Search for Life in the Universe

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Search for Life in the Universe

• Chapter 11
• Extrasolar Habitability
• (Part 1)

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Outline

• Are Habitable Planets Common?
• Distant Suns
• Properties of Stars
• Stellar Types
• Which Stars Make Good Suns?
• Multiple Stars
• Extrasolar Planets
• Detecting Extrasolar Planets
• Astrometry
• Doppler Technique
• Transit
• Direct Detection

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Are Habitable Planets Common?

• Disks of young stars
• Theory disks inevitable ? planets
• Observations 25?50 of stars have disks with a
  few percent of the stellar mass (cf., lt0.2 for
  our planets)
• 223 Extrasolar planets (4/15/2007)
• Radial velocity 211 planets, 183 systems, 20
  multiple
• Microlensing 4 planets
• Imaging 4 planets
• Pulsars 4 planets, 2 systems, 1 multiple
• Characteristics of discovered extrasolar planets
• Massive (mainly a selection effect) Jovian
  planets
• Unlikely to be habitable
• Orbit close to the parent stars
• Highly elliptical orbits common
• Affect habitability of Earth-like planets

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Properties of Stars

• Evolution
• Protostar ? main sequence ? giant or supergiant ?
  white dwarf, neutron star, or black hole
• Habitability in main sequence phase, because it
  lasts longest
• Types
• Universal composition gt98 H He
• Universal burning H ? He ? C O
• Mass only free parameter
• Types OBAFGKM (politically incorrect mnemonic
  Oh, Be A Fine Girl, Kiss Me)
• Lifetimes
• Luminosity steep function of mass ? M(3.5?4)
• Lifetime steep inverse function of mass ?
  M?(2.5?3)

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Which Stars Make Good Suns?

• O much too short
• B little life past accretion phase
• A and F
• Life short but manageable
• UV light a potential problem, but maybe more
  ozone
• G OK (cf., the Sun)
• K and M
• Long life
• Very common
• Less luminous ? smaller habitable zone
• Synchronous orbit problem, unless the atmosphere
  rotates
• Flares main danger is UV, but it also produces
  more ozone
• Brown dwarfs ?

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Multiple Stars

• Orbit
• Planet pulled by two or more stars can have a
  complicated orbit, moving in and out of the
  habitability zones of the stars
• Stable cases
• Stars close, planet orbiting both at a safe
  distance
• Stars far, planet orbiting close to one of them
• Unstable cases
• Any other combination

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Detecting Extrasolar Planets

• Direct detection of the planet
• Difficulty observe a dim planet near a bright
  star ? space missions gt 10 yrs from now
• Absorption spectrum in a planetary atmosphere
  during transit?
• Indirect detection of effect on parent star
• Extrasolar planets around main sequence stars
  discovered in 1995
• Planets around neutron stars discovered
  previously
• Spectroscopy (gt200 cases) detect Doppler shift
  of stellar motion around center of mass
• Astrometry (1 case) detect angular motion

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Astrometry

• Binary or multiple stars
• Very practical, if they can be resolved
• Planets
• Resolution not an issue, observe only the star
• Amplitude problem, e.g., at a distance of 10 ly,
  the amplitude of the solar motion due to Jupiter
  is 0.003??
• Angular motion ? 1/distance ? harder for distant
  stars
• Angular motion ? orbital radius ? easier for
  outlying planets, but orbital period longer and
  region less habitable
• Space Interferometer Mission (SIM, launch 2011?)
• Angular precision 10-6??
• Earth-like planets detect stellar wobble 12
  nearby stars
• Jupiter-size planets detect wobble 3,000 ly away

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Doppler Technique

• Doppler effect
• ??/? vlos/c (nonrelativistic)
• Detection of planets
• Velocities 30 m/s 10-7c
• Accurate spectroscopy possible with iodine cells
  instead of an arc
• Velocity independent of distance
• Velocity ? Mplanet favors massive planets
• Velocity ? 1/?a favors small orbits
• Orbital period ? 1/?a3 shorter period favors
  small orbit (observations take less time)

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