1. Lecture 34 – Exobiology- Life Elsewhere in the Universe What does our knowledge of the solar system tell us about the existence of life elsewhere in the universe?

2. Exobiology: “A scientific discipline without subject matter”. At present no examples known of life outside the Earth Present status of exobiology is intelligent speculation on the processes that led to life on Earth, and the degree to which they are reproduced elsewhere.

3. Astronomical contributions to exobiological speculation: what astronomical factors were necessary for the origin and development of life?

4. A major astronomical constraint: life took a long time to develop

5. On Earth, it took 4 billion years from formation to the appearance of complex, multicellular life (“Cambrian Explosion”). This requires the star to remain relatively constant for a very long time.

6. Fortunately, stars like the Sun are very numerous A rough estimate: 20 billion stars in the Milky Way galaxy

7. Contrary opinions about common life in the universe “Rare Earth Hypothesis”, after book “Rare Earth” by Brownlee and Ward: Asserts that a large number of improbable events were necessary for the development of life on the Earth. These are unlikely to be duplicated elsewhere, so higher life is extremely rare in the universe.

8. An example of a (possible) cosmic oddity necessary for life on Earth

9. Recent research suggests that the Earth’s large moon may have stabilized Earth’s obliquity (tilt), and prevented a chaotic wandering of the Earth’s axis. If Earth’s moon is rare, its stabilizing influence is also rare.

10. Second argument against (intelligent life) elsewhere: Fermi Paradox If higher life, and intelligent life, is common in the galaxy, why don’t we see evidence of their technology? 18 Scorpii – may be several hundred million years older than the Sun

11. We should begin to get answers in the next 10 – 20 years The Terrestrial Planet Finder Mission (TPF) mission: find terrestrial planets and determine if they have life

12. The TPF approach: look for “bioindicators” in the spectra of terrestrial planets Earth has spectral lines of different gases than Mars and Venus