How likely is extraterrestrial life

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How likely is extraterrestrial life? Physics
1910W, Spring 2008 In Physics 236A, 1115-1205
TTh JW Halley 350C Physics 6712 624
0395 woods_at_woods1.spa.umn.edu Course http//www.
physics.umn.edu/courses/1910W
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Format Lecture/discussion in class
periods. Student presentations in the last two
weeks. Prerequisites High school general
science course and elementary algebra. Work load
and grades. A few hours of reading from book and
notes/week. One written exercise per week.
10-15 page term paper and oral presentation. Two
short, unannounced quizzes on reading
material. Grades Exercises 65 , quizzes 10,
paper and presentation 25 .
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Weekly reading assignments and a schedule of
deadlines for the final paper appear in the
syllabus. Make an appointment with the
instructor to select a topic within the first two
weeks. Exercises will usually be distributed
on Thursdays and collected on the
following Thursday.
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• Overview
• The emphasis is on what science can tell
• us about this ancient question.
• I will review evidence from two points of
• view during the next weeks
• Bottom up. We start with well established
• physical and chemical laws and ask what
• they imply about the answer to the question.
• We organise the discussion with the Drake
• Equation.
• 2. Top down. We start with the results of
  experiments
• to try to directly observe life and civilization
• elsewhere (which have shown nothing so far
• Fermi paradox) and ask what they tell us.

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Drake equation. (due to radio astronomer Frank
Drake in the 1950s) Nciv Ngal fstar fplanet
flife Ngal number of stars in a typical
galaxy fstar probability that a star has the
right chemistry for life fplanet
probability that a star with the right
chemistry has a solar system including
earthlike planets with the right climate
for life. flife probability that life
evolves on a habitable earthlike
planet.
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Quick summary Ngal is quite well known. We
will use 1011 We will discuss fstar at some
length. A lot is known about it, and we will use
the number 10-2 Information on fplanet is
rapidly accumulating. We estimate 10-3. More
discussion later. Very little is known about
flife. Estimates range from 1 to less than
10-11! Putting these numbers back in the
equation 10-5 lt Nciv lt
106
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Practically all the huge uncertainty comes
from the fact that we dont know how likely
it is for life to evolve on an earthlike
habitable planet. Question What does it mean
to say that Nciv lt 1 ?