Transcript Life: Definition, Origin, Criteria

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
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 (H2, N2, H2O, CO2, NH3) chemical
reactions will lead to many organic molecules that
form the basic strands of DNA
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?)
Search for Extraterrestrial Life
• The SETI project to search for radio
signals from outer space
Probability of Finding Extraterrestrial
LIfe – The Drake Equation
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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 ?
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
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
Requirements for Planets
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Right distance from the star
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