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Search for Extra-Solar
Planets
Background
1995 first discovered evidence that other
stars have planets.
 As of April 2014, 1780 planets, including
460 multiple planetary systems
 Evidence suggests that a majority of sunlike stars possess them.
 Most of these stellar systems bear little
resemblance to ours.
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Difficulties with the Search
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Planets are very small and very dark
compared to stars…
Even stars appear as nothing more than
pinpoints of light when viewed with even the
largest telescopes
 Planets have only a fraction of the mass of a
star, nuclear fusion reaction that makes stars
“burn” does not take place.
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Planets are found right next to the stars
they orbit
Methods
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Most planets cannot be observed directly,
instead astronomers must observe stars
and look for the minute effects that orbiting
planets have upon them.
Radial Velocity
 Transit Photometry
 Microlensing
 Astrometry
 Direct Imaging
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Radial Velocity
Until the launch of the planet hunting
spacecraft Kepler in 2009, radial velocity
was the most effective method for locating
extrasolar planets.
 The vast majority of Exoplanets detected
from Earth were discovered by this
method.
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Radial Velocity
also known as Doppler spectroscopy
 when a star is orbited by a planet it
responds to the gravitational tug of its
smaller companion.
 these slight movements affect the star's
normal light spectrum.
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If the star is moving towards the observer, its
spectrum would appear slightly shifted
towards the blue; if it is moving away, it will be
shifted towards the red.
Radial Velocity
Drawbacks:
 Can only detect planets accurately that are
“edge-on”
 Cannot accurately determine the mass of
a distant planet.
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This is a serious problem because mass is
used for distinguishing between planets and
small stars.
The sinusoid is the characteristic shape of the radial velocity graph of a star
rocking to the tug of an orbiting planet. exoplanets.org
Transit Photometry
Measures the minute dimming of a star as
an orbiting planet passes between it and
the Earth.
 The passage of a planet between a star
and the Earth is called a "transit."
 Dimming detected at regular intervals and
lasting a fixed length of time, indicates that
a planet is orbiting the star.
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Transit Photometry
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The dimming directly reflects the size ratio
between the star and the planet
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A large planet transiting a small star will have
a more noticeable effect.
The size of the host star can be known
with from its spectrum, photometry
therefore gives astronomers a good
estimate of the orbiting planet's size.
Transit Photometry
Using both methods, scientists can
calculate the planet's density, an important
step towards assessing its composition.
 Additionally, the light from the star passing
through the planet's atmosphere is
absorbed to different degrees at different
wavelengths. Scientists can recreate the
absorption spectrum and deduce the
atmosphere's composition.
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Transit Photometry
The Kepler mission, launched in March of
2009, uses photometry to search for
extrasolar planets from space.
 The spacecraft's sensitivity is such that it
has already detected thousands of
planetary candidates, including several
that are Earth-sized and orbiting in their
star's habitable zone.
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Transit Photometry
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Drawbacks:
The distant planet must pass directly between
it's star and the Earth; the orbital plane must be
almost exactly "edge-on" to the observer.
Transits last only a tiny fraction of its total orbital
period.
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A planet might take months or years to complete its
orbit, but the transit would probably last only hours or
days.
Astronomers need to observe repeated transits
occurring at regular intervals.
An artist's impression of a Jupiter size extrasolar
planet passing in front of its parent star
Microlensing
Microlensing is the only method capable of
discovering planets at great distances
from the Earth.
 Microlensing can find planets orbiting stars
near the center of the galaxy, thousands of
light-years away.
 Microlensing, is most sensitive to planets
that orbit in moderate to large distances
from their star.
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Microlensing
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When the light emanating from a star passes
very close to another star (“the lensing star”), the
gravity of the lensing star will slightly bend the
light rays from the source star.
If the source star is positioned precisely behind
the lensing star, this effect is multiplied.
If a planet is positioned close enough to the
lensing star, the planet's own gravity bends the
light stream and temporarily produces a third
image of the source star.
Microlensing
This effect appears as a temporary spike
of brightness, lasting several hours to
several days
 Such spikes indicate the presence of a
planet.
 The precise characteristics of the
microlensing light-curve, its intensity and
length, allow scientists to deduce the
planet’s total mass, orbit, and period.
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Microlensing
Drawbacks:
 microlensing is dependent on rare and
random events
 microlensing events do not repeat
themselves
 the distance of the detected planet and its
star from the Earth is known only by rough
approximation
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The microlensing process. In the fourth image from the right the planet
adds its own microlensing effect, creating the two characteristic spikes
in the light curve.
Astrometry
Astrometry is the science of precision
measurement of stars' locations in the sky.
 Planet hunters look for a minute but
regular wobble in a star's position. If such
a periodic shift is detected, it is almost
certain that the star is being orbited by a
companion planet.
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Astrometry
Until recently, the level of precision
required to detect the slight shifts in a
star's position was at the outer edge of
technological feasibility
 The Keck telescopes in Hawaii, the largest
in the world, are being fitted for
astrometrical measurements
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Astrometry
Astrometry is most effective when the
orbital plane is "face on" (perpendicular) to
an observer's line of sight
 Excels in detecting planets of long periods,
orbiting further away from their star.
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Astrometry
Drawbacks:
 Atmospheric interference limits the
accuracy of ground-based measurements
 Can only detect the component of a star's
wobble that moves it side to side
 Can only be used for relatively close stars
 A star must be observed continuously for
years or even decades
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No confirmed planets discovered by this
method, due to the precision required
An artist's conception of Gaia spacecraft. Launched Dec. 19, 2013.
Direct Imagining
Direct imaging of exoplanets is extremely
difficult, and in most cases impossible.
 Being small and dim, planets are easily
lost in the brilliant glare of the giant stars
they orbit.
 There are special circumstances in which
a planet can be directly observed.
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Direct Imaging
For humans “seeing is believing”, this is
the only method that allows us to “see”
 Works best for big, bright planets that orbit
at a great distance from their stars.
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Direct Imaging
Drawbacks:
 Only possible on very special
circumstances.
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Hubble Space Telescope image of planet Fomalhaut b orbiting the star
Fomalhaut. (A coronagraph blocks out the star and accounts for the
dark region at the center of the image).
Technology
Radial Velocity = super sensitive
spectrographs
 Transit Photometry = ground based
photometers and Kepler space observatory
 Microlensing = ground based observatories
 Astrometry = Keck Telescope, Gaia space
observatory
 Direct Imaging = visible and infrared. ground
based and space telescopes (Hubble, VLT,
etc)
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Exoplanet Examples
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51 Pegasi b:
First exoplanet discovered around a “Sunlike” star
 Announced: Oct 6, 1995
 Method: Radial Velocity
 Has a mass about half of Jupiter and orbits
much closer than Mercury.
 The discovery of other similar exoplanets
forces scientists to re-examine theories of
solar system formation.
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Exoplanet Examples
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Fomalhaut b:
Orbits star Fomalhaut
 Announced: 2008. Confirmed: 2012
 Method: direct imagining using Hubble
telescope.
 Star is surrounded by a thick disk of gas
and dust. Located the planet in images of
the disk.
 Very luminous; believe that it is
surrounded by a ring system thicker than
that of Saturn.
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Exoplanet Examples
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Alpha Centauri Bb:
Closest exoplanet
 Orbits star Alpha Centauri B
 Announced: October 2012
 Method: radial velocity
 Great debate. Remains unconfirmed as a
planet.
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News update
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http://www.nasa.gov/ames/kepler/nasas-keplermission-announces-a-planet-bonanza/
http://www.nasa.gov/ames/kepler/kepler-marksfive-years-in-space/
http://www.nasa.gov/ames/kepler/keplermission-manager-update-loss-of-a-sciencemodule/
http://www.nasa.gov/ames/kepler/keplermission-manager-update-k2-spacecraftoperation-tests-continue/
references
http://www.planetary.org/explore/spacetopics/exoplanets/
 http://exoplanetarchive.ipac.caltech.edu/
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