SuperWASP (the Wide Angle Search for Planets) is one of the world’s leading exoplanet detection programmes. 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)
[image © copyright SuperWASP]

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 (Summer 2009), the SuperWASP data archive contains 994 nights of data comprising 4,935,899 individual images. These images include 27,683,288 unique stars and give rise to lightcurves containing 165,636,715,663 separate data points. So far, the SuperWASP project has announced the discovery of 19 transiting exoplanets – about 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.
[image © copyright SuperWASP]

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.

Find out more

More on the background to the SuperWASP from a 2004 Open2 article

Study with The Open University: Planetary science and the search for life

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The Cosmos: A Beginner's Guide

It's a huge universe - get a primer with The Cosmos: A Beginner's Guide.

On my first morning back from teaching at geology residential school, I find an urgent email requesting material for the BBC News website. It seems they are (quite rightly) excited by the new Open University TV series The Cosmos: a beginner’s guide (7.30 pm, every Tuesday for six weeks beginning 7 August) and would like a news feature about the ‘latest developments in planet hunting’. This means finding planets round other stars, by the way (sometimes called ‘exoplanets’), rather than new planets in our own Solar System.

I worked on this series as an academic consultant, which means suggesting ideas, and ensuring that the science stays essentially correct even if it has to be simplified. It was made easier because the lead presenter, Adam Hart-Davis, has a good knowledge of science, and the co-presenters, my Open University colleague Janet Sumner and Astrium’s Maggie Aderin, have current research reputations in the fields of planetary science and telescope engineering respectively.

It took about six months to make the series, during which time the presenters were sent to various exotic locations, and I got to go as far afield as Leeds (to view the rough cuts of the first two programmes) and Leicester (to be filmed as a contributor, talking about unmanned space exploration and the BepiColombo mission to Mercury).

My personal goal during the planning of the series (which did not acquire the name ‘Cosmos’ until quite late) was to ensure that planetary matters got a fair share of the treatment, rather than being overshadowed by things cosmological like the Big Bang and black holes. I mean, I chair the Open University's planetary science courses, so that's my job, right? Whether or not I succeeded, you’ll have to judge for yourself.

Anyway, to get back to the BBC News website request, it turns out that I’m the only member of the team free to today, so it falls to me to write the ‘news’ story, Hunting for another Earth-like planet, which has by now appeared online.

The trouble is that, for reasons unnecessary to dwell on, the BBC News website was unable to include the ‘planet hunting’ clips from the series that I had persuaded people to sort out for them. "Waste not, want not,” says I, so here you are:

The first clip shows Adam visiting the ‘Super Wide Angle Search for Planets’ (SuperWASP) telescopes on La Palma. This is particularly interesting for me, because colleagues from the Open University Department of Physics and Astronomy are heavily involved in the project.

The SuperWASP array is great for scanning the sky and keeping track of thousands of stars on the off-chance of seeing a dip in the light from one of them when a planet gets in the way. The second clip shows Adam being shown the much bigger William Herschel Telescope that can make more detailed measurements after SuperWASP has done the vital job of initial discovery.

In the third clip, we see Janet visiting the ‘Virtual Planetary Laboratory’ in Los Angeles, where the conditions on planets around other stars are investigated, and demonstrating ‘just right’ conditions in the so-called ‘habitable zone’ around a star.

There is a further clip on YouTube, in which Janet struts her stuff at the very same observatory once used by Edwin Hubble.

About the author

Dave Rothery is a volcanologist and planetary scientist at the Open University. His current research includes studying volcanic eruptions on the Earth and characterising planetary surfaces, especially Mercury.

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