Search for Extra-Solar Planets

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Search for Extra-Solar Planets
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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 sun-like
  stars possess them. 
• Most of these stellar systems bear little
  resemblance to ours.

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Difficulties with the Search

• 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.
• Planets are found right next to the stars they
  orbit

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Methods

• 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.
• 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.

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Radial Velocity

• Drawbacks
• Can only detect planets accurately that are
  edge-on
• Cannot accurately determine the mass of a distant
  planet.
• This is a serious problem because mass is used
  for distinguishing between planets and small
  stars.

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The sinusoid is the characteristic shape of the
radial velocity graph of a star rocking to the
tug of an orbiting planet. exoplanets.org
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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

• The dimming directly reflects the size ratio
  between the star and the planet
• 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.

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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

• 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.
• 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.

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An artist's impression of a Jupiter size
extrasolar planet passing in front of its parent
star
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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

• 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.

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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 planets 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.
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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
• No confirmed planets discovered by this method,
  due to the precision required

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An artist's conception of Gaia spacecraft.
Launched Dec. 19, 2013.
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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).
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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

• 51 Pegasi b
• First exoplanet discovered around a Sun-like
  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

• 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

• 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

• http//www.nasa.gov/ames/kepler/nasas-kepler-missi
  on-announces-a-planet-bonanza/
• http//www.nasa.gov/ames/kepler/kepler-marks-five-
  years-in-space/
• http//www.nasa.gov/ames/kepler/kepler-mission-man
  ager-update-loss-of-a-science-module/
• http//www.nasa.gov/ames/kepler/kepler-mission-man
  ager-update-k2-spacecraft-operation-tests-continue
  /

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references

• http//www.planetary.org/explore/space-topics/exop
  lanets/
• http//exoplanetarchive.ipac.caltech.edu/