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Extrasolar planets
Detection methods
1. Pulsar Timing
•
•
Pulsars are rapidly rotating neutron stars, with extremely
regular periods
Anomalies in these periods indicate the gravitational influence of
a companion.
Detection methods
2. Astrometry
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•
•
•
the oldest method used in the search for extrasolar planets, used
as early as 1943.
Involves measuring the proper motion of a star in the search for an
influence caused by its planets
changes in proper motion are so small that the best current
equipment cannot produce reliable enough measurements.
This method requires that the planets' orbits be nearly
perpendicular to our line of sight, and so planets detected by it
could not be confirmed by other methods.
3. Radial motions: the Doppler shift
Recall the Doppler shift of the wavelength of light due to the
velocity of the source:
obs  rest
1  vr / c
1  vr / c
obs  rest 

z
rest
rest
vr ( z  1) 2  1

c ( z  1) 2  1
 z if z  1
Spectroscopic binaries
Single-line spectroscopic binary: the
absorption lines are redshifted or
blueshifted as the star moves in its orbit
Double-line spectroscopic binary: two
sets of lines are visible
Java applet: http://instruct1.cit.cornell.edu/courses/astro101/java/binary/binary.htm
Spectroscopic binaries: circular orbits
• If the orbit is in the plane of the sky (i=0) we observe no
radial velocity.
• Otherwise (if the orbit is inclined at an angle i relative to
the plane of the sky) the radial velocities are a sinusoidal
function of time. The minimum and maximum velocities
(about the centre of mass velocity) are given by
v1max
 v1 sin i
r
v2max
r  v2 sin i
Extrasolar Planet searches
• A planet orbiting a distant star will
behave like a single-lined binary
system. In principle we can determine
the mass from the Doppler shift of
the star.
•
Assuming circular orbits,
3
Planet
If the star can be accurately
classified (i.e. with a good
spectral classification and a
parallax distance) we can
determine its mass
independently of the orbit.
 
max 3
Star
m
Pv
3
sin
i

2G
mStar  mPlanet 2
1/ 3
max
2/3  P 
mPlanet sin i  vStar
mStar


2

G


Since the mass of the
planet is generally much
less than that of the star
Extrasolar Planet searches
1/ 3
max
2/3  P 
mPlanet sin i  vStar mStar 

 2G 
E.g. the star HD73256:
From Hipparcos data (and detailed
stellar modelling) we know
Mstar~1.05 Msun
• From the light curve we measure
P=2.54858 days and vmax=269.8 m/s.
• Sinusoidal shape means e~0
So the planet is very massive (1.86 MJ) and very close to the star (0.037 AU: a
tenth as big as Mercury’s semimajor axis of 0.3871 A.U.)
Why?
Extrasolar Planet searches
mPlanet sin i  1.86M Jupiter
a  0.037 A.U .
The maximum velocity shift is only ~270 m/s. The Doppler shift is therefore:



v
 9 10 7
c
which is very small. For example the Ha line is redshifted by only 0.00059 nm!
The spectral resolution must therefore be very high. Detecting smaller planets,
farther away from the star, is an even more difficult task.
Break
Detection methods
4. Gravitational microlensing
 This effect occurs when the gravitational field of a planet and its
parent star act to magnify the light of a distant background star.
 The key advantage of gravitational microlensing is that it allows low
mass (i.e. Earth-mass) planets to be detected using available
technology.
 A notable disadvantage is that the lensing cannot be repeated
because the chance alignment never occurs again.
5. Transit methods
• Detects a planet's
shadow when it
transits in front of its
host star.
• Can be used to
measure the radius of
a planet.
Transits
• Imagine viewing the Earth-Sun system from a distant star. By
how much will the Sun fade during a transit of the Earth? How
about during a transit of Jupiter?
6. Circumstellar disks
Young main sequence stars often still have disks, even after the
molecular cloud has been dispersed.
Infrared-emitting dust disk around
b-Pic. The central star has been
subtracted.
The dust disk around Vega. At least
one large planet is known to exist
within this disk.
Circumstellar Disks
7. Direct detection
Infrared image of the star GQ Lupi
orbited by a massive, young
(therefore warm) planet at a distance
of approximately 20 times the
distance between Jupiter and our
Sun.
2005 image of 2M1207 (blue) and its
planetary companion, one of the
first exoplanets to be directly
imaged
7. Direct Detection
The albedo of the Earth is about AV=0.4. How bright is it in visible
(reflected) light, relative to the Sun? How do they compare at
infrared wavelengths, where Earth emits thermal radiation?
A picture of Earth,
from the surface of
Mars, just before
sunrise.
HD 209458b was the first
transiting planet discovered,
the first extrasolar planet
known to have an atmosphere,
the first extrasolar planet
observed to have an
evaporating hydrogen
atmosphere, and the first
extrasolar planet found to
have an atmosphere containing
oxygen and carbon.
Extrasolar planet searches
As of December 2005, 170 planets have been detected outside
our solar system (in 146 systems).
See http://exoplanets.org/
Most of these have a<1 AU
and masses >MJupiter
Future missions
Keck Interferometer
Large Binocular Telescope Interferometer
SIM PlanetQuest
Kepler
Terrestrial Planet Finders
Spitzer Space Telescope
19
Space Interferometry mission
http://planetquest.jpl.nasa.gov/SIM/sim_index.cfm
• Interferometer with 9m baseline
• 5 year mission; estimated launch in 2010
• will determine the positions of stars several hundred times
more accurately than anything previously possible
• Will search for terrestrial planets around the nearest ~250
stars, with astrometry accurate to 1 mas.
Kepler
• http://www.kepler.arc.nasa.gov/
• NASA mission to hunt for planets using a one-meter diameter
telescope photometer to measure the small changes in brightness
caused by transiting planets.
• Transits by terrestrial planets produce a periodic change in a
star's brightness of about 1/10,000, lasting for 2 to 16 hours.
• Scheduled to launch 2008, 4 year mission
• sensitivity limits of radial velocity
surveys, astrometric surveys,
microlensing surveys, and spacebased transit techniques.
• The shaded areas show the
expected progress towards the
detection of Earth-like planets by
2006 and 2010.
• The filled circles indicate the
planets found by radial velocity
surveys (blue), transit surveys
(red), and microlensing surveys
(yellow).
• The discovered extrasolar
planets shown in this plot
represent the reported findings
up until 31 August 2004.
Next Lecture: Extraterrestrial life