the skin on an orange. Not much room for planets.
Planets that orbit very close to their parent star are often tidally locked
and present one face to the star at all times. The atmosphere of such a
planet would freeze on the cool side that faces away from the star; this
does not promote life. On the other hand, huge hot blue stars have a farther
and wider ecosphere. Of course, judging from our solar system, planets are
spaced further apart the farther they are from the star, so the wider ecosphere
is cancelled by this effect. These larger stars also burn their fuel faster
and don't last very long. They are usually so short-lived that life does
not even get a chance to start before the star goes nova or supernova and
destroys everything in the system. In our solar system, with our average-sized
yellow sun, we have two (Earth and Mars) or maybe three (Venus) planets
within the ecosphere. A conservative guess for the number of planets within
the "life zone" or ecosphere is one. n(e) = 1.
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The next factor, f(l), is where things become a little sticky.
The problem is that we only have a few examples of planets where conditions
are right for life to evolve. As stated above, Venus, Earth, and Mars all
could have had, at one time, proper conditions. We know life evolved on
Earth, and there is now tantalizing evidence for primitive life existing
on Mars billions of years ago. A conservative guess for this number is 0.2,
or one in five planets with proper conditions will evolve life. f(l) = 0.2.
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