Life: Definition, Origin, Criteria

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Q4, A1143, Au15, Pradhan Curve 5
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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
  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

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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

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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?)

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Search for Extraterrestrial Life

• The SETI project to search for radio signals
  from outer space

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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) gt 10 ?

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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

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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

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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