As an exoplanet transits its host star, some of the light from the star is absorbed by the atoms and molecules in the planets atmosphere, causing the planet to seem bigger; plotting the planets observed size as a function of the wavelength of the light produces a transmission spectrum 1 . Measuring the tiny variations in the transmission spectrum, together with atmospheric modelling, then gives clues to the properties of the exoplanets atmosphere. Chemical species composed of light elementssuch as hydrogen, oxygen, carbon, sodium and potassiumhave in this way been detected in the atmospheres of several hot giant exoplanets [2][3][4][5] , but molecules composed of heavier elements have thus far proved elusive. Nonetheless, it has been predicted that metal oxides such as titanium oxide (TiO) and vanadium oxide occur in the observable regions of the very hottest exoplanetary atmospheres, causing thermal inversions on the dayside 6, 7 . Here we report the detection of TiO in the atmosphere of the hot-Jupiter planet WASP-19b. Our combined spectrum, with its wide spectral coverage, reveals the presence of TiO (to a confidence level of 7.7σ), a strongly scattering haze (7.4σ) and sodium (3.4σ), and confirms the presence of water (7.9σ) in the atmosphere 5,8 .Hot Jupiters are gas-giant exoplanets with sizes like that of Jupiter but much shorter orbital periods. WASP-19b is the shortest-period hot Jupiter to be discovered so far 9 , and has an excessively bloated radius, owing to the extreme radiation that it receives from its host star; as a result of this radiation, the planets effective temperature is more than 2,000 K (obtained via secondaryeclipse measurements 10 ). It is thought that high atmospheric temperatures imply the presence of metal oxides such as TiO, but despite extensive searches 11, 12 a definitive detection of metal oxides in exoplanetary atmospheres has proved elusive.We observed three transits of European Southern Observatorys Very Large Telescope (VLT), using the low-resolution FORS2 spectrograph. By using three of FORS2s grisms600B (blue), 600RI (green) and 600z (red), thereby covering the entire visible-wavelength domain (0.431.04 µm)together with the multi-object spectroscopy configuration, we were able to obtain relatively high-resolution, precise, broadband transmission spectra. Such results were made possible through optimized observing strategies 13 and careful design of the observing mask used for the multi-object observations: this has slits about 30 wide, which minimized differential losses owing to variations in telescope guiding and seeing conditions. The observations presented here were made between 11 November 2014 and 29 February 2016.For each set of observations, we obtained a series of spectra for the main target (WASP-19), as well as for several comparison stars. After standard data-reduction steps, we integrated those spectra for the largest common wavelength domain and 10-nm bins, to produce the 'white' and 'spectrophotometric' light curves, respectively. To correct for the imp...

We present FORS2 (attached to ESO's Very Large Telescope) observations of the exoplanet WASP-17b during its primary transit, for the purpose of differential spectrophotometry analysis. We use the instrument in its Mask eXchange Unit (MXU) mode to simultaneously obtain low resolution spectra of the planet hosting star, as well as several reference stars in the field of view. The integration of these spectra within broadband and smaller 100Å bins provides us with 'white' and spectrophotometric light curves, from 5700 to 8000Å. Through modelling the white light curve, we obtain refined bulk and transit parameters of the planet, as well as wavelength-dependent variations of the planetary radius from smaller spectral bins through which the transmission spectrum is obtained. The inference of transit parameters, as well as the noise statistics, is performed using a Gaussian Process model. We achieve a typical precision in the transit depth of a few hundred parts per million from various transit light curves. From the transmission spectra we rule out a flat spectrum at >3σ and detect marginal presence of the pressure-broadened sodium wings. Furthermore, we detect the wing of the potassium absorption line in the upper atmosphere of the planet with 3-σ confidence, both facts pointing to a relatively shallow temperature gradient of the atmosphere. These conclusions are mostly consistent with previous studies of this exo-atmosphere, although previous potassium measurements have been inconclusive.