the search for extra-solar planets with thanks to dr martin hendry / prof webster cash...
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The Search The Search for Extra-for Extra-Solar PlanetsSolar Planets
With thanks to Dr Martin Hendry
http://www.astro.gla.ac.uk/users/martin/teaching/
Prof Webster CashAstrophysical & Planetary Sciences
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?How common are planets?
o How did planets form?How did planets form?
o Can we find Earth-like planets?Can we find Earth-like planets?
o Do they harbor life?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
25 Aug 2008 5
The Basic Problem:
Stars are very bright and their glare makes it difficult to see fainter objects
near them
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
2211 rmrm
P
SSPS m
mrrrr 1
e.g. ‘Jupiter’ at 30 l.y.
radiansd
rSS
kg109.1
kg100.227
30
P
S
m
m
deg105.1 7
The Sun’s “wobble”, mainly due to Jupiter, seen from 30 light years away = width of a tennis ball in London
SIM Planet Quest Just Cancelled!
Suppose line of sight is in orbital plane
Direction to Earth
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
Suppose line of sight is in orbital plane
Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally
Detectable via the Doppler Effect
Can detect motion from shifts in spectral lines
Absorption
e -
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
Star
Laboratory
Limits of current technology:
-1sm1v
c
v
0
Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours
Doppler formula
Wavelength of light as measured in the laboratory
Change in wavelength
Radial velocity
Speed of light
millionth3000
51 Peg – the first new planetDiscovered in 1995
Doppler amplitude
How do we deduce planet’s data from this curve?
-1sm55v
PSS mmT
G 3/23/1
2v
We can observethese directly
We can infer this from spectrum
Complications
Elliptical orbits
Complicates maths a bit, butotherwise straightforward
radius semi-major axis
Orbital plane inclined to line of sight
We measure only
If is unknown, then we obtain a lower limit to
( as )
Multiple planet systems
Again, complicated, but exciting opportunity (e.g. Upsilon Andromedae)
Stellar pulsations
Can confuse signal from planetary ‘wobble’
obssinv iS
i
obssinvv iSS Pm
1sin i
Change in brightness from a planetary transit
Brightness
Time
Star
Planet
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
Gravitational lens
Lens’ gravity focuses the light of the background star on the Earth
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.
Time
The shape of the curve tells about the mass and position of the object which does the lensing
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:
-1ms474sinv iS
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
Right Now
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
Telescope big enough to collect enough light from planetOcculter big enough to block star
– Want low transmission on axis and high transmission off axisTelescope far enough back to have a properly small IWANo outer working angle: View entire system at once
NWD Starshade JWSTTarget Star
Planet
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
JupiterSaturn
Uranus
Neptune
Zodiacal Light
Galaxies
10 arcseconds
Simulated Image of Earth
Planet Finding with Starshades
Five Random Systems from Raymond Database
The higher resolution of ATLAST brings weak signals out of the noise
ATLAST
JWST
Spectroscopy
• R > 100 spectroscopy will distinguish terrestrial atmospheres from Jovian with modeling
O2
H2O
CH4
NH3
S. Seager
Earth Viewed at Improving Resolution
100 km300 km3000 km 1000 km
TRUE PLANET IMAGING
Conclusion By 2025
O2
H2O
By 2013
Demonstration Program 2010-2013Study Planets with Small Starshade 2018Full Up New Worlds Observer 2027Planet 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)