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					      Extrasolar Planets

             Please check:
          http://exoplanet.eu/
(the Extrasolar Planets Encyclopaedia)
          Planets & Brown Dwarfs
  Tentative definition:
• Planets are objects not sufficiently massive (<0.013 M sun) to
  sustain quasi-equilibrium fusion of deuterium in their cores

• Brown dwarfs are intermediate objects between planets and stars
   – large enough for deuterium fusion
   – not large enough to sustain fusion of ordinary hydrogen
   – central temperatures are lower than 6·106 K (M<0.08 Msun)

• Planets and brown dwarfs
   – radiate a major part of their energy as gravitational contraction
     energy
   – shrink and cool (after initial warming) as they age
   – in contrast to stars there is no unique relation between luminosity
     and mass
         Circumstellar Disks
• High-resolution imaging of nearby stars and
  star-forming regions has revealed a large
  number of flattened dust structures (disks)
  around protostars or young main-sequence
  stars
• In the case of protostars, this dust is
  interpreted as a tracer of the accretion disks
• In the case of evolved stars like  Pictoris,
  the dust must be resupplied by the collisional
  evolution of a population of macroscopic
  bodies (similar to our transneptunian
  population?)       Often called Debris disks
Protoplanetary disks




  Disk diameters of the order of 50-100 AU
           Evolved disks
• Typically observed through their IR
  emission as disks surrounding young
  main-sequence stars
• They appear to be a phenomenon of
  common occurrence
• They are typically much larger than our
  planetary system including the Kuiper
  Belt
       Example of evolved disk




The beta Pictoris disk is warped by the gravitational interaction of
one or more unseen companions of planetary or brown dwarf type
  Exoplanet Detection Methods
• Radial velocity (largest number, indirect)
• Pulsar timing (historically the first, indirect)
• Transit search (produces many candidates but
  requires spectroscopic confirmation)
• Microlensing (direct)
• Imaging (direct)

• Astrometry (failed so far, but may become
  important with space-based measurements)
           Detection statistics
• Total number: 312 exoplanets
           (Oct. 2008)
• Radial velocity: 293 planets
 - 251 systems (2211 ,212, 73, 14, 15)
 - 51 single planets also detected by transits
• Microlensing:      8 planets
• Imaging:          6 planets
• Timing:    5 planets
 - 3 systems (2 single, 1 triple)
   System
  properties
     (1)
  (spring 2006)

• No system like
  that of the Sun
 Very often a
  giant planet
  close to the star
  (“hot Jupiters”)
         System properties (2)
• Unlike the Solar
  System, exoplanets
  often have large
  eccentricities
• Explanations are
  sought in terms of
  orbital perturbations
  (Kozai cycles, stellar
  encounters)
           Radial velocities




– radial motions of solar type stars are measured from
  the Doppler shifts of a large number of narrow
  absorption lines in the optical spectrum
– useful to a distance of about 160 light years
– A large majority of exoplanets were detected by this
  method
Exoplanet mass determination




• Measure: period P and radial velocity half amplitude
  v sin i
• Identifying v with v sin i yields a lower limit for Mp
• Detection is easiest if P is short, vp is large or M is
  small
                                v
                     M p  M 
                                vp
       Exoplanet mass distribution
• The distribution of M  sin i
  for exoplanets observed
  through radial velocity
  variations peaks near or
  below Jupiter’s mass
• The number of Neptune-
  sized exoplanets is
  increasing
• There is a strong bias          A large number of Neptune-sized
  against discovering low-        exoplanets speaks against the
  mass planets                    gas disk instability mechanism
            Transit observations




• Photometry: precise observations of the variation of
  light from the star as a planet transits its surface as seen
  from the Earth  exact mass determination since the
  sin i ambiguity is resolved (sin i = 1) + size determination
• Spectroscopy: variation of absorption lines in the stellar
  spectrum are due to gases in the planetary atmosphere
       Transit observations (2)
• Transits have been observed for 51 confirmed
  exoplanets. Some of these were first established
  by the radial velocity technique. Masses range
  from a little more then 1 MNep to about 10 MJup.

• Many more candidates exist, awaiting
  confirmation by the detection of radial velocity
  modulation. Several confirmations have failed,
  proving that some “transits” were due to e.g.
  stellar variability.
     Transit observations (3)
• The densities of giant exoplanets are found to
  be ~ 1 g/cm3
• Models of the interior are generally similar to
  our giant planets, but with important
  differences
              Microlensing
– photometric observation of the increase in
  brightness of a distant star due to gravitational
  lensing of its light (general relativity) by a
  massive object passing by the line of sight
  between the observer and the star
– small variations of the week-month-long lensing
  event yield information on the presence of
  planets around the star
– thousands of stars are being monitored towards
  the Magellanic clouds and Galactic bulge
– the best method for detecting Earth-mass
  planets
– six planets detected so far by the OGLE transit
  survey; two more by the MOA survey
                 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 (future ESA/Darwin
  project)

– six planets around stars; one free-floating planet
  and many brown dwarfs have been imaged
                       2M1207b
• First extrasolar planet to be
  directly observed (by infrared
  imaging)
• Orbits the brown dwarf 2M1207
• Four times the mass of Jupiter
• Distance from star ~50 AU

• The infrared spectrum of the
  planet indicates the presence of
  water molecules in its
  atmosphere


                                     Image from ESO/VLT
                   Pulsar Planets
• The millisecond pulsar PSR 1257+12 was the first star with an
  exoplanet system detected (1992)
• The technique was similar to the radial velocity technique: the
  timing of the pulses is affected by the orbital motion of the star
  around the center of mass
• The pulsar mass is 0.3 Msun, and the three planets have
  masses ~ 4 ME (the two outer) and 0.02 ME (the innermost)
• These planets are believed to have formed from the ejecta of
  the supernova explosion that formed the pulsar

• The pulsar PSR B1620-26 in the globular cluster Messier 4 is a
  binary component along with a white dwarf star; this system is
  orbited by a planet detected by the pulse timing method
• This planet is Jupiter-sized and is thought to have formed
  around a solar-type star that has evolved into the white dwarf
             Detection biases
• Small and distant planets are difficult to detect
   – Detection is biased towards large planets in small orbits
   – Earth-like planets still remain to be found
   – Uranus-Neptune size planets, and somewhat smaller, are now
     being discovered
           Hot Jupiter problem
• Often, large planets are located close to the stars
   – Giant planets form at large stellar distances
   – Need for a dynamical migration process: gap clearing in the gas
     disk may lead to inward migration as the disk drags the planet
     along (Type II migration)
   – Hot surfaces and extended atmospheres: survival problem
                       HD 209458b
• First detected by radial velocity
  in 1999; then observed in
  transit across the star
•   Semi-major axis   0.045 AU (circular)
•   Mass              0.69 ± 0.05 MJ
•   Radius            1.32 ± 0.05 RJ
•   Temperature       1,130 ± 150 K


• Envelope of H, C and O around the
  planet reaches a kinetic temperature
  of 10,000 K
• The heavier C and O atoms are
  blown off of the planet by the extreme
  "hydrodynamic drag" created by its
  evaporating H atmosphere
Which stars can host planets?
• Stars that are severely metal-deficient appear
  to lack planetary systems



                                    This is a problem
                                    for the gas disk
                                    instability theory
                                    of planetary
                                    formation!
        Future
       prospects
• Corot and Kepler missions will
  improve on Earth-based transit
  searches

• NASA SIM interferometry
  mission may detect stellar
  wobbling due to distant gas
  and ice giants

• ESA Darwin mission: search
  for and spectroscopic
  characterization of Earth-like
  planets (ozone, oxygen, water)

				
DOCUMENT INFO
Lingjuan Ma Lingjuan Ma MS
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