What is a planet

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What is a planet?

• Planets are objects not sufficiently massive
  (lt0.013 Msun) to sustain quasi-equilibrium fusion
  of deuterium in their cores
• Brown dwarfs are intermediate objects
• large enough for deuterium fusion
• not large enough to sustain fusion of ordinary
  hydrogen
• central temperatures are lower than 6106 K
• Planets and brown dwarfs
• radiate a major part of their energy as
  gravitational contraction energy
• shrink and cool (after an initial warming) as
  they age
• in contrast to stars there is no unique relation
  between luminosity and mass

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Protoplanetary disks
Disk diameters of the order of 50-100 AU
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Evolved planetary disk
The beta Pictoris disk is warped from the
gravitational interaction of one or more unseen
companions of planetary or brown dwarf type
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Detection methods

• As extrasolar planets are extremely faint, most
  methods are indirect, i.e. rely on the influence
  on the star which they orbit
• Radial velocity
• radial motions of solar type stars are measured
  from a large number of narrow absorption lines in
  the optical spectrum
• useful to a distance of about 160 light years
• majority of exoplanets detected with this method

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• Pulsar timing
• cyclic variation in the frequency of emitted
  radio signals due to the doppler effect caused by
  an orbiting planet
• a few planets detected

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• Astrometry
• oldest method, used since 1943
• the wobble induced in the plane-of-sky motion of
  the star by planets is measured by accurately
  observing its position
• no success
• Photometric detection of transits
• precise observations of the variation of light
  from the star as a planet transits its surface as
  seen from the Earth
• five planets

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• Microlensing
• photometric observation of the increase in
  brightness of a distant star due to relativistic
  bending of its light by a massive object passing
  through the line of sight between the observer
  and the star
• small variations of the week-month-long lensing
  event yields information on the presence of
  planets around the star
• thousands o stars moitored towards Magellanic
  clouds or galactic bulge
• best method for detecting Earth-mass planets
• one planet
• Imaging
• direct detection of planets close to stars by
  observations in the thermal infrared, where the
  brightness contrast is reduced a factor 1000
  compared to the case in the optical
• interferometric nulling
• possibly one planet around a star one
  free-floating planet and many brown dwarfs have
  been detected
• Artificial signals
• Radio frequency searches (SETI)
• Relies on existence of advanced, technological,
  self aware life forms

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Properties of extrasolar planets

• Global statistics (fall 2005)
• 133 planetary systems
• 156 planets
• 18 multiple planet systems
• 1 system with four planets
• 3 systems with three planets
• No system similar to the Suns

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• Small planets are difficult to detect
• Detection methods biased towards large planets in
  small orbits
• Earth-like planets are still to be found
• Uranus-Neptune size planets are now being
  discovered

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• Often, large planets are located close to stars
• Giant planets form at large stellar distances
• Requires a dynamical migration process
• Hot surfaces and extended atmospheres

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51 Pegasi B

• First planet discovered around a sun-like star
  outside of the solar system
• Radial velocity method
• Detection from regular velocity changes in the
  star's spectral lines of around 70 metres per
  second
• Semi-major axis 0.052 AU (circular)
• Orbital period 4.23077 d
• Mass gt0.468 0.007 MJ
• Greater radius than Jupiter despite its lower
  mass
• Superheated 700 K atmosphere
• It is the prototypical hot Jupiter
• Orbital migration to present position

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PSR 125712

• Pulsar located 2630 light years away
• Believed to be orbited by at least four planets.
• These were the first extrasolar planets ever
  discovered
• Millisecond pulsar found to have anomalies in the
  pulsation period, which led to investigations as
  to the cause of the less than regular pulses.
• Pulsar mass 0.3 Msun, rotational period 0.0062
  seconds
• Three planets discovered in 1992, possibility of
  a fourth
• innermost planet has a mass of 0.020 ME, with an
  average orbital distance of 0.19 AU, an orbital
  period of 25.262 days, and an orbital
  eccentricity of 0.0
• second planet has a mass of 4.3 ME, with an
  average orbital distance of 0.36 AU, an orbital
  period of 66.5419 days, and an orbital
  eccentricity of 0.0186
• third planet has a mass of 3.9 ME, with an
  average orbital distance of 0.46 AU, an orbital
  period of 98.2114 days, and an orbital
  eccentricity of 0.0252
• small object has an upper mass limit of 0.2
  MPluto and an upper size of R lt 1000km.

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PSR B1620-26c

• PSR B1620-26c is a planet orbiting the pulsar PSR
  B1620-26 in the globular cluster Messier 4, about
  12,400 light years from Earth
• Orbits a binary pair of stars. One, the pulsar,
  is a neutron star. The second is a white dwarf
  with a mass of 0.34 solar masses
• In the early 1990s, it was found that a third
  object was needed to explain the observed Doppler
  shifts of the system
• Planet orbits at a distance of 23 AU
• The triple system is just outside the core of the
  globular cluster M4. The age of the cluster has
  been estimated to be about 12.7 billion years
• Likely that PSR B1620-26c is also about 12.7
  billion years old

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PSR B1620-26c
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HD 209458b

• Spectroscopic radial velocity studies first
  revealed the presence of a planet around HD
  209458 on November 5, 1999
• 1.7 drop in HD 209458's brightness was measured,
  which was later confirmed as being due to a
  transit. Each transit lasts about three hours,
  and about 1.5 of the star's face is covered by
  the planet during the transit
• Semi-major axis 0.045 AU (circular)
• Orbital period 3.52474541
• Inclination 86.1 0.1
• Mass 0.69 0.05 MJ
• Radius 1.32 0.05 RJ
• Density 370 kg/m³
• Temperature 1,130 150 K
• Probably a gas giant

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HD 209458b

• Envelope of hydrogen, carbon and oxygen around
  the planet that reaches a kinetic temperature of
  10,000 K
• The heavier carbon and oxygen atoms are being
  blown off of the planet by the extreme
  "hydrodynamic drag" created by its evaporating
  hydrogen atmosphere
• The hydrogen tail streaming off of the planet is
  200,000 kilometers long

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2M1207b

• First extrasolar planets to be directly observed
  (by infrared imaging)
• Observations over time show with high confidence
  that the two objects (2M1207 and 2M1207b) show
  similar proper motion in the sky and are
  therefore part of the same stellar system
• Orbits the brown dwarf 2M1207A
• Five times the mass of Jupiter
• Distance from star gt55 AU
• Orbital period 2450 years
• Estimated surface temperature 1250 K
• The infrared spectrum of the planet indicates the
  presence of water molecules in its atmosphere
• Perhaps not a true planet formed around the star
  but rather a faint brown dwarf formed from
  individual gravitational collapse of in the same
  gaseous nebula

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2M1207b
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• Future prospects
• Corot mission will improve on Earth-based
  transit searches
• Earth-like planets Kepler mission
• Distant gas and ice giants
• Darwin mission search for and spectroscopic
  characterization of Earth-like planets (ozone,
  oxygen, water)