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

A large planet orbiting around a star should produce what kinds of spectral lines in the star? a)Strongly blueshifted b)Strongly redshifted c)Strongly blueshifted and strongly red shifted d)Weakly blueshifted and weakly redshifted e)Strongly redshifted and weakly blueshifted

How do most currently detected exoplanets compare to the Earth? a)They are very similar to the Earth b)They have larger masses c)They have larger masses and smaller orbits d)They have larger masses and smaller, more eccentric orbits e)They have a wide range of properties and can’t really be categorized

How does a Jupiter sized planet have an orbit at 0.1 AU? a)It formed at 0.1 AU b)It formed at 5 AU and is ejected inward due to a close encounter with another planet c)It formed at 1 AU and the star expands until it is 0.1 AU away d)It is formed at 5 AU and move inward due to friction with the disk e)It is formed at 30 AU and moves inward due to encounters with the Kuiper Belt

What methods could be used to detect exoplanets? a)Looking for period shifts in stellar spectral lines b)Looking for diming of starlight as the planet transits the star c)Looking for the motion of the star in the sky as it is pulled by its planets d)a and b only e)a, b, and c

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