Extra-Terrestrial Life and the Drake Equation Astronomy 311 Professor Lee Carkner Lecture 25

Final Exam  Monday, 3 pm, SC102  Two hours long  Bring pencil and calculator  Same format as other tests  Matching, multiple choice, short answer  About 50% longer  Covers entire course

The Drake Equation  In 1961, astronomer Frank Drake developed a formula to predict the number of intelligent species in our galaxy that we could communicate with right now   No one agrees on what the right values are   Solving the Drake equation helps us to think about the important factors for intelligent life

The Drake Equation N=R * X f p X n e X f l X f i X f c X f L  N =  R * = Number of stars in the galaxy  f p =  n e = Average number of suitable planets per star  f l = Fraction of suitable planets on which life evolves  f i =  f c = Fraction that can communicate  f L = Lifetime of civilization / Lifetime of star

R * -- Stars   Our best current estimate: R * =3 X (300 billion)   We are ruling out life around neutron stars or white dwarfs or in non- planetary settings (nebulae, smoke rings, etc.)

The H-R Diagram

Extra-Solar Planets

f p -- Planets  What kind of stars do we need?   High mass stars may become a giant before intelligent life can develop   Need medium mass stars (stars like the Sun)   Can we find planets?  Circumstellar disks that produce planets are common  Exoplanets have now been found  We have just begun the search for planets

The Carbonate-Silicate Cycle Water + CO 2 (rain) Ocean Carbonate + silicate (Sea floor rock) CO 2 Volcano Atmosphere Carbonate + water (stream) CO 2 + silicate (subvective melting)

n e -- Suitable Planets  What makes a planet suitable?   Must be in habitable zone   Simulations of inner planet formation produce a planet in the habitable zone much of the time  Heat may also come from another source like tidal heating (Europa)

n e -- Unsuitable Planets  The Moon --  Mars -- Has atmosphere but too small to have plate tectonics  Jupiter -- Too large, has no surface  Venus --  Earth at 2 AU -- CO 2 builds up to try and warm planet, clouds form, block sunlight

The Miller-Urey Experiment

f l -- Life   Complex molecules containing carbon, (e.g. proteins and amino acids)   Organic material is also found in carbonaceous chondrites and comets

f i -- Intelligence   On Earth life evolved from simple to complex over a long period of time (~3-4 billion years)   Impacts (e.g. KT impact)  Climate Change (e.g. Mars drying up)  Life on Earth has gone through many disasters (e.g. mass extinctions), but has survived

f c -- Communication  Even intelligent life may not be able to communicate   What could keep intelligent life from building radio telescopes?   Airworld (floating gasbags can’t build things)   Social, cultural or religious reasons  Lack of curiosity or resources

f L -- Lifetime   Lifetime of a star like the Sun = 10 billion years (1 X )   How long does a civilization last for?

f L -- Destroying Civilization  What could destroy a civilization?   Environmental or technological disaster    Space colonization greatly reduces risk or extinction

N  Multiply these factors together to get N   The galaxy is a disk 100,000 light years across  If you evenly distribute the civilizations across the galaxy, how close is the nearest one?  N ~ 1  N ~ 10D ~ light years  N ~ 1000D ~  N ~ 100,000D ~ 590 ly  N ~10,000,000D ~ 

Summary: Life in the Galaxy  Medium size, medium luminosity star with a planetary system  A planet of moderate mass in the habitable zone  Organic compounds reacting to form simple life  Life evolving over billions of years with no unrecoverable catastrophe  Intelligent life building and using radio telescopes  A long lived civilization