Announcements
• Pick up graded homework
• Total homework score so far (out of 26) is
written in corner
• Projects will be graded by Thursday
• Bring questions!
• Final exam: MTWTh next week, in testing
center; allow two hours; format will be the
same as midterms; content will cover entire
course, with emphasis on galaxies and
cosmology
• I will hold office hours MTWTh next week
(times TBA)
Since this course began . . .
• Earth has spun on its axis about 100 times, causing the sun, stars, etc.
to appear to rise and set each time;
• The moon has orbited the earth 3.5 times, going through its phases;
• Earth has completed more than 1/4 of its orbit around the sun, allowing
us to see different stars and planets at night and causing changes in the
sun’s position in our sky and the seasons;
• Mercury has completed about one orbit around the sun; Venus about
half an orbit; Mars only 1/6; outer planets much less;
• Nearby stars have typically moved about 1 AU with respect to our sun,
causing their positions in the sky to shift by a fraction of an arc-second;
• Our solar system has completed a little over one billionth of its orbit
around the galactic center;
• The observable universe has expanded (and grown older) by one part in
40 billion.
Book Recommendation
365 Starry Nights, by Chet Raymo
My favorite guide to the constellations: hand-drawn, grouped
by season, with a miniature lesson on mythology or astronomy
for each night of the year.
Course Outline
 Naked-eye astronomy
 Crash course in physics
 Our solar system
 The stars
 Structure and history of
the
universe
Life in the Universe
2 December 2003
Today:
• History of life on earth
• Search for life in the solar system
• Life in other solar systems? How to tell?
• Why aren’t they here (or are they)?
History of Life on Earth
• Bacteria (simple cells) > 3 billion
years ago
• Complex cells about 1 billion years
ago
• Multicellular life 700 million years
ago
• Humans < 1 million years ago
• Agriculture 10,000 years ago
• Radio communication 100 years
ago
• Space travel 40 years ago
The planets, to scale
The Terrestrial Worlds
Life on Mars?
• Past robotic visits found
no clear sign of life
• Allen Hills meteorite
inconclusive
• Still much debate over
past climate, whether
water was ever abundant
• More robotic visits
coming soon!
Europa: Water beneath ice
Water is probably kept warm by tidal friction.
Could this be a place to look for life?
Titan (Saturn’s largest moon)
Opaque atmosphere of nitrogen, methane, smog.
Surface (cold!) could have liquid methane, other
hydrocarbons. We’ll get a look in January 2005!
What about other solar systems?
• Planets seem to be common around other stars
• Hot, massive stars probably don’t live long
enough for life (as we know it) to develop
• Cool, low-mass stars are unlikely to have earthlike planets with liquid water
• Best bet: sun-like stars, which are still fairly
common (perhaps 1010 in our galaxy).
• But we can’t go there (in the foreseeable
future), so how can we learn if there’s life?
3 ways to look for life in other solar systems:
• Look for clues in spectra. For example, an atmosphere
with abundant oxygen would probably indicate the
presence of life, and may be detectable with future
instruments…
• Look/listen for artificial signals from extraterrestrial
civilizations…
• Speculate…
Search for Extraterrestrial Intelligence
Jodi Foster in “Contact”
Dr. Jill Tarter, SETI Institute
Speculations are easy…
• Simple life may be relatively
common in the universe…
• Multicellular life is probably much
less common…
• Intelligent life is probably still less
common…
• Technologies comparable to ours
would be rarer still..
• What are the chances that we would
discover such a technology, or that
they would discover us?
Or have they?
(And if you believe everything
you see on Fox…)
Seriously, why aren’t they here?
• Maybe they aren’t there!
• Or maybe interstellar travel is just too difficult…
• Or maybe they just aren’t interested.
The bottom line?
We really have no idea whether life exists
elsewhere, let alone intelligent life or advanced
civilizations.
Everyone seems to have an opinion, but by and
large, these opinions are based upon personal
wishes, not facts.
Part of being “scientific” is setting personal wishes
aside and admitting when you just don’t know.