|
Description
CHEOPS (CHaracterising ExOPlanets Satellite), launched in December 2019, is a space mission dedicated to exoplanet follow-up with the capacity to perform photometric measurements at the20 ppm level. As CHEOPS observes in a broad optical passband, it can be used provide insights on the reflected light from exoplanets, and constrain the short-wavelength thermal emission for the hottest of planets through the observation of occultations and phase curves. We here report the first observation of an occultation by CHEOPS: that of the hot Jupiter WASP-189b, a M~2M_J_ planet orbiting an A-type star. We detect the occultation of WASP-189b at high significance in individual measurements, and derive an occultation depth of dF=87.9+/-4.3 ppm based on four occultations. We compare this measurement to model predictions and find that, when assuming inefficient heat redistribution, they are consistent with an unreflective atmosphere, heated to a temperature of 3435+/-27K. We furthermore present two transits of WASP-189b observed by CHEOPS. These transits have an asymmetric shape that we attribute to gravity darkening of the host star due to its high rotation rate. We use these measurements to refine the planetary parameters, finding a ~25% deeper transit compared to the discovery paper and updating the radius of WASP-189b to 1.619+/-0.021R_J_. We further measure the projected orbital obliquity to be lambda=86.4(+2.9,-4.4)deg, a value in good agreement with a previous measurement from spectroscopic observations, and derive a true obliquity of Psi=85.4+/-4.3deg. Finally, we provide reference values for the photometric precision attained by the CHEOPS satellite: for the V=6.6 mag star, and using a 1-hour binning, we obtain a residual RMS between 10 and 17ppm on the individual light curves, and 5.7ppm when combining four visits.
|
