Thanks to observations performed at the Haute-Provence Observatory, Bouchy et al. recently announced the detection of a 2.2-day orbital period extra-solar planet that transits the disk of its parent star, HD189733. With high level of confidence, we find that Hipparcos likely observed one transit of HD189733b in October 1991, and possibly two others in February 1991 and February 1993. Using the range of possible periods for HD189733b, 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%. Thanks to the 15-year temporal baseline available, we can measure the orbital period of the planet HD189733b with a particularly high accuracy. We obtain a period of 2.218574 (+0.000006/-0.000010) days, corresponding to an accuracy of ~1 second. Such accurate measurements might provide clues for companions presence.

7 pages, to be published in Astronomy & Astrophysics