A posteriori detection of the planetary transit of HD189733 b in the Hipparcos photometry

Aims. Using observations performed at the Haute-Provence Observatory, the detection of a 2.2-day orbital period extra-solar planet that transits the disk of its parent star, HD 189733, has been recently reported. We searched in the Hipparcos photometry Catalogue for possible detections of those transits. Methods. Statistic studies were performed on the Hipparcos data in order to detect transits of HD 189733 b and to quantify the significance of their detection. Results. With a high level of confidence, we find that Hipparcos likely observed one transit of HD 189733 b in October 1991, and possibly two others in February 1991 and February 1993. Using the range of possible periods for HD 189733 b, we find that the probability that none of those events are due to planetary transits but are instead all due to artifacts is lower than 0.15%. Using the 15-year temporal baseline available, we can measure the orbital period of the planet HD 189733 b with particularly high accuracy. We obtain a period of 2.218574(-0.000010)(+ 0.000006) days, corresponding to an accuracy of similar to 1 s. Such accurate measurements might provide clues for the presence of companions.