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The Search for
Extra-Solar
Planets
Prof Webster Cash
Astrophysical & Planetary Sciences
With thanks to Dr Martin Hendry
http://www.astro.gla.ac.uk/users/martin/teaching/
THE BIG QUESTIONS
• What is Reality?
• What are we?
• Are we alone?
How do we even get a handle
on these questions?
Extra-Solar Planets
 One of the most active and
exciting areas of astrophysics
 Nearly 4000 exoplanets discovered
since 1995
Some important questions
o How common are planets?
o How did planets form?
o Can we find Earth-like planets?
o Do they harbor life?
1. How can we detect extra-solar planets?
 Planets don’t shine by themselves; they just
reflect light from their parent star

Exoplanets are very faint
Earth is about 10Billion times fainter than
the Sun
The Basic Problem:
Stars are very bright and their glare
makes it difficult to see fainter objects
near them
25 Aug 2008
5
1. How can we detect extra-solar planets?
 They cause their parent star to ‘wobble’, as
they orbit their common centre of gravity
Johannes Kepler
Isaac Newton
Star + planet in circular
orbit about centre of
mass,  to line of sight
Star + planet in circular
orbit about centre of
mass,  to line of sight
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?…
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?…
Centre of mass condition
m1r1  m2r2
 mS 

r  rS  rP  rS 1 
 mP 
e.g. ‘Jupiter’ at 30 l.y.
mS  2.0 1030 kg
mP  1.9 10 kg
27
rS
 S  radians
d
 1.5 107 deg
SIM Planet Quest
Just Cancelled!
The Sun’s “wobble”, mainly due to Jupiter, seen from 30
= width of a tennis ball in London
light years away
Suppose line of sight is in
orbital plane
Direction
to Earth
Suppose line of sight is in
orbital plane
Star has a periodic motion
towards and away from
Earth – radial velocity
varies sinusoidally
Direction
to Earth
Suppose line of sight is in
orbital plane
Detectable via the
Doppler Effect
Star has a periodic motion
towards and away from
Earth – radial velocity
varies sinusoidally
Can detect motion from shifts in spectral lines
Absorption
e-
Electron absorbs
photon of the precise
energy required to
jump to higher level.
Light of this energy
(wavelength) is
missing from the
continuous spectrum
from a cool gas
e-
Star
Laboratory
Stellar spectra are
observed using prisms
or diffraction gratings,
which disperse starlight
into its constituent
colours
Doppler formula
Change in
wavelength
Radial
velocity

v

0 c
Wavelength of light as
measured in the laboratory
Limits of current technology:

0
Speed
of light
 300 millionth

v  1 ms -1
51 Peg – the first new planet
Discovered in 1995
Doppler amplitude
v  55 ms
-1
How do we deduce planet’s
data from this curve?
 2 G 
2 / 3
vS  
m
mP

S
 T 
1/ 3
We can observe
these directly
We can infer this
from spectrum
Complications
 Elliptical orbits
Complicates maths a bit, but
otherwise straightforward
radius
semi-major axis
 Orbital plane inclined to line of sight
We measure only
v S sin i obs
If i is unknown, then we obtain a
lower limit to mP
( v S  v S sin i obs as sin i  1 )
 Multiple planet systems
Again, complicated, but exciting
opportunity (e.g. Upsilon Andromedae)

Stellar pulsations
Can confuse signal from planetary ‘wobble’
Change in brightness from a planetary transit
Brightness
Star
Planet
Time
Another method for finding planets is gravitational lensing
The physics behind this method is based on Einstein’s General Theory of
Relativity, which predicts that gravity bends light, because gravity causes
spacetime to be curved.
This was one of the first
experiments to test GR:
Arthur Eddington’s 1919
observations of a total
solar eclipse.
Another method for finding planets is gravitational lensing
If some massive object passes between us and a background light source, it can
bend and focus the light from the source, producing multiple, distorted images.
Background stars
Lens’ gravity focuses the
light of the background star
on the Earth
Gravitational lens
So the background star
briefly appears brighter
Even if the multiple images are too close together to be resolved separately,
they will still make the background source appear (temporarily) brighter.
We call this case gravitational microlensing. We can plot a light curve showing
how the brightness of the background source changes with time.
The shape of the
curve tells about
the mass and
position of the
object which
does the lensing
Time
Even if the multiple images are too close together to be resolved separately,
they will still make the background source appear (temporarily) brighter.
We call this case gravitational microlensing. We can plot a light curve showing
how the brightness of the background source changes with time.
If the lensing star
has a planet which also
passes exactly between
us and the background
source, then the light
curve will show a second
peak.
Even low mass planets can
produce a high peak (but for
a short time, and we only
observe it once…)
Could in principle detect Earth mass planets!
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
Artist’s impression of ‘Hot
Jupiter’ orbiting HD195019
‘Hot Jupiters’ produce Doppler
wobbles of very large amplitude
e.g. Tau Boo:
v S sin i  474 ms -1
Right Now
1.
The Doppler wobble technique will not be sensitive enough to
detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will
continue to detect more massive planets
2.
The ‘position wobble’ (astrometry) technique will detect
Earth-type planets – Space Interferometry Mission after 2010
(done with HST in Dec 2002 for a 2 x Jupiter-mass planet)
3.
The Kepler mission (launch
2008?) will detect transits
of Earth-type planets, by
observing the brightness
dip of stars
Transit Detection by OGLE III program in 2003
But the Future is in Direct Imaging….
External Occulters
• Let’s Resurrect an Old Idea
–
Spitzer (1962) appears to be the first
• Just Keep the Starlight Out of the Telescope
Occulter Diagram
Planet
Target Star
NWD Starshade
JWST
Telescope big enough to collect enough light from planet
Occulter big enough to block star
– Want low transmission on axis and high transmission off axis
Telescope far enough back to have a properly small IWA
No outer working angle: View entire system at once
Fly the Telescope into the
Shadow
Dropping It In
Note: No Outer Working Angle
New Worlds Observer
Simulated Solar System
The First Image of Solar System
Uranus
Galaxies
Zodiacal Light
Jupiter
Saturn
Neptune
10 arcseconds
Simulated Image of Earth
Planet Finding with Starshades
Five Random Systems from Raymond Database
JWST
The higher resolution of ATLAST brings weak signals out of the noise
ATLAST
Spectroscopy
• R > 100 spectroscopy will distinguish
terrestrial atmospheres from Jovian with
modeling
HO
2
O2
CH4
NH3
S. Seager
TRUE PLANET
IMAGING
3000 km
1000 km
300 km
Earth Viewed at Improving
Resolution
100 km
Conclusion
By 2025
By 2013
H2O
O2
Demonstration Program 2010-2013
Study Planets with Small Starshade 2018
Full Up New Worlds Observer 2027
Planet Imager – 2035?
Lectures Complete
•
•
•
•
•
Final Exam
1:30-4:00pm Wednesday 17th Here.
Just like the mid-terms except twice as long
Covers everything (comprehensive)
A bit extra on last four lectures
• One or two longer essays
• Review Session by Josh Monday 5:30-6:30 here
• I will do office hours 12:00-1:30 Wednesday for last
minute questions (Duane F913)