Extrasolar planets

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Extra-solar planets

• Dr. Matt Burleigh
• www.star.le.ac.uk/mbu

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Radial Velocity Technique(Doppler Wobble)

• Star planet orbit common centre of gravity
• As star moves towards observer, wavelength of
  light shortens (is blue-shifted)
• Light red-shifted as star moves away

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Transits

• Planets observed at inclinations near 90o will
  transit their host stars

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Super WASP

• Wide Angle Search for Planets (by transit method)
• First telescope located in La Palma, second in
  South Africa
• Operations started May 04
• Data stored and processed at Leicester
• 20 new planets detected!
• www.superwasp.org
• www.wasp.le.ac.uk

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First directly imaged planets

• 3 planets around HR8799, 130 light years away
  (40pc)
• Young (60million years old)
• Three planets at 24, 38 and 68AU separation
• In comparison, Jupiter is at 5AU and Neptune at
  30AU)
• Masses of 7Mjup, 10Mjup and 10Mjup
• Also, a 3Mjup planet around Fomalhaut, at a
  separation of 120AU

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What we know about extra-solar planets

• 328 planets now found
• 34 multiple systems
• 52 transiting planets can directly measure
  radii
• Unexpected population with periods of 2-4 days
  hot Jupiters
• Planet with orbits like Jupiter discovered (eg 55
  Cancri d)
• Is our solar system typical?

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Extra-solar planet period distribution

• Notice the pile-up at periods of 2-4 days /
  0.04-0.05AU
• The most distant planets discovered by radial
  velocities so far are at 5-6AU
• Imaging surveys finding very wide orbit planets

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Extra-solar planet mass distribution

• Mass distribution peaks at 1-2 x mass of Jupiter
• Lowest mass planet so far 5.5xMEarth
• Super-Jupiters (few MJup) are not common
• Implications for planet formation theories?
• Or only exist in number at large separation?
• Or exist around massive stars?

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Selection effects

• Astronomical surveys tend to have built in biases
• These selection effects must be understood
  before we can interpret results
• The Doppler Wobble method is most sensitive to
  massive, close-in planets, as is the Transit
  method
• Imaging surveys sensitive to massive planets in
  very wide orbits (10AU)
• These methods are not yet sensitive to planets as
  small as Earth, even close-in
• As orbital period increases, the Doppler Wobble
  method becomes insensitive to planets less
  massive than Jupiter
• The length of time that the DW surveys have been
  active (since 1989) sets the upper orbital period
  limit
• Only now are analogues of Jupiter in our own
  Solar System going to be found
• But imaging surveys can find the widest planets

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What we know about extra-solar planetsMass
versus semi-major axis

• Blue exoplanets
• Red solar system
• Many of the known solar systems have
  Jupiter-mass planets in small orbits,
• Selection effect of Doppler surveys
• But almost no super-Jupiters are found in close
  orbits
• Real, not a selection effect

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What we know about extra-solar planets
Eccentricity vs semi-major axis
- most extra-solar planets are on orbits
much more eccentric than the giant planets in
the solar system bad news for survivability
of terrestrial planets
- planets close to the star are tidally
circularized
observational bias
extra-solar planets
- planets on circular orbits do exist far
away from star - the planets in our own system
have small eccentricities ie STABLE

solar system planets
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Statistics of the Doppler Wobble surveys Summary

• Of 2000 stars surveyed
• 5 have gas giants between 0.02AU and 5AU
• Trends suggest 10 of stars have planets in
  orbits 5-7AU
• 0.85 have hot Jupiters
• Real effect

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MetallicityThe abundance of elements heavier
than He relative to the Sun

• Overall, 5 of solar-like stars have radial
  velocity detected Jupiters
• But if we take metallicity into account
• 20 of stars with 3x the metal content of the
  Sun have planets
• 3 of stars with 1/3rd of the Suns metallicity
  have planets

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Metallicity

• Does this result imply that planets more easily
  form in metal-rich environments?
• If so, then maybe planet hunters should be
  targeting metal-rich stars
• Especially if we are looking for rocky planets
• This result also implies that chances of very old
  lifeforms ( few billion years) in the Universe
  are slim
• With less heavy elements available terrestrial
  planets may be smaller and lower in mass than in
  our solar system
• Is there a threshold metallicity for life to
  start (e.g. ½ solar)?

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Planetary migration

• Hot Jupiters form several AU out from their
  parent star, where temperature is low enough for
  ice and dust to form
• They then migrate inwards and stop when the
  planet formation disk is finally cleared
• If migration time
• Planets fall into star
• Excess of planets at 0.03-0.04AU is evidence of a
  stopping mechanism in some cases
• Nature of stopping mechanism unclear tides?
  magnetic cavities? mass transfer?
• Large planets will migrate more slowly
• Explanation for lack of super-Jupiters in close
  orbits

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Planetary migration terrestrial planets

• Migrating giant planets will be detrimental to
  terrestrial planet survivability, if they both
  form at same time
• Planets interior to a migrating giant planet will
  be disrupted and lost
• Of course, these small planets may also migrate
  into star!
• If terrestrial planets can only survive when
  migration doesnt take place through their
  formation zone (few AU),
• then 3-20 of planet forming systems will
  possess them
• Alternatively, terrestrial planet formation may
  occur after dissipation of gas in proto-planetary
  disk (after 107 years)
• Disruption by a migrating giant planet unlikely
• Almost all planet-forming stars will have
  terrestrial planets

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Towards other Earths
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Towards Other Earths Habitable Zones
Left courtesy Prof. Keith Horne,
St.Andrews Right courtesy Prof. Barry Jones, Open

• Habitable zone defined as where liquid water
  exists
• Changes in extent and distance from star
  according to stars spectral type (ie
  temperature)
• It is possible for rocky planets to exist in
  stable orbits of habitable zones of known hot
  Jupiter systems
• If they were not previously cleared out by
  migration

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