Webb’s Coronagraphs and their Role in Infrared Exoplanet Detection

The James Webb Space Telescope of NASA plays an important role in identifying exoplanets, notably in the infrared spectrum. Exoplanets circling other stars can be difficult to find directly, which is a very difficult task. However, Webb has coronagraphic modes for its Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI), which can handle this difficult task. For these coronagraphs to function, light from far-off stars must be blocked while passing through to the sensors. Only light from nearby planets must be.

The purpose of Webb’s coronagraphs is to eliminate unwanted starlight in both the pupil and image planes. The “four-quadrant phase masks” developed by MIRI alter the phase of the light such that it interferes destructively with itself. Most image plane masks eliminate starlight by blocking it in the image, which is not the case with pupil plane masks. Unfortunately, the image plane masks are unable to completely cover the star because of the fact that light is a wave. Webb employs extra pupil plane masks—also known as Lyot stops—to filter out the remaining starlight.

The coronagraphic masks on Webb’s NIRCam and MIRI instruments are five and four, respectively. Using these masks, objects can be observed up to 30 arcseconds away from the star and as close as 0.13 arcseconds away. A few Astronomical Units (au) to hundreds of au are things that the coronagraphs can find in the vicinity of close stars. The infrared capabilities and large primary mirror of Webb make its coronagraphs especially well-suited for examining weak objects in the infrared. This complements other instruments already monitoring at other wavelengths.

Giant extrasolar planets still in formation can be found using Webb’s coronagraphs. They are also capable of imaging protoplanetary disks where planets are still developing. Also, they can image the dense circumstellar disks of debris produced by comets and asteroids in exoplanetary systems. The extragalactic study of host galaxies with bright, active galactic nuclei is another application for Webb’s coronagraphs.

Due to its large primary mirror and infrared capabilities, Webb’s coronagraphs are highly suitable for studying faint objects in the infrared. Therefore, Webb’s astronomers are going to eliminate the final traces of light using a range of “point spread function (PSF) subtraction methods.” This procedure entails measuring and deducting the residual starlight pattern from the scientific image. The result is a noisy pattern that finally caps the faintest exoplanet that can be seen. The “contrast” limit is the disparity in brightness between the star and the least observable planet. The NIRCam and MIRI coronagraphs on Webb showed contrasts at a separation of 1 arcsecond that were better than 10-5 and 10-4, respectively, during commissioning.

Webb’s coronagraphs are a crucial tool for finding exoplanets. However, they won’t be able to reveal all of a planetary system’s mysteries. Future missions must be properly tailored for next-generation coronagraphs. This will help in finding planets similar to our own around neighboring Sun-like stars. NASA is already investigating this.