Observations made in November 2023 with the coronagraph on the JWST MIRI instrument have not only detected and measured the mid-infrared intensity of the four young giant planets orbiting the star HR8799 but have also captured the first image of the inner dust disk.
Direct imaging of exoplanets allows the study of their atmosphere and environment by measuring their intensity across different wavelengths. Several such systems are already known thanks to ground-based instruments equipped with extreme adaptive optics on 8 to 10-meter (26 to 33 feet) telescopes. However, the James Webb Space Telescope opens access to the mid-infrared range, a wavelength spectrum that had not been explored before.
Images of the HR 8799 system revealing 4 young giant planets at 10.65 and 11.40 microns (1 and 2), and the inner dust disk at 15.50 and 23 microns (3 and 4), the latter being brighter than the planets at the highest wavelengths.
© Anthony Boccaletti Indeed, exoplanets are located very close to their host stars, which are also much brighter, making them harder to observe. To detect them, a coronagraph is needed, a system used to dim starlight and reveal the planets in their orbit.
A French team involving scientists from CNRS Terre & Univers designed a new type of coronagraph in the 2000s: the four-quadrant phase mask. Three of these masks, as well as a "Lyot" mask (named after the inventor of the solar coronagraph in the 1930s), are installed in the MIRI instrument, whose imaging system was designed and built in France. This system enables, for the first time, the acquisition of exoplanet images at wavelengths from 10 to 20 microns.
Thanks to this technology, based on mid-infrared photometric measurements and combined with near-infrared observations from the ground, a finer estimate of these planets' diameter (ranging between 1 and 1.5 Jupiter radii) is now possible. This new estimate appears to be more consistent with planetary evolution models, considering their age of 30 Myr.
Furthermore, while only three of these planets were expected to be detected, the sensitivity of the MIRI coronagraph also detected a fourth one, even closer to the star HR8799. The presence of ammonia in the atmosphere, which was being sought in the coolest of the four planets, could not be confirmed. Lastly, the inner disk composed of micrometric dust had never been imaged before. Its presence suggests that there are likely no other giant planets within 15 astronomical units of the star.
Other young exoplanetary systems will be observed with the MIRI coronagraph, specifically looking for the signature of ammonia in planets with temperatures below 1000 Kelvin, thereby further contributing to our understanding of the mechanisms of exoplanet formation.
Reference:
Imaging detection of the inner dust belt and the four exoplanets in the HR 8799 system with JWST's MIRI coronagraph Anthony Boccaletti
et al.
Astronomy & Astrophysics (A&A), 686, A33 (2024)
https://doi.org/10.1051/0004-6361/202347912