Life: Definition, Origin, Criteria

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					    Life: Definition, Origin, Criteria
• What is the scientific definition of life?
Collection of atoms  Organic molecules
• Where did life originate, on the Earth or
• 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
• 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
•   Number of technologically advanced
 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
• Limited to no more than 10 billion stars
    Requirements for Planets
• 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

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