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 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”

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.