Transcript Extra-Solar Planets

Searching for Alien
Worlds
Methods of Searching for Alien Planets
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Pulsar Timing
Astrometry
Radial Velocity
Transits
Lensing
Imaging
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.
Pulsar Timing
Some neutron stars beam light along their magnetic poles.
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are needed to see this picture.
Pulsar Timing
If the “searchlight” points towards earth, we see a pulsar. The
timing of a pulsar’s light is normally extremely regular and
precise.
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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
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.
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 star’s wobble via the Doppler effect as it moves towards
us and away from us during the wobbling (radial velocity).
astrometry
radial velocity
Astrometry
If one observed the Sun from a distance of 10 pc, it’s 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.
0.001 arcsec
Astrometry demo
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 star’s velocity over
time with the Doppler effect.
Doppler shift demo
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YUV420 codec decompressor
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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, you’ve found a planet.
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
It’s like a very hot Jupiter!
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.
Planets around Sun-like stars
Large
planets
10% of stars have gas giant planets in
their inner solar system.
Hard to detect planets that are far
from their stars or have small masses
Small
planets
Small
orbits
Large
orbits
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.
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 won’t be available for another 20 years.
Transits
If a planet’s 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.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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.
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.
Gravitational Lensing: 8 planets found
Large
planets
Small
planets
Small
orbits
Large
orbits
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)
“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
Ground-based image of a star
Image with Hubble Space Telescope
Image with Hubble Space Telescope
12 Jupiter masses
200 AU
120 AU
3 Jupiter masses
http://planetquest.jpl.nasa.gov/index.cfm
mass
age