1. 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

2. 365 Starry Nights, by Chet Raymo
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.

3. Course Outline Naked-eye astronomy Crash course in physics
Our solar system
The stars
Structure and history of the  universe

4. The Observable Universe
Galaxy
Dark era
Quasar
Background radiation (3000-degree ionized gas)

5. We can use redshifts to map the universe…
You are here!

6. Virgo Cluster

7. M83

8. Inside the Milky Way

9. The Outer Solar System On this scale: 1 A.U. = 7 pixels Orbital radii:
Neptune
On this scale:
1 A.U. = 7 pixels
Orbital radii:
Jupiter 5.2 A.U.
Saturn 9.5 A.U.
Uranus 19 A.U.
Neptune 30 A.U.
Uranus
Saturn
Jupiter

10. The Inner Solar System On this scale: 1 A.U. = 120 pixels
Mars
Earth
On this scale:
1 A.U. = 120 pixels
Diam. of Sun = 1 pixel
Diam. of moon’s orbit = 1/2 pixel
Diam. of earth = 1/100 pixel
Venus
Mercury
Sun

11. Earth and Moon to scale

12. Home!

13. Life in the Universe
6 December 2006

14. 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)?

15. Earth: Life

16. 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

17. The planets, to scale

18. The Terrestrial Worlds

19. 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

20. Europa: Water beneath ice
Water is probably kept warm by tidal friction.
Could this be a place to look for life?

21. Titan (Saturn’s largest moon)
Opaque atmosphere of nitrogen, methane, smog. Surface (cold!) could have liquid methane, other hydrocarbons. Huygens probe landed in Jan

22. 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?

23. 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…

24. 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!)

25. 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?

26. Or have they?
(And if you believe everything you see on Fox…)

27. 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.

28. 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.