Imaging detection of the inner dust belt and the four exoplanets in the HR8799 system with JWST’s MIRI coronagraph

Context. The MIRI instrument onboard JWST is now offering high-contrast imaging capacity at mid-IR wavelengths, thereby opening a completely new field of investigation to characterize young exoplanetary systems. Aims. The multiplanet system HR8799 is the first target observed with MIRI’s coronagraph as part of the MIRI-EC Guaranteed Time Observations exoplanets programme, in Nov. 2022. We obtained deep observations in three coronagraphic filters from ∼ 10 to 15 μm (F1065C, F1140C, F1550C), and one standard imaging filter at ∼ 20 μm (F2100W), with the goal to extract the photometry of the four planets, as well as to detect and investigate the distribution of circumstellar dust. Methods. Using dedicated observations of a reference star, we tested several algorithms to subtract the stellar diffraction pattern while preserving the fluxes of planets, which can be significantly affected by over-subtraction. Measuring correctly the planet’s flux values requires accounting for the attenuation by the coronagraphs as a function of their position, and to estimate the normalisation with respect to the central star. We tested several procedures to derive averaged photometric values and error bars. Results. These observations have enabled us to obtain two main results. First of all, the four planets in the system are well recovered, and their mid-IR fluxes, combined with near-IR flux values from the literature, are compared to two exoplanet atmosphere models, ATMO and Exo-REM. As a main outcome, the MIRI photometric data points imply larger radii (0.86 or 1.07 RJ for planet b) and cooler temperatures (950 or 1100K for planet b), especially for planet b, in better agreement with evolutionary models. Second of all, these JWST/MIRI coronagraphic data also deliver the first spatially resolved detection of the inner warm debris disk, the radius of which is constrained to about 15 au, with flux densities comparable, but lower than former unresolved spectroscopic measurements with Spitzer. Conclusions. The coronagraphs of MIRI cast a new vision of known exoplanetary systems which differs significantly from more shorter wavelength high-contrast images delivered by extreme adaptive optics from the ground. Inner dust belts and background galaxies become predominant at some mid-IR wavelengths, potentially causing confusion for the detection of exoplanets. Future observing strategy and data reduction should take such features into account.