Extrasolar Planets

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Extrasolar Planets
James J Marie, Astronomy 2005
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Other Solar Systems

• For a long time, we knew of only one solar
  system The one in which we live.
• In 1995, the first definitive planet beyond our
  solar system was discovered!
• As of October 2005, there are 145 known
  planetary systems beyond our solar
• system.
• These newly discovered solar systems, contain
  at least 169 planets.
• The philosophical implications of extrasolar
  planets are very profound.
• Knowing that planets are common increases the
  chance for life elsewhere
• in the universe and perhaps even intelligent
  life.

James J Marie, Astronomy 2005
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Detection of Extrasolar Planets

• Extrasolar planets were not directly observed
  in a telescope.
• Presently, it is too difficult to directly
  observe extrasolar planets.
• Observing an Earth-like planet orbiting a
  nearby star is similar to observing
• a firefly near a street lamp 3,000 miles away!
• A Sun-like star would be a billion times
  brighter than the reflected light from
• any planet.
• The small flecks of planetary light are
  overwhelmed by the glare of scattered
• starlight.
• Consequently, all extra solar planets have been
  detected indirectly using very
• advanced technology.

James J Marie, Astronomy 2005
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Detection Methods

• Extrasolar planets are detected by measuring
  the gravitational pull of the
• planet on the host star.
• As the planet orbits the star,
• the star wobbles ever-so-slightly
• from the pull of the planet.
• The wobble of the star is
• detected by the slight doppler
• shift of its spectral lines back
• and forth.

James J Marie, Astronomy 2005
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Stellar Motion

• The doppler shift is translated into a periodic
  velocity (back and forth motion).

• It may take several years to record one
  wobble cycle of a star.

James J Marie, Astronomy 2005
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Doppler Technique

• The doppler technique allows the orbital period
  of the planet to be determined.
• The planets average distance from the star and
  orbital eccentricity can also be
• determined.
• Sometimes, the data may indicate more than one
  planet!
• The doppler technique also give an estimate on
  the mass of the unseen planet.
• In most cases, the doppler shift can only
  provide a lower limit on the mass of
• the unseen planet.
• The uncertainty in the mass arises because the
  unseen planets orbit may be
• tilted from our point of view on Earth.

James J Marie, Astronomy 2005
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Extrasolar Planets Orbital Plane

• If the orbit of the extrasolar planet is seen
  edge-on, the planets mass can be
• calculated with high precision.

James J Marie, Astronomy 2005
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Transits

• If the orbital plane of the extrasolar planet
  is oriented approximately edge-on,
• a transit of the planet in front of the star
  may be observed.
• As the planet passes directly in front of the
  star as seen from the Earth, the
• stars brightness drops slightly due to the
  blocked light.
• This drop in brightness during a transit can be
  used to determine the actual
• size of the planet.
• If the size and mass of the unseen planet are
  known, then its density can be
• calculated.
• Most extrasolar planets have a density
  comparable to the jovian planets in our
• solar system.
• NASA is planning a mission called Kepler to be
  launched in 2007 to look for
• Earth sized planets using the transit method.

James J Marie, Astronomy 2005
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Transit of an Extrasolar Planet
James J Marie, Astronomy 2005
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51 Pegasi

• The first definitive extrasolar planet ever
  discovered was found orbiting the
• main sequence star 51 Pegasi which is 47.9
  light years away.
• The discovery by Michel Mayor and Didier Queloz
  was announced October 6,
• 1995.
• The host star, 51 Pegasi, is a Sun-like star
  with a mass of 1.04 solar masses.
• The planet belongs to an unsuspected class of
  planets known as epistellar
• jovians or hot jupiters with remarkably
  small orbits.
• Known facts about the planet

Mass 0.47 MJ where MJ
mass of Jupiter Semi-major axis 0.052
AU Orbital Period 4.23 days
James J Marie, Astronomy 2005
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51 Pegasi
James J Marie, Astronomy 2005
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51 Pegasi B

• An artists rendering of
• 51 Pegasi B.
• This view gives an idea of the
• large size of 51 Pegasi B.

James J Marie, Astronomy 2005
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51 Pegasi B

• Early speculation about 51 Pegasi B suggested
  that it might be a titanic
• terrestrial world with the mass of Jupiter.

James J Marie, Astronomy 2005
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51 Pegasi B

• It seemed impossible for there to be enough
  rocky material in a protoplanetary
• disk to form a super massive planet at such a
  small distance.
• Yet, it couldn't possibly be a gas giant,
  because a jovian atmosphere would
• surely evaporate into space under the intense
  stellar radiation.

So what exactly is 51 Pegasi B?
James J Marie, Astronomy 2005
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The 51 Pegasi B Puzzle

• Mathematical computer simulations of
  protoplanetary disks showed that gas
• giants like Jupiter could migrate inward
  towards their stars, either due to drag
• against disk material or by gravitational
  perturbations with the disk.
• Tidal forces and disk clearing near the star
  would tend to park such migrating
• planets at orbital distances quite similar to
  those of 51 Pegasi B.
• It appears that 51 Pegasi B and other similarly
  discovered planets originally
• formed far from their stars but then spiraled
  inward to their current orbits.
• But how can gas giants survive so close to
  their parent stars without having
• their atmospheres stripped away?
• Calculations of the temperature of 51 Pegasi
  B's atmosphere showed that the
• planet could hold on to a Hydrogen/Helium
  atmosphere for astronomically
• significant timescales.

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51 Pegasi B

• 51 Pegasi B is the archetypical hot jupiter.
  Heated both inside and out to
• temperatures high enough to vaporize
  silicates, its atmosphere would be a
• bottomless inferno.

James J Marie, Astronomy 2005
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Eternal Sunset on 51 Pegasi B

• 51 Pegasi B is tidally locked to its star so
  that its sun appears motionless.

James J Marie, Astronomy 2005
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Beta Pictorus

• In 1993 a circumstellar disk of gas and dust
  was found to be orbiting the star,
• Beta Pictorus.
• Beta Pictorus is located 62.9 light years away
  and is 1.75 times more massive
• than the Sun.
• Beta Pictorus is 8.7 times brighter than our
  Sun.

• Spectra of the dust ring indicates the
• possible existence of comet like objects.
• A Jupiter sized planet has been
• discovered orbiting within the central
• zone.

James J Marie, Astronomy 2005
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Beta Pictorus

• The Hubble Space Telescope has photographed the
  200 billion mile dust
• disk and reveals that the disk is slightly
  warped.
• The warp is thought to be caused by the
  gravitational pull of a planet.

James J Marie, Astronomy 2005
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Beta Pictorus

• The Jupiter sized planet
• discovered at Beta Pictorus
• orbits at a distance of
• 20 AU.
• The orbit of the planet is
• nearly circular.
• The mass of the planet is
• about 5 times the mass of
• the Earth.
• Evidence indicates that other
• planets may also orbit this
• star.

James J Marie, Astronomy 2005
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First 77 Extrasolar Planets
James J Marie, Astronomy 2005