NASA’s James Webb Space Telescope is designed to study exoplanets using a variety of observing modes, including coronagraphs. Webb’s coronagraphs use NIRCam and MIRI instruments to block the light from a distant star and allow faint planet light to reach its sensors. The coronagraphs can observe objects as close as 0.13 arcseconds from the star, and as distant as about 30 arcseconds from the star. Webb’s large primary mirror and infrared capabilities mean that its coronagraphs are uniquely suited to study faint objects in the infrared and complement other instruments currently observing at other wavelengths. Despite the coronagraphs, Webb’s astronomers will still use “point spread function (PSF) subtraction methods” to remove the last remnants of starlight. Webb’s coronagraphs will mainly be used to detect giant extrasolar planets that are still warm from being formed and dense circumstellar disks of debris generated by the asteroids and comets in these exoplanetary systems. Webb’s coronagraphs can even be used for extragalactic astronomy. A future mission fully optimized around next-generation coronagraphs is required to image planets like our own around nearby Sun-like stars.
Unlocking the Secrets of Distant Worlds: How Webb’s Coronagraphs Reveal Exoplanets in the Infrared
Christopher Stark, the Deputy Observatory Project Scientist from NASA’s Goddard Space Flight Center, recently shared insights on one of the methods used by NASA’s James Webb Space Telescope to investigate distant exoplanets. One of the central aspects of the telescope’s scientific objectives is the examination of these exoplanets, and Webb has several observing modes to achieve this. One particular mode is the ability to directly detect some of these planets.
Directly detecting planets around other stars is not an easy task. Even the nearest stars are incredibly far away, which means their planets appear to be separated by only a fraction of the width of a human hair held at arm’s length. At these tiny angular scales, the planet’s faint light is often lost in the glare of its host star when attempting to observe it.
Fortunately, Webb has the perfect tool for this task – the Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) coronagraphic modes. Webb’s coronagraphs block out the light from the distant star, while allowing the faint planet light to pass through and reach its sensors. This is similar to how we use our car’s visor during sunrise or sunset to help us see the cars in front of us. However, Webb’s use of a coronagraph is far more advanced.
Webb’s coronagraphs are a vital part of the telescope’s scientific objectives. The telescope can use the Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) coronagraphic modes to study the planets orbiting other stars, also known as exoplanets. The coronagraphs allow the telescope to block out the light from a distant star, making it easier to observe the exoplanet.
Webb’s NIRCam and MIRI coronagraphic images of the exoplanet HIP 65426 b showcase how the coronagraphs work. The images feature a white star symbol that indicates the location of the star blocked out by the coronagraphs. Unlike other exoplanet images, HIP 65426 b does not display Webb’s hallmark six-spiked diffraction pattern due to the pupil plane coronagraph masks.
Overall, Webb’s coronagraphs play an essential role in the telescope’s ability to observe and study distant exoplanets. With these tools, scientists can unlock the secrets of these worlds and gain a better understanding of our place in the universe.
How Webb’s Coronagraphs Optimize Performance and Remove Starlight
Webb’s optics have several important locations called “planes,” including the image and pupil planes. The image plane is where the distant sky, including all astrophysical objects, is in focus. On the other hand, the pupil plane allows the surface of the primary mirror to be in focus, which was used to make Webb’s “selfie.” All of Webb’s coronagraphs mask out unwanted starlight in both the image and pupil planes to optimize performance.
Most of Webb’s image plane masks resemble opaque spots or bars that remove starlight by simply blocking it in the image. However, MIRI’s “four-quadrant phase masks” shift the wave-tops of one part of the wave of light so it cancels out with another part through a process called “destructive interference.”
Due to the wave nature of light, the image plane masks can’t entirely block the star. Therefore, Webb uses additional pupil plane masks, also called Lyot stops, to remove much of the remaining starlight. These pupil plane masks look different from the hexagonal primary mirror (the telescope “pupil”). As a result, objects imaged with the coronagraphs do not exhibit Webb’s hallmark six-spiked diffraction pattern.
Webb’s 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. The coronagraphs can observe objects as close as 0.13 arcseconds from the star and as distant as about 30 arcseconds from the star, which roughly translates to circumstellar distances ranging from a few Astronomical Units (au) to hundreds of au around nearby stars.
Webb’s coronagraphs play a significant role in optimizing the telescope’s performance and removing starlight to provide clear and detailed images of distant exoplanets. The different masks used in both the image and pupil planes help block out unwanted light and provide scientists with valuable data to unlock the secrets of distant worlds.
Revealing Distant Worlds: The Limitations of Webb’s Coronagraphs and Future Prospects
Webb’s coronagraphs play a crucial role in detecting and studying exoplanets. However, despite the masks used in both the image and pupil planes, they don’t entirely remove a star’s light. To remove the remaining remnants of light, Webb’s astronomers use a variety of “point spread function (PSF) subtraction methods.” By measuring the pattern of the residual starlight and subtracting it from the science image, they can limit the faintest detectable exoplanet expressed in terms of “contrast,” the ratio in brightness between the faintest detectable planet and the star. During commissioning, Webb’s NIRCam and MIRI coronagraphs demonstrated contrasts better than 10^-5 and 10^-4 at 1 arcsecond separation, respectively.
Webb’s large primary mirror and infrared capabilities make its coronagraphs uniquely suited to studying faint objects in the infrared. They will complement other instruments currently observing at other wavelengths, including Hubble’s STIS coronagraph and multiple instruments on ground-based observatories. Webb’s coronagraphs will detect giant extrasolar planets that are still warm from being formed, as well as dense circumstellar disks of debris generated by asteroids and comets in these exoplanetary systems. They can even be used for extragalactic astronomy to study host galaxies that contain bright active galactic nuclei.
However, Webb’s coronagraphs have limitations. They won’t be able to reveal all the secrets of a planetary system. To image planets like our own around nearby Sun-like stars, scientists need to detect planets just one ten billionth the brightness of the star. This will require a future mission fully optimized around next-generation coronagraphs. NASA’s upcoming Nancy Grace Roman Space Telescope will carry a technology demonstration instrument to test next-generation coronagraph technology. Following the recommendations of the 2020 Astrophysics Decadal Survey, NASA is laying the groundwork for further technology development for a Habitable Worlds Observatory mission concept, a telescope that would be as large as Webb, operating in the same wavelengths as Hubble, but designed to find truly Earth-like exoplanets around other stars and search them for signs of life.
In summary, while Webb’s coronagraphs have proven to be successful in detecting and studying distant exoplanets, they have their limitations. The hunt for Earth-like exoplanets and the search for signs of life continue to drive scientific inquiry and push the boundaries of what is possible in the field of astronomy. The future prospects of next-generation coronagraphs and the Habitable Worlds Observatory mission concept provide hope for even greater discoveries in the years to come.
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