by Ludwig Scheibe (TU Berlin), March 2025
Overview
The James Webb Space Telescope JWST, often simply called Webb, is a powerful infrared telescope. As the name suggests, Webb operates in space, where it is not exposed to the disruptive influences of the Earth’s atmosphere and can observe continuously without having to take day and night into account. It is seen as the continuation of the Hubble mission, which has been running since 1990, and serves many areas of astronomy: it observes distant galaxies, measures stellar properties, provides information about the early universe shortly after the Big Bang and, of particular interest to this website, images extrasolar planets and investigates their composition using spectroscopy.
Artistic depiction of the James Webb Space Telescope. Credit: NASA.
Webb was jointly developed and built by the US American Space Agency NASA, the European pendant ESA and the Canadian equivalent CSA. Like the Hubble telescope, Webb is operated by the Space Telescope Science Institute (STScI) in Baltimore. It is named after James Webb, who was NASA’s administrator from 1961 to 1968 and played a key role in the development of the agency.
On 25th December 2021, Webb was launched on an Arianne 5 rocket and reached its final orbit about a month later, on 24th January 2022. Since then, Webb has expanded astronomical research in many fields. Every year, researchers have the opportunity to apply for observation time for the coming year as part of the General-Observer-Programms. The proposals are then reviewed and the most promising are selected.
The Instrument
The honeycomb-shaped primary mirror is probably the most iconic element of the Webb telescope. It has a diameter of 6.5 meters and consists of 18 individual hexagonal elements. Each element can be controlled individually, which allows the entire mirror to be adjusted with high precision. Using this, Webb captures light and focuses it onto the secondary mirror, not unlike a satellite dish. Webb observes infrared light, i.e. light with a wavelength that is too long for us humans to see.
The light is then fed into four different instruments, which split it into its individual wavelengths and analyze it in more detail. Three of the instruments – NIRSpec, NIRISS, NIRCam – cover the wavelength range between around 0.6 and 5 micrometers, the so-called “near infrared”, and the fourth, MIRI, the “mid infrared” range between around 5 and 30 micrometers wavelength. Incidentally, NIRSPec and MIRI were built with significant European participation. You can read more about the individual instruments on the STScI Website.
An important challenge is to keeping the telescope cool and protecting it from Solar radiation. For this reason, the actual observation apparatus is always turned away from the Sun and a five-layer protective screen keeps out the Sun’s rays.
Design of the James Webb telescope. On the left is the side facing away from the Sun (“cold”) and shows the primary and secondary mirror as well as the multi-layer solar shield and the module containing the four instruments. On the right is the side facing the Sun (“hot”), which shows the antenna for communication with the Earth, the solar panel and the module for the main electronics and drive technology (bus). Credit: STScI
Where does Webb observe from?
Unlike its predecessor mission Hubble, Webb does not orbit Earth. Instead, it orbits the Sun together with the Earth, specifically on the so-called Lagrange point 2. At this point, ca. 1.5 million kilometers from Earth on the extended line connecting Earth and the Sun, the different forces acting on the telescope nearly cancel out and it can be held at this position using very little fuel.
Schematic view of Earth’s orbit and the five stable Lagrange points L1 to L5. Webb orbits the L2 point on the right. Credit: NASA, STScI
Results from the world of exoplanets
Unlike missions like Kepler and PLATO, which primarily have the goal to find new exoplanets by surveying a large part of the sky, James Webb does not discover new planets. Instead, it allows us to characterize known planets in an unprecedented level of detail.
Already among thee first four press releases by Webb in July 2022 was a result from exoplanets: The accurate transmission spectrum of the hot Jupiter Wasp-96 b’s atmosphere shows characteristic traces of various gases, among them water vapour.
Transmission spectrum of hot Jupiter Wasp-96 b, measured by the James Webb Space Telescope. It shows strength of absorption in relation to the light wavelength. White: measured points, blue: spectrum of the best-fitting computer model. Credit: NASA, ESA, CSA, STScI
And things have continued ever since. Since the start of scientific operations, over a quarter of the successful applications for observation time have been in the field of exoplanet research. Many of these were in the field of spectroscopy, as with Wasp-96 b. Another exoplanet, WASP-17b, whose spectrum Webb observed, showed small quartz crystals, like tiny grains of sand, in the clouds of this hot Jupiter.
But Webb has also made headlines in other areas. In 2023, for example, Webb measured the thermal radiation of Trappist-1 b, a rocky planet that is only about as warm as Mercury. Previously, this had only been achieved very rarely with these small worlds and only when they were hot. This measurement of Trappist-1 b suggests that the planet probably has no significant atmosphere. This is an indicator of how well planets around this type of star can retain their atmosphere.
Artist’s impression of rocky exoplanet Trappist-1 b, orbiting a small red dwarf star. According to James Webb observations, it is most likely a bare rock without an atmosphere. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
Currently, Webb is in the third of five planned operation years. The mission can most likely be extended for another five years, with some hope for more. So, we can Derzeit ist Webb im dritten von fünf geplanten Betriebsjahren. So, we can look forward to seeing what other spectacular results the James Webb Space Telescope will bring us.
To learn more, visit the JWST webpage at NASA or the Webb telescope page at STScI.