The Search for ExtraSolar Planets

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The Search for Extra-Solar Planets
Dr Martin Hendry Dept of Physics and Astronomy
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• Extra-Solar Planets
• One of the most active and
• exciting areas of astrophysics
• About 150 exoplanets discovered
• since 1995

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• Extra-Solar Planets
• One of the most active and
• exciting areas of astrophysics
• About 150 exoplanets discovered
• since 1995

• What we are going to cover
• How can we detect extra-solar planets?
• What can we learn about them?

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• 1. How can we detect extra-solar planets?
• Planets dont shine by themselves they just
• reflect light from their parent star
• Exoplanets are very faint

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2nd problem Angular separation of star and
exoplanet is tiny
Distance units Astronomical Unit mean
Earth-Sun distance For interstellar
distances Light year
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Star
e.g. Jupiter at 30 l.y.
Planet
Earth
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e.g. Jupiter at 30 l.y.
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Exoplanets are drowned out by their parent
star. Impossible to image directly with current
telescopes (10m mirrors)
Keck telescopes on Mauna Kea, Hawaii
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• 1. How can we detect extra-solar planets?
• They cause their parent star to wobble, as
• they orbit their common centre of gravity

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• 1. How can we detect extra-solar planets?
• They cause their parent star to wobble, as
• they orbit their common centre of gravity

Isaac Newton
Johannes Kepler
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• 1. How can we detect extra-solar planets?
• They cause their parent star to wobble, as
• they orbit their common centre of gravity

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• 1. How can we detect extra-solar planets?
• They cause their parent star to wobble, as
• they orbit their common centre of gravity

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Star planet in circular orbit about centre of
mass, to line of sight
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Star planet in circular orbit about centre of
mass, to line of sight
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Star planet in circular orbit about centre of
mass, to line of sight Can see star
wobble, even when planet is unseen. But how
large is the wobble?
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Centre of mass condition
Star planet in circular orbit about centre of
mass, to line of sight Can see star
wobble, even when planet is unseen. But how
large is the wobble?
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Centre of mass condition
Star planet in circular orbit about centre of
mass, to line of sight Can see star
wobble, even when planet is unseen. But how
large is the wobble?
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Detectable routinely with SIM (launch date 2009)
but not currently
The Suns wobble, mainly due to Jupiter, seen
from 30 light years away
width of a 5p piece in Baghdad!
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Suppose line of sight is in orbital plane
Direction to Earth
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Suppose line of sight is in orbital plane Star
has a periodic motion towards and away from Earth
radial velocity varies.
Direction to Earth
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Suppose line of sight is in orbital plane Star
has a periodic motion towards and away from Earth
radial velocity varies
Detectable via the Doppler Effect
Can detect motion from shifts in spectral lines
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Star
Laboratory
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Stellar spectra are observed using prisms or
diffraction gratings, which disperse starlight
into its constituent colours
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Stellar spectra are observed using prisms or
diffraction gratings, which disperse starlight
into its constituent colours
Doppler formula
Radial velocity
Change in wavelength
Wavelength of light as measured in the laboratory
Speed of light
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Stellar spectra are observed using prisms or
diffraction gratings, which disperse starlight
into its constituent colours
Doppler formula
Radial velocity
Change in wavelength
Limits of current technology
Wavelength of light as measured in the laboratory
Speed of light
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51 Peg the first new planet
Discovered in 1995 Doppler wobble
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51 Peg the first new planet
Discovered in 1995 Doppler wobble How do
we deduce planets data from this curve?
We can infer this from spectrum
We can observe these directly
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When we plot the temperature and luminosity of
stars on a diagram most are found on the Main
Sequence
Surface temperature (K)
25000 10000 8000 6000 5000 4000
3000
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-10
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Deneb
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Rigel
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Betelgeuse
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Antares
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-5
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Arcturus
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Aldebaran
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Regulus
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102
0
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Vega
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Mira
Sirius A
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Stars on the Main Sequence turn hydrogen into
helium. Stars like the Sun can do this for about
ten billion years
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Pollux
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Procyon A
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Luminosity (Sun1)
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Altair
Sun
Absolute Magnitude
1
5
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10-2
10
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Barnards Star
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Sirius B
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10-4
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Procyon B
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O5 B0 A0 F0 G0 K0
M0 M8
Spectral Type
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Main sequence stars obey an approximate mass
luminosity relation ? We can, in turn,
estimate the mass of a star from our
estimate of its luminosity
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Summary Doppler Wobble method
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In recent years a growing number of exoplanets
have been detected via transits temporary
drop in brightness of parent star as the planet
crosses the stars disk along our line of sight.
Transit of Mercury May 7th 2003
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Change in brightness from a planetary transit
Brightness
Star
Planet
Time
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Ignoring light from planet, and assuming star is
uniformly bright Total brightness during
transit e.g. Sun Jupiter Earth
Total brightness outside transit
? Brightness change of 1
? Brightness change of 0.008
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What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000 29 exoplanets
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What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000 29 exoplanets
Nov 2005 150 exoplanets
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What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000 29 exoplanets
Up-to-date summary at http//www.exoplanets.org
Now finding planets at larger orbital semimajor
axis
Nov 2005 150 exoplanets
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What have we learned about exoplanets?
Discovery of many Hot Jupiters Massive
planets with orbits closer to their star than
Mercury is to the Sun Very likely to be gas
giants, but with surface temperatures of several
thousand degrees.
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What have we learned about exoplanets?
Discovery of many Hot Jupiters Massive
planets with orbits closer to their star than
Mercury is to the Sun Very likely to be gas
giants, but with surface temperatures of several
thousand degrees.
Mercury
Hot Jupiters produce Doppler wobbles of very
large amplitude
Artists impression of Hot Jupiter orbiting
HD195019
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Looking to the Future

• NASA Terrestrial Planet Finder
• ESA Darwin

2015 launch
These missions plan to use interferometry to
blot out the light of the parent star,
revealing Earth-mass planets
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Looking to the Future

• NASA Terrestrial Planet Finder
• ESA Darwin

2015 launch
Spectroscopy will search for signatures of life-
Spectral lines of oxygen, water carbon dioxide in
atmosphere?
Simulated Earth from 30 light years
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The Search for Extra-Solar Planets
What (or who) will we find?