First step to detect an extrasolar planet using simultaneous observations with the VLTI instruments AMBER and MIDI

Aims. Performed in November 2007 as a part of the MIDI Guaranteed Time Observation Exoplanet Programme, the observation of Gliese 86b constituted the first attempt at an exoplanet detection with the VLTI instrument MIDI. It is also a technical achievement since it motivated the first VLTI observation using AMBER and MIDI simultaneously. Methods. Fringes were obtained for both instruments with the aim of reaching sufficient precision on the low differential phase signal of Gliese 86b. The principle is to correct the phase measured in N-band from the water vapour dispersion using the fringes in K-band. In N-band, the source, Gliese 86, has an estimated magnitude of 3.8. With a separation of 0.11 AU, a flux ratio of about 10(-3) is expected between the planet and the star. According to the measurement principle and the planet signal signature, the effective expected interferometric phase is a curved-like function of the wavelength with a mean amplitude of about 0.03 degrees. Results. Based on the MIDI phase measurements of the calibrator HD 9362, our study shows that a precision on the curvature measurement of about 0.33 degrees is currently reached. Consequently, we stand at a factor 10 above the phase signal from the planet. The AMBER data, obtained in parallel, were too noisy to extrapolate and to remove the corresponding dispersion in N band at the required level of precision. However, we report the set of data obtained, we discuss the calibration process involved, and we estimate its theoretical efficiency.