SuperWASP (the Wide Angle Search for Planets) is the world’s leading ground-based exoplanet detection programme, having discovered 75 exoplanets to date. Exoplanets are simply planets that orbit stars other than the Sun, and SuperWASP works by looking for those exoplanets that transit in front of their parent star.
Planets produce virtually no light of their own, so if they happen to pass in front of their star (from our viewpoint), then they will block out a tiny fraction of the star’s light, resulting in a slight dimming of the light that we see. Unfortunately, even planets as big as Jupiter will block out less than 1% of the light from a star like the Sun, and for a planet the size of the Earth, the fraction of starlight blocked out is a hundred times smaller still.
As a planet passes in front of its parent star, as seen from our viewpoint, so the brightness of the star is reduced slightly. (Not to scale)
And if this is not difficult enough, only those planetary orbits that happen to line up exactly with our line of sight will cause a transit in the first place. Orbits can be orientated at any angle, but only those within about 1 degree or so of our line of sight will cause the dip in the starlight that we can observe.
Despite all these difficulties, programmes like SuperWASP have been remarkably successful at finding exoplanets. One of the keys to SuperWASP’s success is that it can image a huge area of the sky in a single snapshot. SuperWASP in fact comprises two installations – one in the northern hemisphere on La Palma in the Canary Islands, and one in the southern hemisphere at the South African Astronomical Observatory.
Each installation consists of eight cameras on a robotic telescope mount, and each camera can take images of the sky covering an area over two hundred times that of the full Moon. This means that SuperWASP can take images of around a million stars in a single exposure.
One of the SuperWASP telescopes showing the 8 cameras on the robotic mount. [Image © copyright SuperWASP]
(For those who like the technical details, SuperWASP uses Canon 200mm focal length, f/1.8 focal ratio ‘papperazzi-style’ lenses with an aperture of 11cm each. They are backed by high quality e2v CCD detectors with 2048x2048 pixels, resulting in an image scale of 13.7 arcseconds per pixel.)
The way to find transiting exoplanets is to take many, many images of the same stars over and over again. Over the course of an observing season lasting around eight months, SuperWASP may take thousands of images of each star field, accumulating several terabytes of data in the form of images of the sky. The brightness of each star on each image is then carefully measured, resulting in a so called lightcurve of each star – its brightness variation with time.
Sophisticated computer programs then examine these millions of lightcurves looking for those that show possible repeating dips that signify the presence of a planet orbiting the star.
Not all the dips found are due to planets though. Some of the dips may just be due to random noise in the detectors or effects of the weather, and some may be due to other astronomical phenomena such as the presence of another nearby star. Therefore there is a process of carefully weeding out these so-called ‘false positives’ and then following up the remaining candidates with other, larger telescopes to verify that they are indeed transiting exoplanets.
At the time of writing (January 2012), the SuperWASP data archive contains 1849 nights of data comprising 10,640,175 individual images. These images include 30,874,261 unique stars and give rise to lightcurves containing 321,144,914,057 separate data points. So far, the SuperWASP project has announced the discovery of 75 transiting exoplanets – over one-third of the total number of transiting exoplanets that are known – but there are many more SuperWASP planets that are at various stages of confirmation and whose discovery will be presented in the coming months.
The first 15 transiting exoplanets discovered by SuperWASP, shown to scale, compared with the Sun and Jupiter (bottom right). Each image illustrates the colour and size of the star and the relative size of the transiting planet in each case.
The WASP Consortium consists of astronomers primarily from the Queen’s University Belfast, Keele University, Leicester University, The Open University, St Andrews University, the Isaac Newton Group (La Palma), the Instituto de Astrofısica de Canarias (Tenerife) and the South African Astronomical Observatory. The SuperWASP-N and WASP-S Cameras were constructed and operated with funds made available from Consortium Universities and the UK’s Science and Technology Facilities Council.
This article was originally published in July 2009; it was revised in January 2012 to reflect the latest figures from the project.