Download Astronomy of extrasolar planetary systems

Astronomy of extrasolar
planetary systems
Methods and results of searches
for planets around other stars
Course layout - methods
• Introduction and history of searches for
planets
• Doppler spectroscopy and classical
astrometry
• Optical interferometry
• Transit photometry and coronography
• Gravitational microlensing
• Timing of pulsars and white dwarfs
Course layout - results,
theories, interpretation
• Results of planet searches
• Theories of Solar System formation
• Theories of formation and evolution of
protoplanetary disks
• Interpretation of the results of planetary
searches so far
• Future of astronomy of extrasolar
planets
How to define a planet?
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Stars “burn” hydrogen, brown
dwarfs do the same to deuterium,
planets radiate away their energy
while contracting
The observed distribution of planet
masses shows a deficiency of
masses larger than ~12-13 Mjup (1
Mjup=0.001 Msun)
A lower mass limit above which a
star can burn deuterium is ~13 Mjup
This agreement between theory
and observations suggests a
planet definition of a body of mass
< 13 MJup
Classification of planets
Gas giants
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In the Solar System: Jupiter and Saturn
Have extended gas envelopes
Have masses on the order of Mjup
Chemical composition different from that
of the Sun
Gas had to be present in large quantities
at the time of gas giant formation
Terrestrial Planets
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Prototypes: Venus, Earth, Mars
These planets are dense and rocky
They are located close to the Sun
compared with the distant gas giants
A review of planet detection methods
Indirect detection methods: I
Doppler spectroscopy
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Observed quantity: radial component
of stellar velocity in the star’s motion
around the center of mass of the star
- planet system
So far, most of the extrasolar planets
(>150) have been detected with this
method
Astrometry
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Observed quantity: stellar motion in
the plane of the sky
Very likely the most efficient method
in the future (optical interferometers
on the ground and in space: Keck,
VLTI, SIM, GAIA, TPF)
Indirect detection methods: II
Transit photometry
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Observable: decrease of stellar brightness,
when planet moves across the stellar disk
Condition of observability: planetary orbit
must be (almost) perpendicular to the plane
of the sky
Several “hot Jupiters” have been detected
using this method
Earth-mass planets can possibly be detected
with this method using space-based
telescopes (COROT, Kepler)
Gravitational microlensing
One observes a light curve of the star, which
is lensed by the gravitational field of a star
located along the line-of-sight between the
lensed star and the observer. The presence
of a planet around the lensing star modifies
the light curve of the background star
An appropriate geometry occurs very
infrequently (need to observe millions of
stars) and the phenomenon is not repeatable
The method is sensitive to Earth-mass
planets
Indirect detection methods: III
Pulsar timing
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This method uses pulsars as
clocks (microsecond timing
precision)
The observed quantity is a
change of pulse arrival time
caused by the pulsar’s motion
around the center of mass of
the system
Precision of the pulsar clocks
makes it possible to detect
planets down to asteroid
masses
The first extrasolar planets
have been detected with the
aid of the timing method.
Direct methods: I
Adaptive optics
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Jupiter observed from the
distance of 10 pc would be
105 weaker than the Sun,
separated from it by about
one millisecond of arc
So far, adaptive optics has
made it possible to detect
brown dwarfs in binary
systems
Direct detection of planets
requires that the stellar light
be efficiently blocked off
Direct detection methods: II
Coronagraphy
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This method of blocking off the
stellar light has been originally
worked out by Lyot
The method removes 99% of the
stellar light, and it leaves out ~50%
of light from the hypothetical planets
Nulling interferometry
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This method removes the stellar
light observed with an
interferometer by placing a dark
interference fringe on it (destructive
interference)
Both methods are very
promising, especially, when
used with orbital telescopes
A comparison of some search projects
Some history
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The earliest, well-known planet
search episode was a long-term
effort by van de Kamp to detect
planets around the Barnard star
It was only in the 1970s that
Gatewood and Eichhorn have
shown, that van de Kamp’s
“detections” were the results of
instrumental errors
Around the same time, reports
have been published on possible
detections of planets around the
Crab pulsar and another pulsar,
PSR B0329+54. In both cases,
the results have been convincingly
shown to be due to irregularities in
the pulsar rotation
More history
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1983 was an important year in
planet detection history. At that
time, Smith and Terrile have
detected the first circumstellar
debris disk (ß Pictoris). It was a
spectacular confirmation of the
prediction based on the known
structure of the Solar System:
disks and then planets should
be a natural byproduct of the
stellar birth
Another seriously considered
possibility for planet detection
was a confirmed detection of a
companion to star HD114762 by
Latham et. al in 1989. However,
that object appears to be a
brown dwarf rather than a planet
First extrasolar planets
Planets around a pulsar
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Discovered in1991 by Wolszczan using
the pulse timing method. Initially, it was
two planets with masses of ~4 Earth
masses, orbiting the millisecond pulsar,
PSR B1257+12. Confirmed in 1994 by
detecting gravitational perturbations
between planets B and C caused by a
mean motion resonance
Currently, three terrestrial-mass planets
and a large asteroid (~2 masses of
Ceres) are known in this system
The first planet around a Sun-like star
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Discovered in1995 by Mayor and Queloz
using the Doppler spectroscopy method.
This Jupiter-mass planet circles a solartype star, 51 Pegasi, once every 4.2
days. So far, >150 giant planets have
been detected around other stars,
including “hot Neptune” planets with
masses <20 Earth masses