FITS images of HR8799 observed with MIRI

We tested several procedures to derive averaged photometric values and error bars. These observations have enabled us to obtain two main results. First, the four planets in the system are well recovered and we were able to compare their mid-IR fluxes, combined with near-IR flux values from the literature, to two exoplanet atmosphere models: ATMO and Exo-REM.

As a main outcome, the MIRI photometric data points imply larger radii (1.04 or 1.17R_J_ for planet b) and cooler temperatures (950 or 1000K for planet b), especially for planet b, in better agreement with evolutionary models. Second, 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 15au, with flux densities that are comparable to (but lower than) former unresolved spectroscopic measurements with Spitzer.

The coronagraphs coming from MIRI ushers in a new vision of known exoplanetary systems that differs significantly from shorter wavelength, high-contrast images delivered by extreme adaptive optics from the ground. Inner dust belts and background galaxies become dominant at some mid-IR wavelengths, potentially causing confusion in detecting exoplanets. Future observing strategies and data reductions ought to take such features into account.