Announcements
• Turn in Homework 14 (5 questions)
• Today is the deadline for the Monty
Python challenge!
• Observatory session tonight, weather
permitting
• Review session on Friday
• Final Exam next Monday or Tuesday:
2 hours, 100 points, comprehensive,
with an emphasis on galaxies and
cosmology
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
The Observable Universe
Galaxy
Dark era
Quasar
Background
radiation
(3000-degree
ionized gas)
We can use redshifts to map the universe…
You
are
here!
The Outer Solar System
Neptune
On this scale:
Uranus
1 A.U. = 7 pixels
Saturn
Jupiter
Orbital radii:
Jupiter
5.2 A.U.
Saturn
9.5 A.U.
Uranus
19 A.U.
Neptune
30 A.U.
The Inner Solar System
Mars
Earth
Venus
Mercury
On this scale:
1 A.U. = 120 pixels
Diam. of Sun = 1 pixel
Sun
Diam. of moon’s orbit =
1/2 pixel
Diam. of earth = 1/100
pixel
Earth and Moon to scale
Life in the Universe
6 December 2006
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)?
Earth: Life
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 45 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
• Robotic exploration has
discovered pretty good
evidence for past surface
water
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. Huygens probe landed in Jan. 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 earth-like 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”
(heard a message from
an alien civilization and
got to go for a visit!)
Dr. Jill Tarter, SETI Institute
(hasn’t heard any messages
yet!)
Speculations are easy…
• Simple life may be relatively common in the
universe…
• Multicellular life is probably much less
common, since it may require rather rare
conditions (stable environment for billions of
years)…
• Intelligent life is probably still less common…
• Technological civilizations comparable to
ours would be rarer still, but may develop far
superior technologies in a cosmic
nanosecond…
• What are the chances that we would
discover such a civilization, 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.