Launched on December 25, 2021, the James Webb telescope completed its test phase in July 2022. The scientific programs have begun and are already producing their first results, including the first image of an exoplanet obtained in the mid-infrared, HIP 65426 b. This giant exoplanet is very young, about 15 million years old, and is located 90 astronomical units from its star. With an estimated mass of about 7 Jupiters, it was discovered using the European instrument Sphere at the Very Large Telescope (Chile) in 2017. Its direct observation in the infrared is now possible, thanks to the James Webb instruments.

With the help of the instruments NIRCAM and MIRI, HIP 65426 b was just observed as part of an Early Release Science program, led by an international collaboration including several French researchers, notably from the CEA-Irfu.

MIRI is the only mid-infrared instrument of the James Webb Space Telescope. Its imaging system was designed in part by a consortium of French laboratories led by the Irfu. It features a coronagraphic system that strongly attenuates the light coming from the central star, making it possible to image exoplanets and study their atmosphere. MIRI's coronagraphs, which are based on a technological innovation developed at the Observatoire de Paris-PSL, are achieving better performances than initially expected. The recent publication of these results shows that stellar light can be attenuated by a factor of 10,000 to 100,000. These coronagraphs operate at wavelengths of 10.65 μm, 11.40 μm, and 15.50 μm, specifically chosen to probe the atmosphere of giant exoplanets, to identify molecules such as ammonia, and to complement ground-based near-infrared observations.

The data gathered on exoplanet HIP 65426 b provide the first reliable temperature measurement of its atmosphere: 1,400°C, or the temperature of a lighter's flame. The scientists expect small grains of silicate dust to form and remain suspended in such an atmosphere.

Thus, the JWST has demonstrated its potential to study the physicochemical properties of these extrasolar planets in detail and to better understand their formation. The images could also reveal new, as yet unknown planets in these systems.

These spectacular images are the first from the ERS 1387 program, which is entirely dedicated to direct observations of nearby planetary systems. In particular, researchers will study a system that is even younger (by 5 million years), around which there is still a lot of gas and dust, to determine if these dusts contain frozen water, like comets in our own solar system.