Extra-Solar Planets

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Searching for Alien Worlds
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Methods of Searching for Alien Planets

• Pulsar Timing
• Astrometry
• Radial Velocity
• Transits
• Lensing
• Imaging

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Wobbling Stars
Planets cause stars to wobble. There are 3 ways
to detect the wobbling of stars caused by
planets pulsar timing, astrometry, and radial
velocity.
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Pulsar Timing
Some neutron stars beam light along their
magnetic poles.
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Pulsar Timing
If the searchlight points towards earth, we see
a pulsar. The timing of a pulsars light is
normally extremely regular and precise.
But if a planet orbits the pulsar, the pulsar
wobbles back and forth, which causes the time
between pulses to change. So we can search for
planets around pulsars by checking whether their
pulses occur at irregular intervals.
Pulsar timing demo
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Pulsar Timing

• In 1994, Penn State Prof. Alex Wolzczan
  discovered the first planet outside the solar
  system through timing of a pulsar. This pulsar
  has 3 small planets with masses of 0.02, 4.3, and
  3.9 M? (M?mass of Earth). These planets probably
  formed after the supernova explosion from the
  debris that was left behind.

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Astrometry Radial Velocity
In addition to pulsar timing, there are 2 other
methods of detecting planets through the wobbles
they induce in their stars. We can see the star
physically wobbling on the sky (astrometry), or
we can detect the stars wobble via the Doppler
effect as it moves towards us and away from us
during the wobbling (radial velocity).
astrometry
radial velocity
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Astrometry
If one observed the Sun from a distance of 10 pc,
its the size of its wobble on the sky (due
mostly to Jupiter and Saturn) would be less than
0.002?. In comparison, because of blurring by
the atmosphere, stars have sizes of about 1? on
the sky.
It is very difficult to measure such small
wobbles from the ground, so no planets have been
discovered with astrometry, but future space
telescopes should be more successful.
Astrometry demo
0.001 arcsec
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Radial Velocity
As the star wobbles back and forth due to the
orbit of an unseen planet, its velocity relative
to us changes slightly. So planets can be
detected by measuring the change in a stars
velocity over time with the Doppler effect.
Doppler shift demo
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Radial Velocity
If you plot the velocity of a star over a long
period of time, and find that the velocity
changes regularly, youve found a planet.
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Radial Velocity

• The first planet around a Sun-like star (51 Peg)
  was discovered via radial velocity monitoring in
  1995.

• The data imply the presence of a planet with
• - a roughly circular orbit
• - a distance of 0.052 A.U.
• - a mass of 0.46 MJup Its like a very hot
  Jupiter!

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Radial velocity monitoring has been the most
successful method of finding planets, discovering
more than 400 planets so far. For more than 40
stars, multiple planets have been discovered.
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Planets around Sun-like stars
Large planets
10 of stars have gas giant planets in their
inner solar system.
Current detection limit for radial velocity
Hard to detect planets that are far from their
stars or have small masses
Small planets
Large orbits
Small orbits
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Searching for Earth-like planets

• Larger planets produce faster wobbles in the
  stars that they orbit, which are easier to
  detect. Most of the planets discovered outside
  the solar system are gas giants that are unlikely
  to harbor life. Smaller, rocky planets like Earth
  induce only very slow wobbles in their stars,
  which are very hard to detect.

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Searching for Earth-like planets

• The smallest planet found so far has a mass of
  about 2 Earth masses. A 5 Earth mass planet
  appears to be near the edge-on of the habitable
  zone, so it could have liquid water on its
  surface. The next step is to search for evidence
  of life on this planet (e.g., oxygen in its
  atmosphere), but the necessary technology
  probably wont be available for another 20 years.

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Transits
If a planets orbit happens to be perfectly
edge-on from our point of view, it will pass
directly between us and its star. When this
happens, the light from the star will decrease
very slightly (less than 1). About 60 planets
have been found by watching stars to see if they
dim periodically in this way.
Since stars are so far away, they appear as only
points of light. As a result, we cannot see the
planet moving across the face of the star as
shown in this movie. We only detect the dimming
of the total light from the star.
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Gravitational Lensing
If a star/planet moves exactly in front of a
background star, the brightness of the background
star can be greatly magnified by the
gravitational lens effect.
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Gravitational Lensing
In principle, the gravitational lens technique
can detect planets of any mass. However, once
the event is over, the planet is lost (since we
are only seeing the background source). It is
impossible to learn anything more about the
planet.
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Gravitational Lensing 8 planets found
Large planets
Small planets
Large orbits
Small orbits
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Amazing Amateurs
Auckland Observatory (0.4-meter) Bronberg
Observatory (0.4-meter ) Catino Austral
Observatory (0.4-meter) CTIO (1.3-meter) Farm
Cove Observatory (0.4-meter) Hunters Hill
Observatory (0.4-meter) MDM Observatory,
(2.4-meter) Mt Lemmon Observatory
(1.0-meter) Palomar Observatory (60-inch) Perth
(0.3-meter) Southern Stars Observatory
(0.3-meter) Vintage Lane Observatory
(0.4-meter) Wise Observatory (1.0-meter)
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It just shows that you can be a mother, you can
work full-time, and you can still go out there
and find planets. -Jenny
McCormick Farm Cove Observatory
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Ground-based image of a star
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Image with Hubble Space Telescope
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Image with Hubble Space Telescope
12 Jupiter masses
200 AU
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120 AU
3 Jupiter masses
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http//planetquest.jpl.nasa.gov/index.cfm
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mass
age