Imaging detection of the inner dust belt and the four exoplanets in the HR 8799 system with JWST’s MIRI coronagraph

Context. The MIRI instrument on board JWST is now offering high-contrast imaging capacity at mid-IR wavelengths, thereby opening a completely new field of investigation for characterizing young exoplanetary systems. Aims. The multiplanet system HR 8799 is the first target observed with MIRI’s coronagraph as part of the MIRI-EC Guaranteed Time Observations (GTO) exoplanet program, launched in November 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). The goal of this work is to extract photometry for the four planets and 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. To obtain correct measurements of the planet’s flux values, the attenuation by the coronagraphs as a function of their position must be accounted for, as well as an estimation of 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, 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 1.17 RJ for planet b) and cooler temperatures (950 or 1000 K 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 15 au, with flux densities that are comparable to (but lower than) former unresolved spectroscopic measurements with Spitzer. Conclusions. 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.</p

The Mid-infrared Instrument for JWST and Its In-flight Performance

The Mid-Infrared Instrument (MIRI) extends the reach of the James Webb Space Telescope (JWST) to 28.5 μm. It provides subarcsecond-resolution imaging, high sensitivity coronagraphy, and spectroscopy at resolutions of λ/Δλ ∼ 100–3500, with the high-resolution mode employing an integral field unit to provide spatial data cubes. The resulting broad suite of capabilities will enable huge advances in studies over this wavelength range. This overview describes the history of acquiring this capability for JWST. It discusses the basic attributes of the instrument optics, the detector arrays, and the cryocooler that keeps everything at approximately 7 K. It gives a short description of the data pipeline and of the instrument performance demonstrated during JWST commissioning. The bottom line is that the telescope and MIRI are both operating to the standards set by pre-launch predictions, and all of the MIRI capabilities are operating at, or even a bit better than, the level that had been expected. The paper is also designed to act as a roadmap to more detailed papers on different aspects of MIRI.</p