The fully unfolded James Webb Space Telescope in space is shown in this artist's concept. Credit: Adriana Manrique Gutierrez, NASA animator
The James Webb Space Telescope's scientific objectives include the study of exoplanets. Christopher Stark, NASA's Deputy Observatory Project Scientist at NASA's Goddard Space Flight Center, spoke on one of Webb's methods to investigate these distant worlds.
The James Webb Space Telescope of NASA has many different methods of observation to study exoplanets that orbit other stars. One method, in particular, is that Webb may directly detect some of these planets. Even the closest stars are still so far away that their planets appear to be separated by a fraction of the width of a human hair held at arm's length when trying to observe them.
The NIRCam and MIRI coronagraphic modes at Webb's lab are excellent, although we may need to adjust our visors to see the cars ahead of us.
Because of the pupil plane coronagraph masks, Webb's hallmark six-spiked diffraction pattern does not exist. A. Pagan (STScI) and ASA/ESA/CSA
The "image plane" is where Webb's objects are in focus, and the "pupil plane" was used to record Webb's "selfie." All of Webb's image plane masks, resembling opaque spots or bars, remove starlight simply by blocking it in the image. A separate technique called "destructive interference" is used to cancel out starlight.
NASA's coronagraphic image plane mask hardware, consisting of two wedge-shaped bars and three round spots (from left to right), is on the right. Three phase-shifting four quadrant phase masks and one round spot
The image plane masks can't completely block the star, therefore Webb creates additional pupil plane masks, also known as Lyot stops, to remove much of the remaining starlight. These pupil plane masks contrast with the hexagonal primary mirror (the telescope "pupil") as a result of Webb's six-spiked diffraction pattern.
Illustration of the NIRCam pupil plane mask/Lyot stop for the round image plane mask (left) and the bar image plane mask (right)
The NIRCam instrument has five coronagraphic masks, each of which can be configured to observe at different wavelengths, ranging from 1.7 to 5 microns. Webb's MIRI instrument has four coronagraphic masks that operate at fixed wavelengths between 10 and 23 microns. This roughly translates to circumstellar distances between Earth and the Sun.
Webb's coronagraphs don't precisely remove a star's light, although they will carefully apply a number of "point spread function (PSF) subtraction techniques." What remains is a noisy-looking pattern, which ultimately limits the faintest detectable exoplanet, according to the term "contrast."
After PSF subtraction, the left side shows an example of residual starlight. The star is located in the center of the image. The faintest detectable planet in an observation
Webb's extensive primary mirror and infrared capabilities make it ideal for studying faint objects in the infrared, as well as other instruments currently observing at other wavelengths, including Hubble's STIS coronagraph and many ground-based observatories. These include giant extrasolar planets that are still warm from being formed, as well as protoplanetary disks in which planets are still forming, as well as extragalactic galaxies that are still active.
NASA is already working on a Habitable Worlds Observatory mission that would be as large as Webb, operating at the same wavelengths as Hubble, but capable of detecting true Earth-like exoplanets around other stars.
Christopher Stark, a NASA Goddard deputy observatory project scientist, was the author.
