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					         Extrasolar Planets

          Philippe Thébault
Paris Observatory/Stockholm Observatory
Plato   Aristoteles



Reminder:The classic scenario of
       planetary formation


                                   Planets should
                                    be a common
Part I: direct exoplanet detections
So, let’s do it….
           Difficulties for a direct detection

• Extrasolar systems are far away
- - Closest star at 4 light years. ~ 40 000 billions of km!
- - Earth-Sun distance = 150 millions of km
   => min. angular distance of a Star-Earth system = 0.77”
                         (1/5000 °)

   •The planet’s brightness is very low

 Lplan / L*  (Rp/a)2 (Rp planet’s radius, a distance to the star)
                   (Rp/a)2 = 6.10-9 for Jupiter
                  (Rp/a)2 = 1.10-9 for the Earth
        Direct detection of brown drwarfs

  Gliese 229. Lcomp / L* = 5000 , distance = Soleil-Pluto.
A Jupiter would be 14 times closer to its star the luminosity
         ratio 200 000 times weaker ! (HST, 1995)
     Direct detection of exoplanets around
                M-dwarfs stars

2M1207: MPl~5MJup

                          5 candidates so far, but 4 of
                          them might in fact be brown

  (Chauvin et al. 2003)
     nothing yet around solar-type stars

•so far:
non-detection in surveys puts upper limit of ~6% on
the fraction of stars with planets>5MJup with
separation > 50AU (Lafreniere et al., 2007)

Future studies have to focus on young giant planets,
since they are more luminous (x1000!) than the final
ones, due to ongoing gravitational contraction.
 last minute: First direct detection of an
exoplanet around a young solar-type star?
Lafreniere et al.2008 (submitted)

         MPl ~8MJup

            ~330AU (!)

Age~ 5x106 years                    •Is it bound to the star or a
                                    free-floating planet?
Part II: circumstellar disc detections
                     Circumstellar discs

 Spectroscopic detection through I.R. excess (IRAS, ISO)
 Direct imaging: near IR, visible (ESO, HST, …)
               50 % of Y.S.O. are surrounded by discs
Class 0: Md  0.5 M☼ time scale  104 years (protostars)
Class I:   Md  0.1 M☼ time scale  105 years (CTTS)
Class II: Md  0.01 M☼ time scale  106 years (WTTS)
Class III: Md < 0.01 M☼ time scale  107 years   (debris discs)

               (Reminder: : Minitial Solar-System > 0.03 M☼ )

      Confirmation of the « classical » scenario
« protoplanetary »

                                                      Debris discs

                     a bevy of circumstellar discs!
      “proto-planetary” discs around “YSOs”

Young (<107yrs) and massive (~100M) discs, with Mgas/Mdust~100
Protoplanetary discs: wavelength vs radius probed

 Circumstellar disks have been studied at all wavelengths from optical to cm

                                    Different wavelengths probe different
                                    locations in the disc; e.g., thermal
                                    emission from an optically thin disk,
                                    assuming black body grains:

                                     Tdust = 278.3 L*0.25/r0.5
                                     peak = 2898m/T = 10.4r0.5/L*0.25
                                     rprobed = 0.012L*0.5 AU

    NIR=0.1AU, MIR=1AU, FIR=30AU, SUB=1000AU (though smaller as not
    observed at peak)
Circumstellar discs: Observational techniques

Some general things we want to know about a disk:

• Total flux = photometry
    • One wavelength shows disk is there
    • Two wavelengths determines dust temperature
    • Model fitting with multiple wavelengths (Spectral Energy Distribution)

• Composition = spectroscopy
    • Can be used like multiple photometry
    • Also detects gas and compositional features

• Structure = imaging
    • Give radial structure directly and detects asymmetries
    • But rare as high resolution and stellar suppression required
Accretion diagnostics:T.Tauri Stars
Discs around pre-main sequence stars were
first inferred from evidence of strong emission
lines in visible/UV spectra: H, HeI, OI, NaD,

Explained as magnetospheric accretion
columns in which a circumstellar accretion
disc is disrupted by the stellar magnetic field
and accreting material falls onto the stellar
surface along magnetic field lines

Stars with accretion lie in clusters (T associations)

Modelling emission lines gives mass accretion rate
of around 10-8Msun/yr, falling with age

Accreting stars are known as: classical T Tauri
stars (CTTS)
           Spectral Energy Distribution (SED)

To understand disc structure, unless   Meeus et al. (2001): HAeBes
the disc is resolved, one must
interpret the spectral energy
distribution (SED): flux (F) vs
wavelength () [or F vs  where

parts of the
                  Deriving dust masses:
                 sub-mm/mm photometry
Sub-mm/mm observations are the best way of deriving dust mass:
• unaffected by uncertainties in Tdust
• discs are optically thin so most of the mass is seen
• larger grains contain most of the mass
• little contribution to flux from stellar photosphere

The basic equation is:
  Mdust = Fd2/[B(T) ]
where d is distance,  = 1.5Q/(D)  0(0/) is the mass opacity and a
value of 0=0.17m2/kg is often used for 0=850m with =1
NOTE this is dust mass, and disc mass is sometimes quoted as
(and/or you may wish to use) an additional 100 gas/dust factor
This also means F  -2- so that measuring spectral slope (F  -) gives 
       older and less massive discs:
“debris” discs around Main Sequence Stars
      Discoveries of debris discs, the IR trilogy:
IRAS (ESA, 1983): all sky survey.
Resolution~0.5’-2’. First IR-excess detection: Vega
(“vega-type” stars). ~170 IR-excess detections =>
~15% of stars with debris discs

ISO (ESA, 1996): pointed telescope.
Resolution~1.5”-90”. 22 new detections. ~17% of
stars with discs. Spectral type dependancy: A:40%,
F:9%, G:19%, K:8%.

Spitzer (NASA, 2003): pointed telescope,
currently operating.
Resolution~1”-10”. Truckload of new results
coming out.
ISO/IRAS/Spitzer compared sensitivities
Imaging of debris discs: visible/near IR

              -Pictoris (1984)
β-Pictoris: the crowned queen of debris discs

     0.5µm          0.5µm      1-2µm

      10-20µm          850µm
Some more…
and some more…
             debris discs: temporal evolution

     Not so

                        (from Su et al., 2006)
              What we’re really
What we see    interested in

Part III: indirect exoplanet detections
                          hitting the jackpot:
                       the Doppler-shift method

                                                       from / measurement it
perturbed motion of the star
                                                       follows V*/c => Mp.sin(i)

                                                   -      Some examples
                                                           VSun(Jupiter) ~ 13 m/s
                                                           VSun(Earth) ~ 0.1 m/s
                               Unseen planet           Present day resolution ~ 3m/s

                                                             Possible detection of
                                                            “exo-Jupiters” !
      1st detection (OHP, 1995)
                                        More than 250 planets
                                           detected so far

                                       Most of them are GIANT
                                       planets very CLOSE to
                                               their star
          Time (in planetary orbits)
(from Marcy, « SIM » science proposal)
       the Transit method

           Leclipse / L*  (Rp/R*)2
       (Rp/R*)2 = 0.01 for Jupiter
     (Rp/R*)2 = 0.0001 for the Earth

much more favourable than direct detection
              Detection by transit         (2)

BUT     small chances to have an edge-on line of sight
                0.1% for a Jupiter-Sun couple
                0.5% for an Earth-Sun couple

THUS    Large and methodic search
So far 52 positive

                     (HD149026, Sato et al., 2005)
      atmosphere/exosphere of HD209458

Charbonneau et al. (2002) used HST spectra of
NaD line both in and out of transit to detect the
additional absorption of 0.02% during transit due
to sodium in the atmosphere of the planet;
models of planetary atmospheres (and stellar limb
darkening) produce significantly deeper absorption

Vidal-Madjar et al. (2003) detected
atomic hydrogen in absorption
(15%) in stellar Lyman  line ->
beyond Roche limit so from
escaping hydrogen atoms; also
escaping oxygen and carbon (Vidal-
Madjar et al. 2004)
water in the atmosphere of HD189733b

                         (Tinetti et al., 2007)
Methane in HD189733b’s atmosphere

      (Swain et al., 2008)
Transits from space: CoRoT (France, ESA)
                     •0.3m photometer
                     •Search for planetary transits
                     •Launch: october 2006
                     •Duration: 2 ½ years
                     •Polar orbit at 896 km
                     •Search for planetary transits
                     •12000 stars simultaneously
                     observed during 150 days
                     (total: 60 000 stars)
                     •may detect « Super-Earths »
                     …but no « exo-Earths »
                CoRoT: 5 planets so far…

                             CoRoT-Exo-1b: 1.03MJup, a=0.0254AU

CoRoT-Exo-4b: synchronous orbits
with its star!

                             tentative detection: 1.7 M? (awaits
                        detection by astrometry

           Principle
    To detect the star’s motion induced by
               unseen planet(s)

           Difficulties
    -       very high resoltion required:
                   <0.001”                        Trajectory of the Sun induced by
                                                   the planets as seen from 10 pc
             Impossible from the ground
                            - Possible from a satellite

    So far, 1 succesful mission : Hipparcos, but not good enought

                                      GAÏA (soon)

Lingjuan Ma Lingjuan Ma MS
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