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

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: anthony.boccaletti@obspm.fr
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁶ HFML – FELIX. Radboud University PO box 9010, 6500 GL Nijmegen, The Netherlands
⁷ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁸ Department of Astrophysics, University of Vienna, Türkenschanzs-trasse 17, 1180 Vienna, Austria
⁹ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹⁰ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹¹ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹² Université Paris-Saclay, CEA, Département d’Électronique des Détecteurs et d’Informatique pour la Physique, 91191 Gif-sur-Yvette, France
¹³ LERMA, Observatoire de Paris, Université PSL, Sorbonne Univer-sité, CNRS, Paris, France
¹⁴ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Kapteyn Institute of Astronomy, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁶ European Space Agency, Space Telescope Science Institute, Baltimore, MD, USA
¹⁷ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹⁸ School of Physics & Astronomy, Space Research Centre, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
¹⁹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
²⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²¹ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
²² School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland
²³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Received: 8 September 2023
Accepted: 27 February 2024

Abstract

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 R_J 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.