1. Q4, A1143, Au15, Pradhan: Curve +5%

2. Life: Definition, Origin, Criteria What is the scientific definition of life? Collection of atoms  Organic molecules Where did life originate, on the Earth or extraterrestrial? Have we been visited or are we the first? Development of “intelligence” Not all civilizations may (a) survive, or (b) explore and develop technology Aliens in vastly different (a) forms of life, (b) phases of evolution

3. Physical definition of life Quite simple! Self-reproducing mechanism  Deoxyribonucleic Acid (DNA) Complex carbon-based molecule but made of simple building blocks with light CHON elements Many organic molecules form naturally out of CHON elements Miller-Urey experiment: Given a primitive atmosphere (H 2, N 2, H 2 O, CO 2, NH 3 ) chemical reactions will lead to many organic molecules that form the basic strands of DNA

5. Extraterrestrial candidates Intra-solar: Mars, Europa, Titan, Enceladus Mars rover missions Europa has very thin atmosphere, but plenty of ice Titan has thick but primitive Earth-like atmosphere Enceladus has water jets (geysers?)

6. Search for Extraterrestrial Life The SETI project to search for radio signals from outer space

7. Probability of Finding Extraterrestrial LIfe – The Drake Equation Number of technologically advanced civilizations N = R(solar) x f(planet) x n(earth) x f(life) x f(intelligence) x f(technology) x L(lifetime) N(Milky Way) > 10 ?

8. How many Solar Systems in the Milky Way? ~ 100 billion stars Planets should form naturally out of stellar ‘debris’ in the disk We can now detect many planets, from Jupiter to Earth size

9. Requirements for the star Solar like Main Sequence star, stable for billions of years Less than 1.5 times massive than the Sun; otherwise too much UV More than 0.3 times the mass of the Sun; large warm region near the star for liquid water Limited to no more than 10 billion stars

10. Requirements for Planets Right distance from the star Habitable “Goldilocks” zone Liquid water Near-circular orbit (why?) Sufficient gravity to hold on to an atmosphere, but not too heavy to suffocate Could be upwards of 10 million Earth-like planets in the Galaxy