There are an estimated 10 billion planetary systems in our galaxy
alone, yet to-date, only a few hundred have been discovered.
Exoplanets are very difficult to detect because they don't emit any
light of their own and are completely obscured by their extremely
bright parent stars. Normal telescope observation techniques cannot be
used. Instead of trying to image/detect exoplanets directly we look for
the physical effects they have on their parent star such as shifts in
position or changes in brightness.
SuperWASP detects planets by looking for 'transits'. These occur when a planet
passes in front of its parent star, temporarily obscuring some of its
light. This can be detected from the Earth as a slight dimming of the
star's luminosity. The dimming can be as little as 1% so extremely
accurate measurements are needed.
The image to the left is
a transit diagram demonstrating the process. As the planet passes in
front of the star it produces a characteristic 'light-curve' whose
shape is affected by the size and orbital distance (and hence orbital
period) of the planet. SuperWASP constantly monitors the brightness of
the stars in its field of view and alerts us of any variations that may
be due to the presence of a planet.
The easiest planets to detect are close orbiting gas giants known as 'Hot
Jupiters'. These are very large planets (about 10 times the size of the
Earth) which orbit extremely closely to their parent star (far closer
than Mercury orbits our Sun). Their size and proximity creates a large
decrease in brightness which is easy to detect. Their close orbital
proximity produces very short orbital periods, sometimes as short as a
few days, which enables the whole transit to be observed from only a
few observations. The image to the right shows the transit of Venus in
front of our own sun which occurred in June 2004 (image courtesy of
Keele University Observatory).
Venus is such a small planet that its transit hardly blocks any light
and so would be very difficult to detect if it were orbiting around another nearby
The SuperWASP observatories each consist of an array of
8 cameras, backed with a high-quality CCD (See Technical
section for more details). These cameras are extremely wide field - upto 2000 times
greater than a conventional astronomical telescope. The cameras
continuously image the night sky, each camera capturing up to
100,000 stars per image (this many are needed to stand any chance of
detecting transiting planets). This amounts to over 50 gigabytes of
observational data per night, per observatory, which is automatically processed by
our custom built computer 'Pipeline'.
The 'Pipeline' first reduces the images by removing errors such as
variations in pixel sensitivity, dirt/scratches on the lenses, noise
etc. This is done by comparing the images to special calibration
images. The diagram below shows three of these images;
1) 'Flat-field' - an image (or several images) of a uniform area of the
sky taken at twilight, used to remove variations in pixel sensitivity
across the CCD, dust and scratches and variations in illumination
across the lens known as vignetting.
2) 'Bias' - a short exposure image with the lens shutter in place, used to estimate
the effect of a voltage difference applied to the CCD to ensure a linear response.
3) 'Dark-current' - a longer exposure with the lens shutter in place,
used to eliminate thermal noise from the CCD (although the CCDs are
cooled to -50C thermal noise is still present).
The 'pipeline' then examines the images and matches each star with an
astronomical catalogue of stars to identify them. Finally a complex
photometric analysis is performed to measure the brightness of every star,
the results of which are stored in the project database hosted by the University of Warwick.
When sufficient observations have been made (over several months),
we perform searches for dips in brightness that might indicate the
presence of a planet passing in front of a star. Unfortunately there are a large
number of phenomenon, other than planets, that can cause changes in stellar brightness.
Very careful analysis is needed to verify the presence of a planet which is performed
with the help of our collaborators from the Geneva Planet Search group. To confirm the
presence of the planet they use sensitive spectrographs to measure the minute shift in
position of the star as the planet orbits around.
Once confirmed, astronomers can then use large instruments such as the Hubble and Spitzer
space telescopes to study these worlds.