Astronomy190 - Topics in Astronomy

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Astronomy190 - Topics in Astronomy

• Astronomy and Astrobiology
• Lecture 19 Extrasolar Planets
• Ty Robinson

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Questions of the Day

• What techniques are used to study extrasolar
  planets?
• What is the difference between direct and
  indirect detection of a planet?
• What are the known extrasolar planets like?

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A Multitude of Worlds

• 365 Planets
• 308 Planetary Systems
• 39 Multiple Planet Systems
• Most are Jovians
• 10 SuperEarths (2-10 Earth masses)
• Not bad for not being able to see anything!

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Indirect Detection of Extrasolar Planets
These techniques use changes in the position or
brightness of a star to infer the existence of a
planet
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• Planet and Star orbit around a common center of
  mass. This can make the star appear to dance,
  even when the planet cant be seen.

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The Doppler Technique
http//planetquest.jpl.nasa.gov
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Detection of Radial Velocity via Spectral Shift
Na _at_ 589.3nm
Source not moving
Observer sees Blueshift
Observer sees Redshift
Source redshifted
Na _at_ 591.5nm

• Radial velocity changes in the star being
  observed are detected via the shift in the
  wavelengths of known absorption features in the
  spectrum of the star. Redshifted if the source
  is moving away from the observer, blueshifted if
  the source is moving towards the observer.

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What We Can Observe
No Planet RV changes small And random
Planet Periodic RV changes

• If there is no planet, then there is no strong,
  periodic change in a stars apparent radial
  velocity.
• Stars with planets show periodic changes in RV

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Period and Orbital Eccentricity
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The Radial Velocity (Doppler) Technique

• uses spectroscopic measurements of starlight to
  detect the redshift or blueshift from the radial
  velocity component of a stars motion
• most successful technique to date
• most sensitive to big planets close to their star
• can be used to infer planetary mass, albeit with
  a sine i ambiguity due to the (usually) unknown
  inclination of the planetary system to the
  observers line of sight
• can determine planetary period and eccentricity

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Transit
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Change in the Stars Brightness

• From http//astro.u-strasbg.fr/goutelas/g2005/chap
  03-moutou.pdf

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The Kepler Mission
Measures stellar brightness changes caused by
transiting terrestrial planets. Monitoring
100,000 stars for 4 years!
Transit Telescope
Launched 2009
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Transit Technique

• detects the faint drop in a stars brightness due
  to a planet passing across the face of the star
• sensitive to a large planet/star area ratio (so
  more sensitive to large planets overall, but has
  improved sensitivity to smaller planets around
  smaller stars)
• more sensitive to planets close to their star
  because these are more likely to be seen in
  transit
• can be used to determine planetary size, if the
  stars size can be estimated (transit does not
  give mass!)
• provides planetary orbital period and distance
• limitation the probability of a transit is low
  for any given planetary system, so transits are
  rare

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Direct Detection of Extrasolar Planets

• Directly detects photons from the planet itself.
• We can either suppress the light from the parent
  star
• Or cleverly subtract the stars light

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Direct Detection

• allows us to directly study a planet
• Extremely difficult!
• star-planet contrast is large
• star and planet often cant be separated on the
  sky (angular resolution)
• star-planet contrast is typically less in the
  infrared

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Removing the Light from the Parent Star

• direct detection techniques also require some
  means to separate or remove the light from the
  parent star so that the light from the smaller,
  fainter planet can be seen
• subtraction techniques light from both planet
  and star, and star alone, are measured and
  subtracted to reveal the planets radiation
• suppression or cancellation techniques light
  from the parent star is cancelled (nulled) or
  blocked out (suppressed) using a special type of
  instrument

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Extrasolar Planet Properties Mass

• Rise towards lower masses (smallest are
  3.5Mearth)
• In this sample, only 13/138 (10) have Mp gt 5Mj

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Hot Jupiters
very easy to detect using transit, radial
velocity techniques form beyond frost line,
migrate inwards habitable moons?
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Questions of the Day

• What techniques are used to study extrasolar
  planets?
• What is the difference between direct and
  indirect detection of a planet?
• What are the known extrasolar planets like?

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Quiz
1 - Describe one way that we constrain the inner
edge of the HZ around the Sun and one way that we
constrain the outer edge.
2 - Describe how Hot Jupiters form.
3 - What is one thing you did not understand from
todays lecture?
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Challenges Angular Separation from Parent Star

• angular resolution improves with shorter
  wavelength and/or bigger optics (lens or mirror)