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Properties of Extrasolar Planets . Roman Rafikov (Institute for Advanced Study). Methods of detection Characterization Theoretical ideas Future prospects. Methods of detection. Methods of detection. 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
Methods of detection Methods of detection
Methods of detection 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. Planet NS WD Pulsar timing m sin i degeneracy (broken in PSR 1257+12 by planetary interaction) First discovered extrasolar system: PSR 1257+12 (Wolszczan and Frail 1992) Second suggested system: PSR 1620-26 (Backer et al. 1993) Similar to Solar System Recently confirmed (Sigurdsson et al 2003) to have a giant planet in a wide eccentric orbit. Similar to nearby EGP systems.
Methods of detection Butler et al 2004 Keck GL 436 Radial Velocity Variations Planetary 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)
Methods of detection Jupiter-size planet passing in front of the star causes stellar flux drop (eclipse) by for ~ hours. Transit probability for AU • 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) First transit detection in previously known planetary system HD 209458 (Charbonneau et al 2000) Planetary Transits • 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.)
Methods of detection Bond et al 2004 Bond et al 2004 Planetary mass Stellar mass Semimajor axis Distance to the system 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 OGLE 2003-BLG-235/MOA 2003-BLG-53 (first planet detection by microlensing)
Methods of detection 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. • 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. 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. Future missions should revolutionize this field (Keck Interferometer, SIM). Astrometry
Characterization Results: characteristics of extrasolar planets
Characterization General Statistics • 120 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!
Characterization • 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 . • Planetary sizes consistentwith expectations except that • Radius of HD 209458 is too large to be accomodated by simple models. • 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. Physical Properties
Characterization 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 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 Simulation from (Vidal-Madjar et al 2003) HD 209458 (a.k.a. “Osiris”) (Vidal-Madjar et al 2004) Planet is safe from evaporation!
Theoretical ideas Theoretical Ideas
Theoretical ideas Bryden • 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? Rasio & Ford 1996 • 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 We don’t really know these things very well Dynamicalpeculiarities
Theoretical ideas 1 M_J planet (Lubow et a al 2003) • 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) Peculiarities of physical structure
Future prospects Future Prospects
Future prospects 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 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.
Future prospects SIM (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
Future prospects TPF (Terrestrial Planet Finder) Goals: - 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. 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.
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.