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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
Pulsar timing
Planetary motion around pulsar causes periodic pulsar displacement
around the center of mass by ~ a M p / M NS ~ 103 light-seconds for
Earth-like planet ( M p ~ 1028 g ) at a  1 AU from the neutron star
( M NS ~ 1.4 M Sun ) – well within current detection capabilities.

m sin i degeneracy

(broken in PSR 1257+12 by planetary interaction)
First discovered extrasolar system:
PSR 1257+12 (Wolszczan and Frail 1992)
A
B
C
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
Mass, M_Earth
Second suggested system:
PSR 1620-26 (Backer et al. 1993)
Planet
NS
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.
Methods of detection
Radial Velocity Variations
Planetary motion moves star around with velocity  orb ( M p / M * ) .
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
e  0.927
(Mayor & Queloz 1995)
Butler et al 2004
Keck
GL 436
Methods of detection
Planetary Transits
Jupiter-size planet passing in front of the star causes stellar flux drop
(eclipse) by ~ ( Rp / R* ) 2 ~ 102 for ~ hours.
Transit probability
~ R* / a ~ 102
for
a 1
AU
• 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.)
• More than 20 transit searches are underway
• 1 planet detected (4 more – OGLE transits )
TrES-1
(Alonso et al
2004)
First transit detection in
previously known planetary
system HD 209458
(Charbonneau et al 2000)
• Great ancillary science for time-variable
phenomena searches (GRB monitoring
campaigns, microlensing searches)
Methods of detection
Microlensing
Bond et al 2004
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
OGLE 2003-BLG-235/MOA 2003-BLG-53
(first planet detection by microlensing)
Planetary mass
Stellar mass
Semimajor axis
Distance to the system
M p  2 MJ
M *  0.36 M Sun
a  3 AU
D  5 kpc
Methods of detection
Astrometry
Jupiter-mass planet at 1 AU moves its star around by
which for
a 1
~ a(M p / M* ) ~ 103 a
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).
Methods of detection
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
Physical Properties
• High masses ~ M J (only recently RV started
probing Neptune mass range).
• Larger number of lower mass planets.
• “Brown dwarf desert” – relative lack of planets
heavier than 13 M J .
• Planetary sizes consistent with 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.
Characterization
HST transit curve
(Brown et al 2001)
HD 209458 (a.k.a. “Osiris”)
• Mass 0.7 M J , period 3.5 d, a  0.05 AU
• Parent star (G0 V) at a distance of 47 pc
• Radius of 1.4 RJ is a challenge for theory
Direct probe of planetary atmosphere
(Vidal-Madjar et al 2004)
• 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
 1010
g/s
• Evaporation time is more than a
Hubble time
Simulation from (VidalMadjar et al 2003)
Planet is safe from evaporation!
Theoretical ideas
Theoretical Ideas
Theoretical ideas
Dynamical peculiarities
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 planetdisk interaction
• Resonant interaction of migrating planets
We don’t really know these things very well
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)
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)
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.
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.
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.