Today, exoplanetary transits are coveted by numerous projects. “For instance, there’s the English consortium WASP (Wide Angle Search for Planets) which has a telescope in every hemisphere, one in La Palma (WASP-North) and one in South Africa (WASP-South)”, explains Michaël Gillon. “Every instrument, equipped with eight mirrors, has a total field of 482 degrees squared, which is equivalent to sixty times that of the full moon! Each field continuously measures the fluxes from several hundreds of thousands of stars at the same time, in the hope of detecting a fall in flux in percent for some of them, which would possibly indicated the presence of a planet. By comparison, a binary system induces reductions in flux of up to 40%. Complete instrumental know-how had to be developed and is constantly being improved to track planetary transits.”

In two years, among the hundreds of thousands of stars placed under high surveillance, WASP-South spotted 300 possible exoplanets using photometry. They then had to be confirmed (or not) with spectroscopy, mainly using Coralie, an instrument installed in La Silla on the Swiss Euler telescope. “Out of the 300 WASP-South possibilities, we’ve confirmed about 20, including WASP-18, which is the subject of our publication in Nature”, continues Michaël Gillon. “The others turned out to be star binary systems, false-positives, atmospheric or systematic disturbances.”

In the WASP harvest, number 18 is not the only one to stand out. There is WASP-17b, which is also at the centre of a recent publication (2) though for quite another reason. It is the least-dense planet known, but that is not all. Since it is the same cloud of gas and dust that gives rise to a star and its trail of planets, we expect a planet to orbit its star in the same direction as its own rotation. This is the case for all the planets in the solar system. This was also the case for all the exoplanets discovered… before WASP-17b, which turns in the opposite direction… “We believe that a “planetary pinball machine” violently modified WASP-17b’s orbit in the past. This implies the presence of either a stellar companion, or another planet in the system. We shall have to test these hypotheses in the future”, explains Michaël Gillon.

(2) Anderson D. R., Hellier C., Gillon M., Triaud A. H. M. J., Smalley B., Hebb L., Collier Cameron A., Maxted P. F. L., Queloz D., West R. G., Bentley S. J., Enoch B., Horne K., Lister T. A., Mayor M., Parley N. R., Pepe F., Pollaco D., Ségransan D., Udry S., Wilson D. M., 2009, “WASP-17b: an ultra-low density planet in a probable retrograde orbit”, A&A (accepted) – arXiv:0908.1553