properties of extrasolar planets roman rafikov (institute for advanced study) methods of detection...
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Properties of Extrasolar Planets
Roman Rafikov
(Institute for Advanced Study)
• Methods of detection
• Characterization
• Theoretical ideas
• Future prospects
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Methods of detection
Methods of detection
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Methods of detection
Pulsar timing
Planetary motion around pulsar causes periodic pulsar displacement around the center of mass by light-seconds for Earth-like planet ( g ) at AU from the neutron star ( ) – well within current detection capabilities.
310~/~ NSp MMa
1a2810~pMSunNS MM 4.1~
First discovered extrasolar system: PSR 1257+12 (Wolszczan and Frail 1992)
A B C
Mass, M_Earth 0.015 3.4 2.8
Eccentricity 0.0 0.018 0.026
Period, days 25.3 66.5 98.2
Sem. axis, AU 0.19 0.36 0.47
Second suggested system: PSR 1620-26 (Backer et al. 1993)
PlanetNS
WD
Recently confirmed (Sigurdsson et al 2003) to have a giant planet in a wide eccentric orbit.
Similar to Solar System
Similar to nearby EGP systems.
m sin i degeneracy (broken in PSR 1257+12 by planetary interaction)
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Methods of detection
Radial Velocity Variations
)/( *MM porbPlanetary motion moves star around with velocity . Jupiter-mass planet at 1 AU causes stellar motion with velocity 30 m/s.
Searching for periodic Doppler shifts can detect planet (like companion in spectroscopic binary).
• Method suffers from m sin i degeneracy
• Velocity noise can be pushed down to 1-2 m/s
• Up to now the most successful technique – more than 100 planets detected
• First detection - 51 Peg (Mayor & Queloz 1995)
927.0e
Butler et al 2004
GL 436Keck
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Methods of detection
Planetary Transits
Jupiter-size planet passing in front of the star causes stellar flux drop (eclipse) by for ~ hours. 22
* 10~)/(~ RRp
Transit probability for AU 2
* 10~/~ aR 1a
• Combined with RV data gives mass (sin i =1)
• The only method which gives planetary size
• Expect signal also in reflected light
• Systematic effects (blends, etc.)
First transit detection in previously known planetary system HD 209458 (Charbonneau et al 2000)
• More than 20 transit searches are underway
• 1 planet detected (4 more – OGLE transits )
• Great ancillary science for time-variable phenomena searches (GRB monitoring campaigns, microlensing searches)
TrES-1 (Alonso et al 2004)
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Methods of detection
Microlensing
Gravitational lensing by binary systems leads to easily recognizable pattern of magnification.
Light curve fitting yields system parameters and may be used for planet searches.
• Cannot repeat observation
• Usually distance is unknown
• Stellar brightness unimportant
Bond et al 2004
Bond et al 2004
OGLE 2003-BLG-235/MOA 2003-BLG-53 (first planet detection by microlensing)
SunMM 36.0*
kpcD 5
Planetary mass
Stellar mass
Semimajor axis
Distance to the system
Jp MM 2
AUa 3
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Methods of detection
Astrometry
Jupiter-mass planet at 1 AU moves its star around by
which for AU at 10 pc results in 100 µas displacement on the sky. aMMa p
3* 10~)/(~
1a
Benedict et al (2002) may have detected astrometric signal from previously known planet Gliese 876b (~ 2 M_J, 0.3 AU) using fine guidance sensors on HST.
• Sensitive to massive and distant planets – requires long time baseline
• In combination with RV data breaks m sin i degeneracy and determines mass and orbit orientation (similar to observations of stars near the Sgr A*)
• Requires exquisite astrometric accuracy.
Future missions should revolutionize this field (Keck Interferometer, SIM).
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Methods of detection
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Characterization
Results: characteristics of extrasolar planets
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Characterization
General Statistics120 planetary systems, 138 planets, 15 multiple systems
• 5 by transits
• 4 by pulsar timing
• 1 by microlensing
• 110 by radial velocity searches
Dynamical Characteristics• Unusual distribution of semimajor axes – many planets very close to the star
- Incomplete beyond several AU
- Very significant within 1 AU (“Hot Jupiters”)
• Predominance of planets with large eccentricities for periods longer than several days
- closest ones are circularized by stellar tides
- e-distribution is close to that of binary stars
Very different from Solar System planets!
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CharacterizationPhysical Properties
• Planetary sizes consistent with expectations except that
• Radius of HD 209458 is too large to be accomodated by simple models.
• High masses (only recently RV started probing Neptune mass range).
• Larger number of lower mass planets.
• “Brown dwarf desert” – relative lack of planets heavier than .
JM~
JM13
• Clear correlation between the metallicity of the parent main-sequence star and the probability of hosting planet.
• Pulsar planet in M4 may have formed in very low-Z environment; planets around PSR 1257+12 may have formed from very high-Z material.
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Characterization
HD 209458 (a.k.a. “Osiris”)
Planet is safe from evaporation!
(Vidal-Madjar et al 2004)
HST transit curve (Brown et al 2001)
• Mass , period 3.5 d, AU • Parent star (G0 V) at a distance of 47 pc• Radius of is a challenge for theory
JM7.0 05.0a
JR4.1
Direct probe of planetary atmosphere
• Na absorption detected in transit – atmospheric signature (Charbonneau et al 2002)
• Extended absorbing region in H-alpha – envelope of escaping hydrogen (Vidal-Madjar et al 2003)
• Evidence for oxygen and carbon in this escaping envelope
• Gas loss rate g/s
• Evaporation time is more than a Hubble time
1010
Simulation from (Vidal-Madjar et al 2003)
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Theoretical ideas
Theoretical Ideas
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Theoretical ideas
Dynamical peculiarities
Tight planetary orbits
• Evidence for planet migration – planets did not form where we observe them
• They moved there as a result of the tidal interaction with the gaseous disk
• Why did they stop? What is the parking mechanism?
Bryden
Large orbital eccentricities
• Possible signature of planet-planet gravitational scattering
• Could have also been caused by the tidal planet-disk interaction
• Resonant interaction of migrating planets
Rasio & Ford 1996
We don’t really know these things very well
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Theoretical ideas
Peculiarities of physical structure
Mass distribution
• Mass distribution arises from interplay of gas accretion and gravitational interaction of growing planet with surrounding gas disk
• Formation of “gap” can probably explain the “brown dwarf desert”
• Current observed overabundance of high mass planets can be purely a selection effect
Metallicity dependence
• Can be accounted for assuming that giant planets form via the gas accretion onto the rocky cores
• Could be a signature of stellar pollution by accreting planetary material (e.g. migrating planets which failed to hit the brake)
1 M_J planet (Lubow et a al 2003)
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Future prospects
Future Prospects
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Future prospects
Transits of terrestrial planets:
- About 50 planets if most have R ~ R_Earth,
Modulation of the reflected light from giant inner planets:
- About 870 planets with periods less than one week.
Transits of giant planets:
- About 135 inner-orbit planet detections along with albedos for about 100 of them.
Kepler
Space-based Photometer ( 0.95-m aperture )
Photometric One-Sigma Noise <2x10-5
Monitor 100,000 main-sequence stars for planets.
Mission lifetime of 4 years.
Launch date – October 2007
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Future prospectsSIM
(Space Interferometry Mission)
Precision astrometry on stars to V=20
Optical interferometer on a 10-m structure
Global astrometric accuracy: 4 microarcseconds (µas)
Typical observations take ~1 minute; ~ 5 million observations in 5 years
Launch date - February 2010
- Search for terrestrial planets around the nearest 250 stars (1 µas)-“Broad survey" for Neptune-size planets around 2000 stars (3 µas)- Giant planets around young stars
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Future prospectsTPF
(Terrestrial Planet Finder)
- Survey nearby stars looking for terrestrial-size planets in the "habitable zone“.
- Follow up brightest candidates with spectroscopy, looking for atmospheric signatures, habitability or life itself.
- Will detect and characterize Earth-like planets around as many as 150 stars up to 15 pc away.
Goals:
Two complementary space observatories: a visible-light coronagraph and a mid-infrared formation-flying interferometer.
TPF coronagraph launch is anticipated around 2014.
TPF interferometer launch is anticipated before 2020.
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Conclusions
• More than 100 planets are now known around main sequence stars and neutron stars, among them 15 multiple systems.
• They are detected by 4 different methods:
- Radial velocity searches
- Pulsar timing
- Transits
- Microlensing
• Most extrasolar planets exhibit unusual dynamical characteristic: small orbits and high eccentricities.
• Their physical properties are similar to those of giant planets in the Solar System (Jupiter and Saturn).
• In the next 10 years at least 3 space borne missions will open up new horizons in this field. More than 30 are currently underway.