Extrasolar Planets and The Search for Life

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Extrasolar Planets and The Search for Life
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The Formation of the Solar-System

• The condensation model explains how the solar
  system formed during the collapse of the solar
  nebula
• It accounts for
• The distinction between Terrestrial and Jovian
  planets
• The different locations of the two planet types
• The fact that orbits are close to circular
• The fact that orbits lie in the same plane

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The Condensation Model

• Rotation of the solar nebula led to a
  protoplanetary disk
• Gas and dust within the disk condensed to form
  planetesimals
• The planetesimals combined to form planets
• The temperature gradient within the disk
  determined the compositions of the planets

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Terrestrial vs. Jovian

• Terrestrial
• Near the sun
• Too hot for light elements to condense
• Therefore, made from heavy elements
• No significant atmospheres

• Jovian
• Further from the sun
• Cool enough for light elements to condense
• Therefore, made from light and heavy elements
• Became large enough to acquire thick hydrogen
  atmospheres

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Extrasolar Planets vs. Brown Dwarfs

• Similar masses ( lt 0.08 Msun, the point at which
  nuclear fusion begins)
• Both generate energy via Kelvin-Helmholtz
  contraction
• However, different formation processes
• Planets form from a protoplanetary disk
• Brown dwarfs form via collapse of a solar nebula,
  like stars

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Detecting Extrasolar Planets

• Five different methods
• Pulsar timing
• Doppler spectroscopy
• Astrometry
• Transit photometry
• Microlensing
• First three rely on the wobble caused by the star
  orbiting the systems centre of mass
• Last two rely on the influence a planet has on
  the amount of light coming from the star

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Extrasolar Planets to Date

• Over 100 detected (most by Doppler spectroscopy)
• Masses similar or greater than Jupiter
• No terrestrial planets
• Orbits close to star, or eccentric
• Probably due to planetary migration

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Conditions for Life

• Above all, life needs liquid water
• Habitable zone (HZ) is range of distances from
  star where water is liquid on surface
• Continuous HZ is the overlap between HZs at
  different times
• Location of HZ depends on a number of factors

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Location of the Habitable Zone

• Depends on
• Distance from star (i.e., how much radiation is
  arriving)
• Planets albedo
• Planets atmospheric composition (e.g., carbon
  dioxide for greenhouse effect)
• Remember that liquid water can exist outside HZ
  (e.g., Europa)

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The Development of Life on Earth

• Definition of life
• React with environment and heal
• Grow by taking nourishment and energy from
  surroundings
• Reproduce, passing along characteristics
• Change genetically, allowing evolution
• Two theories for development
• Chemosynthesis
• Panspermia

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Chemosynthesis

• Chemical building blocks (e.g., amino acids
  Miller Urey experiment)
• Macromolecules (e.g., proteins)
• Prebionts (e.g., coacervate droplets)
• Prokaryotes (no nucleus)
• Autotrophic prokaryotes (made their own food,
  e.g. by photosynthesis)
• Aerobic respirers (used oxygen to burn their
  food)
• Eukaryotes (with nucleus)

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Panspermia

• Micro-organisms from space seeded life on Earth
• A number of variants
• Pseudo-panspermia
• Impact panspermia
• Cosmic ancestry
• Not a serious competitor to chemosynthesis

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Life in Extreme Environments

• Many organisms adapt to extreme environments
• Thermophiles (liking heat)
• Acidophiles (liking acidic environments)
• Psychrophiles (liking cold)
• Halophiles (liking salty environments)
• Demonstrates that life flourishes even in the
  harshest of locations

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Life in the Solar System Mercury and Venus

• Mercury
• Very hot
• No atmosphere
• Maybe ice deposits at poles?
• Strong UV radiation
• Almost no chance of life

• Venus
• Very hot
• Thick atmosphere (runaway greenhouse effect)
• Atmospheric anomalies have led to speculation of
  cloud-based life
• However, chances of life are very slim

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Life in the Solar-System Mars

• Very thin carbon dioxide atmosphere
• Low surface temperatures
• Frozen water at polar caps
• Much evidence for liquid water in the past
• Some evidence for liquid water today
• Some evidence for life in the past (ALH84001)
• Prospects for life today are reasonable

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Life in the Solar System Europa

• Very smooth surface, covered with long cracks and
  ridges
• Surface made of nearly-pure ice
• Probably liquid water ocean beneath surface, kept
  warm by tidal heating
• Possible evidence for life today (red colouring
  of ice cracks)
• Prospects for life today are good

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Life in the Solar-System Titan

• Thick atmosphere of ammonia, nitrogen, methane
  and other hydrocarbons
• Low surface temperature
• Any water will be frozen
• Too cold for life today
• However, life may develop in the future, when the
  sun passes through its red-giant phase, and warms
  the planet up

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The Role of Atmospheric Compositions

• Analyze the atmospheric composition of extrasolar
  planets
• Look for absorption lines caused by
• Oxygen
• Water
• Carbon dioxide
• Ozone
• Case study HD 209458
• HST found sodium in atmosphere of eclipsing planet

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Future Planet Searches

• Kepler
• Look for terrestrial-sized planets orbiting in HZ
• Single spacecraft
• Photometry of 100,000 nearby stars
• Look for dimming caused by transiting planets

• Darwin
• Look for atmospheric signs of life
• 6 spacecraft flying in formation
• Nulling interferometry in infra-red wavelengths

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SETI The Drake Equation

• An equation to calculate the number of
  communicating civilizations in the Galaxy today
• N R fp ne fl fi fc L
• Can lead to many different values, from 1 to 100
  or more
• Important as a conceptual tool

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SETI Radio Searches

• Most searches take place in the water hole (1,400
  MHz)
• First search was Project Ozma (1960)
• Project Cyclops report (1970s)
• HRMS (1991 cancelled)
• SERENDIP (piggyback on Arecibo)
• SETI_at_Home to analyze SERENDIP results
• Arecibo message

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SETI The Fermi Paradox

• Enrico Fermi Where are they?
• Filter
• Socioeconomic
• Cosmic zoo
• Suburbia
• Null
• Null answer is currently most promising
• However, it isnt a reason to stop looking