Webb Confirms Accuracy of Universe’s Expansion Rate Measured by Hubble, Deepens Mystery of Hubble Constant Tension

The rate at which the universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. However, a persistent difference called the “Hubble Tension” is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the big bang afterglow.

A large galaxy takes up the entirety of the image. The galaxy has a bright white core, and several large spiral arms extending out from that core, rotating clockwise. The arms are light blue with many pink speckles and clumps littering the arms. The background is also filled with a smattering of white and pink dots.
Combined observations from NASA’s NIRCam (Near-Infrared Camera) and Hubble’s WFC3 (Wide Field Camera 3) show spiral galaxy NGC 5584, which resides 72 million light-years away from Earth. Among NGC 5584’s glowing stars are pulsating stars called Cepheid variables and Type Ia supernova, a special class of exploding stars. Astronomers use Cepheid variables and Type Ia supernovae as reliable distance markers to measure the universe’s expansion rate.
Download the high-resolution file from the Resource Gallery.
Credit: NASA, ESA, CSA, and A. Riess (STScI).

NASA’s James Webb Space Telescope provides new capabilities to scrutinize and refine some of the strongest observational evidence for this tension. Nobel Laureate Adam Riess from the Johns Hopkins University and the Space Telescope Science Institute presents his and his colleagues’ recent work using Webb observations to improve the precision of local measurements of the Hubble constant.

“Did you ever struggle to see a sign that was at the edge of your vision? What does it say? What does it mean? Even with the most powerful telescopes, the ‘signs’ astronomers want to read appear so small that we struggle too.

“The sign cosmologists want to read is a cosmic speed limit sign that tells us how fast the universe is expanding — a number called the Hubble constant. Our sign is written into the stars in distant galaxies. The brightnesses of certain stars in those galaxies tell us how far away they are and thus for how much time this light has been traveling to reach us, and the redshifts of the galaxies tell us how much the universe expanded over that time, hence telling us the expansion rate.

Graphic titled “Uncrowding Cepheids in the Near-Infrared.” Left: Colorful image of a spiral galaxy is labeled “NGC 5584 (Webb NIRCam + Hubble WFC3).” Part of a spiral arm is outlined with a box. Inside the box is a solid red circle. Middle: Set of two diagrams illustrating zoomed-in views of the boxed region. Top diagram shows Hubble pointing toward a large transparent box. Long edge of the base is labeled “depth.” Box contains numerous overlapping, blue-green spheres of different sizes, with a red sphere near the middle. Bottom diagram shows Webb pointing toward an almost identical box. Spheres in this box are significantly smaller and there is less overlap. The red sphere is labeled “Cepheid.” Right: Two square grayscale images. Top is labeled “HST WFC3-IR.” Bottom labeled “JWST NIRCAM.” Pixels in the top image are noticeably larger than those in the bottom image. Black spots in the bottom image are smaller, more distinct, and more numerous. Click View Description for more details.
This diagram illustrates the combined power of the NASA’s Hubble and Webb space telescopes in nailing down precise distances to a special class of variable star that is used in calibrating the expansion rate of the universe. These Cepheid variable stars are seen in crowded star fields. Light contamination from surrounding stars may make the measurement of the brightness of a Cepheid less precise. Webb’s sharper infrared vision allows for a Cepheid target to be more clearly isolated from surrounding stars, as seen in the right side of the diagram. The Webb data confirms the accuracy of 30 years of Hubble observations of Cepheids that were critical in establishing the bottom rung of the cosmic distance ladder for measuring the universe’s expansion rate. At the left, NGC 5584 is seen in a composite image from Webb’s NIRCam (Near-Infrared Camera) and Hubble’s Wide Field Camera 3.
Download the high-resolution file from the Resource Gallery.
Image Credit: NASA, ESA, A. Riess (STScI), W. Yuan (STScI).

“A particular class of stars, Cepheid variables, has given us the most precise measurements of distance for over a century because these stars are extraordinarily bright: They are supergiant stars, a hundred thousand times the luminosity of the Sun. What’s more, they pulsate (that is, expand and contract in size) over a period of weeks that indicates their relative luminosity. The longer the period, the intrinsically brighter they are. They are the gold standard tool for the purpose of measuring the distances of galaxies a hundred million or more light years away, a crucial step to determine the Hubble constant. Unfortunately, stars in galaxies are crowded together in a small space from our distant vantage point and so we often lack the resolution to separate them from their line-of-sight neighbors.

“A major justification for building the Hubble Space Telescope was to solve this problem. Prior to Hubble’s 1990 launch and its subsequent Cepheid measurements, the expansion rate of the universe was so uncertain astronomers weren’t sure if the universe has been expanding for 10 billion or 20 billion years. That’s because a faster expansion rate will lead to a younger age for the universe, and a slower expansion rate will lead to an older age of the universe. Hubble has better visible-wavelength resolution than any ground-based telescope because it sits above the blurring effects of Earth’s atmosphere. As a result, it can identify individual Cepheid variables in galaxies that are more than a hundred million light-years away and measure the time interval over which they change their brightness.

“However, we also must observe the Cepheids at the near-infrared part of the spectrum to see the light which passes unscathed through intervening dust. (Dust absorbs and scatters blue optical light, making distant objects look faint and fooling us into believing they are farther away than they are). Unfortunately, Hubble’s red-light vision is not as sharp as its blue, so the Cepheid starlight we see there is blended with other stars in its field of view. We can account for the average amount of blending, statistically, the same way a doctor figures out your weight by subtracting the average weight of clothes from the scale reading, but doing so adds noise to the measurements. Some people’s clothes are heavier than others.

“However, sharp infrared vision is one of the James Webb Space Telescope’s superpowers. With its large mirror and sensitive optics, it can readily separate the Cepheid light from neighboring stars with little blending. In the first year of Webb operations with our General Observers program 1685, we collected observations of Cepheids found by Hubble at two steps along what’s known as the cosmic distance ladder. The first step involves observing Cepheids in a galaxy with a known, geometric distance that allows us to calibrate the true luminosity of Cepheids. For our program that galaxy is NGC 4258. The second step is to observe Cepheids in the host galaxies of recent Type Ia supernovae. The combination of the first two steps transfers knowledge of the distance to the supernovae to calibrate their true luminosities. Step three is to observe those supernovae far away where the expansion of the universe is apparent and can be measured by comparing the distances inferred from their brightness and the redshifts of the supernova host galaxies. This sequence of steps is known as the distance ladder.

“We recently got our first Webb measurements from steps one and two which allows us to complete the distance ladder and compare to the previous measurements with Hubble (see figure) Webb’s measurements have dramatically cut the noise in the Cepheid measurements due to the observatory’s resolution at near-infrared wavelengths. This kind of improvement is the stuff astronomers dream of!  We observed more than 320 Cepheids across the first two steps. We confirmed that the earlier Hubble Space Telescope measurements were accurate, albeit noisier.   We have also observed four more supernova hosts with Webb and we see a similar result for the whole sample.

Graphs of the luminosity versus period relationship of cepheids in NGC 5584 (top) and NGC 4258 (bottom), as measured by HST (gray data points) and JWST (red data points). Top graph: The y-axis of luminosity ranges from 27 Vega mag at the bottom to 22 Vega mag at the top, labeled in increments of 0.5. The x-axis of Period is on a log scale ranging from ## days on the right to ## days at the far, labeled at 16, 25, 40, 63, and 100. Bottom graph: y-axis ranges from 26 to 20; x-axis ranges from # to 80, labeled at 5, 10, 20, 40, and 80.
Comparison of Cepheid period-luminosity relations used to measure distances. The red points are from NASA’s Webb, and the gray points are from NASA’s Hubble. The top panel is for NGC 5584, the Type Ia supernova host, with the inset showing image stamps of the same Cepheid seen by each telescope. The bottom panel is for NGC 4258, a galaxy with a known, geometric distance, with the inset showing the difference in distance moduli between NGC 5584 and NGC 4258 as measured with each telescope. The two telescopes are in excellent agreement.
Image Credit: NASA, ESA, A. Riess (STScI), and G. Anand (STScI).

“What the results still do not explain is why the universe appears to be expanding so fast! We can predict the expansion rate of the universe by observing its baby picture, the cosmic microwave background, and then employing our best model of how it grows up over time to tell us how fast the universe should be expanding today. The fact that the present measure of the expansion rate significantly exceeds the prediction is a now decade-long problem called “The Hubble Tension.” The most exciting possibility is that the Tension is a clue about something we are missing in our understanding of the cosmos.

“It may indicate the presence of exotic dark energy, exotic dark matter, a revision to our understanding of gravity, or the presence of a unique particle or field. The more mundane explanation would be multiple measurement errors conspiring in the same direction (astronomers have ruled out a single error by using independent steps), so that is why it is so important to redo the measurements with greater fidelity. With Webb confirming the measurements from Hubble, the Webb measurements provide the strongest evidence yet that systematic errors in Hubble’s Cepheid photometry do not play a significant role in the present Hubble Tension. As a result, the more interesting possibilities remain on the table and the mystery of the Tension deepens.”

Editor’s Note: This post highlights data from a paper that was accepted by The Astrophysical Journal.


  • Adam Riess is a Bloomberg Distinguished Professor at the Johns Hopkins University, the Thomas J. Barber Professor in Space Studies at the JHU Krieger School of Arts and Sciences, a distinguished astronomer at the Space Telescope Science Institute, and a recipient of the 2011 Nobel Prize in Physics.

Mid-Infrared Instrument Operations Update

 On Apr. 21, 2023, the James Webb Space Telescope team shared that one of the MIRI (Mid-Infrared Instrument) observing modes, called Medium-Resolution Spectroscopy (MRS), showed a reduction in the amount of light registered by MIRI’s detectors. Initial analysis of MIRI’s imaging mode did not show a similar effect. However, as part of the team’s investigation into the issue, additional monitoring observations were taken with MIRI imaging. Combined with earlier data, these new calibrations have revealed a reduced signal for MIRI imaging at the longer wavelengths.

This change does not substantially impact MIRI’s science capabilities but will have an impact on the exposure times needed for MIRI imaging.

There is no risk to the instrument, and the effect on imaging is less than the effect in MRS. The team is investigating the cause of this issue. Regular monitoring observations are being taken to continue measuring the response, and the team is providing updated guidance to Webb’s user community to correct for this change. MIRI’s third observing mode, Low-Resolution Spectroscopy, is currently performing normally, and the investigation of MIRI’s fourth mode, Coronographic Imaging, has not yet concluded.

The Webb team has also enacted a plan for long-term monitoring, and are exploring potential mitigations. The observatory is in good health, and each of Webb’s other scientific instruments are unaffected.

For more information, visit the Space Telescope Science Institute.


Webb Reveals Intricate Details in the Remains of a Dying Star

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

NASA’s James Webb Space Telescope obtained images of the Ring Nebula, one of the best-known examples of a planetary nebula. Much like the Southern Ring Nebula, one of Webb’s first images, the Ring Nebula displays intricate structures of the final stages of a dying star. Roger Wesson from Cardiff University tells us more about this phase of a Sun-like star’s stellar lifecycle and how Webb observations have given him and his colleagues valuable insights into the formation and evolution of these objects, hinting at a key role for binary companions.

This image of the Ring Nebula appears as a distorted doughnut. The nebula’s inner cavity hosts shades of blue and green, while the detailed ring transitions through shades of orange in the inner regions and pink in the outer region. The ring’s inner region has distinct filament elements.
NASA’s James Webb Space Telescope has observed the well-known Ring Nebula in unprecedented detail. Formed by a star throwing off its outer layers as it runs out of fuel, the Ring Nebula is an archetypal planetary nebula. This new image from Webb’s NIRCam (Near-Infrared Camera) shows intricate details of the filament structure of the inner ring. There are some 20,000 dense globules in the nebula, which are rich in molecular hydrogen. In contrast, the inner region shows very hot gas. The main shell contains a thin ring of enhanced emission from carbon-based molecules known as polycyclic aromatic hydrocarbons (PAHs). Download the full-resolution version from the Space Telescope Science Institute. Credit: ESA/Webb, NASA, CSA, M. Barlow (University College London), N. Cox (ACRI-ST), R. Wesson (Cardiff University)

“Planetary nebulae were once thought to be simple, round objects with a single dying star at the center. They were named for their fuzzy, planet-like appearance through small telescopes. Only a few thousand years ago, that star was still a red giant that was shedding most of its mass. As a last farewell, the hot core now ionizes, or heats up, this expelled gas, and the nebula responds with colorful emission of light. Modern observations, though, show that most planetary nebulae display breathtaking complexity. It begs the question: how does a spherical star create such intricate and delicate non-spherical structures?

“The Ring Nebula is an ideal target to unravel some of the mysteries of planetary nebulae. It is nearby, approximately 2,200 light-years away, and bright – visible with binoculars on a clear summer evening from the northern hemisphere and much of the southern. Our team, named the ESSENcE (Evolved StarS and their Nebulae in the JWST Era) team, is an international group of experts on planetary nebulae and related objects. We realized that Webb observations would provide us with invaluable insights, since the Ring Nebula fits nicely in the field of view of Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) instruments, allowing us to study it in unprecedented spatial detail. Our proposal to observe it was accepted (General Observers program 1558), and Webb captured images of the Ring Nebula just a few weeks after science operations started on July 12, 2022.

“When we first saw the images, we were stunned by the amount of detail in them. The bright ring that gives the nebula its name is composed of about 20,000 individual clumps of dense molecular hydrogen gas, each of them about as massive as the Earth. Within the ring, there is a narrow band of emission from polycyclic aromatic hydrocarbons, or PAHs – complex carbon-bearing molecules that we would not expect to form in the Ring Nebula. Outside the bright ring, we see curious “spikes” pointing directly away from the central star, which are prominent in the infrared but were only very faintly visible in Hubble Space Telescope images. We think these could be due to molecules that can form in the shadows of the densest parts of the ring, where they are shielded from the direct, intense radiation from the hot central star.

 This image of the Ring Nebula appears as a distorted doughnut. The nebula’s inner cavity hosts shades of red and orange, while the detailed ring transitions through shades of yellow in the inner regions and blue/purple in the outer region. The ring’s inner region has distinct filament elements.
This new image of the Ring Nebula from Webb’s MIRI (Mid-InfraRed Instrument) reveals particular details in the concentric features in the outer regions of the nebulae’s ring. Roughly ten concentric arcs located just beyond the outer edge of the main ring. The arcs are thought to originate from the interaction of the central star with a low-mass companion orbiting at a distance comparable to that between the Earth and Pluto. Download the full-resolution version from the Space Telescope Science Institute. Credit: ESA/Webb, NASA, CSA, M. Barlow (University College London), N. Cox (ACRI-ST), R. Wesson (Cardiff University)

“Our MIRI images provided us with the sharpest and clearest view yet of the faint molecular halo outside the bright ring. A surprising revelation was the presence of up to ten regularly-spaced, concentric features within this faint halo. These arcs must have formed about every 280 years as the central star was shedding its outer layers. When a single star evolves into a planetary nebula, there is no process that we know of that has that kind of time period. Instead, these rings suggest that there must be a companion star in the system, orbiting about as far away from the central star as Pluto does from our Sun. As the dying star was throwing off its atmosphere, the companion star shaped the outflow and sculpted it. No previous telescope had the sensitivity and the spatial resolution to uncover this subtle effect.

“So how did a spherical star form such a structured and complicated nebulae as the Ring Nebula? A little help from a binary companion may well be part of the answer.”

Related Links:


  • Roger Wesson is a research associate in the School of Physics and Astronomy at Cardiff University, UK and a co-investigator on the ESSENcE program.
  • Mikako Matsuura is a reader (equivalent to associate professor) in the School of Physics and Astronomy at Cardiff University, UK and a co-investigator on the ESSENcE program.
  • Albert A. Zijlstra is a professor of astrophysics at the University of Manchester, UK and a co-investigator on the ESSENcE program.

Talking with Webb using the Deep Space Network

NASA’s James Webb Space Telescope is nearly 1 million miles (1.5 million kilometer) away from Earth, orbiting around the Sun-Earth Lagrange point 2. How do we send commands and receive telemetry – the science and engineering data from the observatory – from that far away? We use the DSN (Deep Space Network) to communicate with the observatory. We receive data when we have a contact with Webb using a DSN antenna

Sandy Kwan, the mission interface manager for Webb within the DSN, notes that each mesmerizing Webb image that has graced our screens would not have been possible without the support of the DSN antennas and personnel, the backbone of interplanetary communication.

The DSN has three sites around the world, each positioned 120 degrees apart. There are antennas in Goldstone, California; Canberra, Australia; and Madrid, Spain. This allows us to communicate with Webb at any time of day, as the Earth rotates. The DSN is managed by NASA’s Jet Propulsion Laboratory (JPL) in Southern California. Kari Bosley, the lead Webb mission planner at the Space Telescope Science Institute (STScI), walks us through more of this communication process between Webb and the DSN.

This is the 34 meter antenna at Goldstone, California.  The dish is enormous, taking up the bottom right half of the image. The dish, which is white with lines running through it is turned up to the sky. It has a white base that attaches it to the ground, and a smaller building to its left, partially blocked by shrubs and bush.   Behind it are low mountains and a mostly clear sky, with faint layers of clouds off in the distance behind the mountains.
34-meter antenna at Goldstone, CA. Image credit: Kari Bosley
This is the 70 meter antenna at Goldstone, California.  The dish is enormous, taking up most of the image. The dish, which is white with lines running through it is turned up to the sky. It has a white base that attaches it to the ground.  Four people stand to the left giving a sense of scale.  Behind it are low mountains and a mostly cloudy sky.
70-meter antenna at Goldstone, CA. Image credit: Kari Bosley

“How do we plan contact time with Webb? It’s not as simple as picking up the phone and calling the telescope. In order for Earth to connect with Webb there are a few things that happen prior to scheduling a contact. On average, the Webb mission operations center connects with the observatory at least 2-3 times in a 24-hour period. There are mission planners at STScI where the Mission Operations Center (MOC) is located, mission schedulers at JPL, and of course at the DSN complexes. The mission planners at STScI work together with the mission schedulers at JPL to create contacts with Webb.

“How do we know when we can contact Webb? The Flight Dynamics Facility at NASA’s Goddard Space Flight Center sends the MOC at STScI the view periods in which the observatory is visible from those three different DSN sites. The mission scheduler compares those times to what is available in the scheduling system where other missions are competing for time with their spacecraft. All missions require specific amounts of time to communicate with their spacecraft, and the timing depends on where the spacecraft are in space. There are times when conflicts between multiple missions request the same resource at the same time. When this happens, our mission scheduler at JPL will negotiate with other missions to come to a compromise that satisfies all of the missions. Once all negotiations are complete, schedules are sent to the mission planners up to 6 months in advance. The scheduling for the first 8 weeks is fixed, with no changes allowed unless there is an emergency or important event with a spacecraft. The later periods are subject to continuing negotiations.

“Each of the DSN complexes has different types of antennas, including 70-meter (230-foot in diameter), 34-meter (111-foot in diameter), and 26-meter (85-foot in diameter) antennas. The DSN complexes use the 34-meter antennas to talk with Webb with the 70-meter antennas as a backup. The DSN supports different radio frequency allocations, such as the S-band and Ka-band frequencies that Webb uses. S-band has a lower bandwidth, and we use that to send commands to the spacecraft (e.g., start recorder playback), to receive engineering telemetry to monitor the health and safety of the observatory, and for ranging. Ranging is the process of determining Webb’s position and trajectory by the delay between when the signal is sent up and when it is received back on the ground.

“We use Ka-band to downlink stored science and engineering data, and some telemetry from the spacecraft. If we used S-band to downlink data, it would take many days to download each day’s data. With Ka-band, it takes much less time, and we can usually complete download all of the stored data in a couple of hours. The high gain antenna on Webb is used for Ka-band downlink and the medium gain antenna is used for S-band uplink and downlink when both antennas are pointed directly at the complex for a contact. Most of our contacts are 2-6 hours in length. Normally, we request at least 4-hour contacts. Since DSN hosts almost 40 different missions, scheduling is complicated.

“Infographic about communication between the James Webb Space Telescope and the Deep Space Network. Drawing of the telescope is centered at top, with a large ground-based radio antenna centered underneath it, labeled Deep Space Network (DSN). Three dotted-line arrows indicate communication between the telescope and DSN. One green arrow going up to the telescope, one green arrow going back down to the DSN, and a thicker blue arrow going down to the DSN. Green text at left reads, S-band uplink: 16 kbps, Commanding. S-band downlink: 40 kbps, Ranging. Blue text on the right, corresponding with the thicker blue arrow, reads Ka-band downlink: 28 Mbps, stored science and engineering data, telemetry.
Webb talks to the Deep Space Network of antennas using S-band and Ka-band radio frequencies. For S-band communication, commanding instructions are uplinked at 16 kilobits per second (kbps) and observatory engineering telemetry and ranging are downlinked at 40 kbps. For Ka-band communication, stored science and engineering data and telemetry is downlinked at 28 Megabits per second. Image Credit: STScI

“There are times when our contacts are very short and times when they are longer. In each contact, it is important to downlink as much data as we can since the telescope continually makes science observations and acquires more data. When we are not in contact, the telescope continues to autonomously perform science observations. These data are stored on a solid-state recorder and downlinked on our next contact. After the Webb MOC at STScI receives the data and ingests them into the Barbara A. Mikulski Archive for Space Telescope for processing and calibration, the observers will receive the data from their observations.

Phillip Johnson is an operations controller and command controller in the Webb Mission Operations Center (MOC) at the Space Telescope Science Institute (STScI). He works to ensure the health and safety of the observatory, and work in close concert with the ground systems engineers who keep the MOC in contact with the DSN. Image credit: STScI

“Those interested in seeing the downlink and uplink between NASA missions and the DSN can visit the ‘Deep Space Network Now’ website at https://eyes.nasa.gov/dsn/dsn.html. You can view the missions and resources that are actively being used at DSN.”


About the author:

Kari Bosley is the lead mission planner in the Ground Systems Engineering Branch at the Space Telescope Science Institute. She schedules the activities that are executed onboard the James Webb Space Telescope. She also collaborates with other mission planners and schedulers to obtain contact time for Webb through Deep Space Network. Kari thanks Carl Hansen (Webb spacecraft systems engineer at STScI) for providing information on the subject of ranging and data rates.



Join Celebrations of Webb’s First Year of Science

July 12 marks the first anniversary of science and amazing discoveries from NASA’s James Webb Space Telescope. To celebrate the year of spectacular discoveries, on July 12, 14 and throughout the summer, there will be multiple events online and live across the U.S. where the public can join in.

The schedule of Webb first anniversary events follows:

Wednesday, July 12

6 a.m. EST: Release of a new Webb image online 

Visit www.nasa.gov/webb for the unveiling of a new Webb science image to commemorate the first year of science. High-resolution downloads and supplemental content will be available for download at https://webbtelescope.org/news from the Space Telescope Science Institute.

4– 5p.m. EST: NASA Science Live event online

Two Webb experts will be featured on a NASA Science Live episode, highlighting the year of stunning Webb images and discoveries. They will discuss how Webb has made an impact on exploring the distant universe, characterizing exoplanet atmospheres, and understanding the solar system.

Watch on NASA Live or on NASA Science Live homepage. Ask your questions about the anniversary image or about other interesting Webb discoveries by using the hashtag “askNASA” and you may see your questions answered during this program.

Friday, July 14

1– 4p.m. EST: Webb Anniversary Event at the Enoch Pratt Free Library in Baltimore, Maryland

Webb team members will host this in-person event at the Enoch Pratt Free Library in Baltimore, Maryland. Visitors of all ages can learn about the telescope and how it studies the universe with its infrared eyes. There will be talks about Webb, a Virtual Reality experience, hands-on activities for children, and educational giveaway items.

This Summer:  

Webb Anniversary Community Events, Nationwide

Throughout the summer, locations around the country are hosting Webb Community Events to celebrate the Webb anniversary. These free, public events will be held at schools, libraries, museums, and other community locations in 25 states and Washington, D.C., in July, August, and September. Each event will be different, but all will highlight Webb science and accomplishments with information, activities, or a presentation.




Saturn’s Rings Shine in Webb’s Observations of Ringed Planet

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

The background is mostly dark. At the center is a dark orange-brownish circle, surrounded by several blazing bright, thick, horizontal whiteish rings. This is Saturn and its rings. There are three tiny organ-like dots in the image—one to the upper left of the planet, one to the direct left of the planet, and the lower left of the planet. They are labeled Dione, Enceladus, and Tethys. There is a slightly darker tint at the northern and southern poles of the planet. The rings surrounding Saturn are mostly broad, with a few singular narrow gaps between the broader rings. At the right side of the planet, labels are applied to the rings. The innermost, thicker ring is labeled “C ring.” Next to that, a brighter, wider ring is labeled “B ring.” Traveling farther outward, a small dark gap is labeled “Cassini division” before another thicker ring labeled “A ring.” Within the “A ring,” a narrow faint band is labeled “Encke gap.” The outermost, faintest, thinnest ring is labeled “F ring.”
Image of Saturn and some of its moons, captured by the James Webb Space Telescope’s NIRCam instrument on June 25, 2023. In this monochrome image, NIRCam filter F323N (3.23 microns) was color mapped with an orange hue. Download the full-resolution image, both labeled and unlabeled, from the Space Telescope Science Institute. Credits: NASA, ESA, CSA, STScI, M. Tiscareno (SETI Institute), M. Hedman (University of Idaho), M. El Moutamid (Cornell University), M. Showalter (SETI Institute), L. Fletcher (University of Leicester), H. Hammel (AURA); image processing by J. DePasquale (STScI)

On June 25, 2023, NASA’s James Webb Space Telescope turned to famed ringed world Saturn for its first near-infrared observations of the planet. The initial imagery from Webb’s NIRCam (Near-Infrared Camera) is already fascinating researchers.

Saturn itself appears extremely dark at this infrared wavelength observed by the telescope, as methane gas absorbs almost all of the sunlight falling on the atmosphere. However, the icy rings stay relatively bright, leading to the unusual appearance of Saturn in the Webb image.

This image was taken as part of Webb Guaranteed Time Observation program 1247. The program included several very deep exposures of Saturn, which were designed to test the telescope’s capacity to detect faint moons around the planet and its bright rings. Any newly discovered moons could help scientists put together a more complete picture of the current system of Saturn, as well as its past.

This new image of Saturn clearly shows details within the planet’s ring system, along with several of the planet’s moons – Dione, Enceladus, and Tethys. Additional deeper exposures (not shown here) will allow the team to probe some of the planet’s fainter rings, not visible in this image, including the thin G ring and the diffuse E ring. Saturn’s rings are made up of an array of rocky and icy fragments – the particles range in size from smaller than a grain of sand to a few as large as mountains on Earth. Researchers recently used Webb to explore Enceladus, and found a large plume jetting from the southern pole of the moon that contains both particles and plentiful amounts of water vapor – this plume feeds Saturn’s E ring.

Saturn’s atmosphere also shows surprising and unexpected detail. Although the Cassini spacecraft observed the atmosphere at greater clarity, this is the first time that the planet’s atmosphere has been seen with this clarity at this particular wavelength (3.23 microns), which is unique to Webb. The large, dark, diffuse structures in the northern hemisphere do not follow the planet’s lines of latitude, so this image is lacking the familiar striped appearance that is typically seen from Saturn’s deeper atmospheric layers. The patchiness is reminiscent of large-scale planetary waves in the stratospheric aerosols high above the main clouds, potentially similar to those seen in early Webb NIRCam observations of Jupiter.

When comparing the northern and southern poles of the planet in this image, the differences in appearance are typical with known seasonal changes on Saturn. For example, Saturn is currently experiencing northern summertime, with the southern hemisphere emerging from the darkness at the end of a winter. However, the northern pole is particularly dark, perhaps due to an unknown seasonal process affecting polar aerosols in particular. A tiny hint of brightening towards the edge of Saturn’s disk might be due to high-altitude methane fluorescence (the process of emitting light after absorbing light), emission from the trihydrogen ion (H3+) in the ionosphere, or both; spectroscopy from Webb could help confirm this.

Missions like NASA’s Pioneer 11, Voyagers 1 and 2, the Cassini spacecraft, and the Hubble Space Telescope have tracked Saturn’s atmosphere and rings for many decades. These observations from Webb are just a hint at what this observatory will add to Saturn’s story in the coming years as the science team delves deep into the data to prepare peer-reviewed results. 

Science Credits

NASA, ESA, CSA, STScI, Matt Tiscareno (SETI Institute), Matt Hedman (University of Idaho), Maryame El Moutamid (Cornell University), Mark Showalter (SETI Institute), Leigh Fletcher (University of Leicester), Heidi Hammel (AURA)

Image Processing Credits

J. DePasquale (STScI)

About the Authors

  • Heidi B. Hammel is a Webb interdisciplinary scientist leading Webb’s Cycle 1 Guaranteed Time Observations (GTO) of the solar system. She is the vice president for science at the Association of Universities for Research in Astronomy (AURA) in Washington, D.C.
  • Leigh Fletcher is a professor of planetary science at the University of Leicester in England. Leigh is the principal investigator for several of Webb’s Guaranteed Time Observation Programs, including Program 1247 highlighted here.
  • Matt Tiscareno is a Senior Research Scientist at the SETI Institute, California, where he studies the dynamics of planetary systems, including planetary rings. He is an integral member of the Webb Guaranteed Time Observation team for the study of Saturn.

Unannotated Version of Image:

The background is mostly dark. At the center is a dark orange-brownish circle, surrounded by several blazing bright, thick, horizontal whiteish rings. This is Saturn and its rings. There are three tiny organ-like dots in the image—one to the upper left of the planet, one to the direct left of the planet, and the lower left of the planet. There is a slightly darker tint at the northern and southern poles of the planet. The rings surrounding Saturn are mostly broad, with a few singular narrow gaps between the broader rings. There is an innermost, thicker ring, and next to that is a brighter, wider ring. Traveling farther outward, there is a small dark gap before another thicker ring. In the thicker ring, there is a narrow faint band. There is then an outermost, faintest, thinnest ring.
Download the full-resolution image, both labeled and unlabeled, from the Space Telescope Science Institute.

The Telescope Allocation Committee: Selecting What Webb Observes Next

This week, astronomers around the world are celebrating the announcement of the next cycle of Webb observations. We asked Christine Chen, associate astronomer and JWST Science Policies Group lead at the Space Telescope Science Institute, to describe the selection process to determine the targets Webb will observe.

“On May 10, the Space Telescope Science Institute (STScI), the science operations center for NASA’s James Webb Space Telescope, announced the scientific program for Cycle 2, the second year of regular operations. This announcement was the culmination of a peer-review process to select the most scientifically compelling programs, which began with the submission of observing and archival proposals on January 27.

“For every year of regular operations, STScI plans to issue a Call for General Observer and Archival proposals from the international astronomical community to solicit ideas for new observations and archival studies to be executed in the upcoming year. Archival proposals request support to analyze already existing observations, develop theoretical models to interpret observations, and/or develop scientific software to facilitate data analysis. For Cycle 2, a record-breaking 1,600 proposals were submitted by more than 5,450 scientists from 52 countries including the United States, ESA (European Space Agency) member states, and Canada. The proposals covered all topics in astronomy and astrophysics from solar system bodies, exoplanets, supernova remnants, and merging neutron stars to nearby and distant galaxies, supermassive black holes at the centers of galaxies, and the large-scale structure of the universe. Together, the submitted proposals requested more than 35,000 hours of telescope time, far exceeding the 5,000 hours of telescope time available to be allocated.

“To select the programs that will be executed, STScI recruits hundreds of members of the international astronomical community to serve on the Telescope Allocation Committee (TAC). Each reviewer is assigned to a topical panel reflecting their scientific expertise. The peer-review process is carried out such that the proposers don’t know who is reviewing the proposals, and the reviewers don’t know who wrote the proposals, a process called Dual-Anonymous Peer Review (DAPR). STScI instituted DAPR in 2016 in support of the Hubble Space Telescope Cycle 26 TAC and has found that DAPR has decreased a previously-seen disparity in proposal selection rate for male and female investigators and has encouraged many more students to apply for telescope time.

“Once the proposals have been submitted, the STScI JWST Science Policies Group sorts the proposals by type and/or size and by scientific category. Very small proposals, are graded asynchronously by external panelists, whereas larger programs are reviewed by discussion panels. Each panel is given an allocation of telescope time, for which it can recommend observing programs.

“Reviewers are asked to grade each proposal based on three criteria: (1) impact within subfield, (2) out-of-field impact, and (3) suitability for the observatory For external panels, proposals are ranked using submitted grades. For discussion panels, proposals are first triaged using submitted grades because there is not enough time to discuss all of the submitted proposals. At the TAC meeting, the discussion panelists review the strengths and weaknesses of all of the proposals that survive triage, and regrade and re-rank the proposals. The highest ranked proposals are recommended for allocation of telescope time and/or funding. For the Large, Treasury, and Legacy Archive proposals, the panel chairs also receive and incorporate expert reviews from the community and from their discussion panels. In addition, reviewers provide feedback for the proposers detailing the perceived strengths and weaknesses.

“For this mission, the STScI director is the allocating official. Therefore, all of the recommendations from the TAC are advisory to the director. Once the director approves the programs, STScI notifies proposers of the outcome for their proposals and begins implementation of the awarded observations. The selected Cycle 2 program that was just announced contains lots of exciting and ground-breaking science. You can learn more about the breath of research areas and questions to be answered with Webb observations by reading the abstracts of the selected programs. Eventually, all of the observations in the approved programs will become publicly available in the archive, enabling additional new discoveries that may not have been foreseen by the original proposers.”

About the Author:
Christine Chen is an associate astronomer in the Science Mission Office at the Space Telescope Science Institute. She leads the JWST Science Policy Group that issues Calls for Proposals to the astronomical community to conduct research using Webb and organizes Dual Anonymous Peer Review of the proposals submitted by the astronomical community in response to these calls.


Mid-Infrared Instrument Operations Update

All 17 observing modes of the James Webb Space Telescope undergo routine performance monitoring and calibration. This month, while performing calibration by comparing the brightness of standard stars that have been well-cataloged by other observatories to what Webb’s Mid-Infrared Instrument (MIRI) was receiving, team members noticed a discrepancy in the data.

Further analysis of MIRI’s Medium Resolution Spectroscopy (MRS) mode revealed that at the longest wavelengths, the throughput, or the amount of light that is ultimately registered by MIRI’s sensors, has decreased since commissioning last year. No effect has been seen for MIRI imaging, and there is no risk to the instrument. All other observation modes – within MIRI and each of Webb’s other scientific instruments – remain unaffected.

NASA and its partners are developing a systematic plan to approach, analyze, and then explore the issue. The Webb team will continue MIRI observations as planned. The team will gather all relevant ground test and flight data to fully assess MRS performance. Further test observations will be taken to completely characterize the nature of the issue using this particular mode of observation. Next, a plan for long term-monitoring will be enacted, while the team continues to investigate the cause, identify risks, and explore mitigations that would potentially improve performance. One possible mitigation strategy includes taking slightly longer exposures at the affected wavelengths to increase the signal to noise.

Webb Shows Areas of New Star Formation and Galactic Evolution

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

On Oct. 11, 2022, NASA’s James Webb Space Telescope spent over 20 hours observing the long-studied Hubble Ultra Deep Field for the first time. The general observer program (GO 1963) focused on analyzing the field in wavelengths between approximately 2 and 4 microns.

A rectangular image that appears to be two separate square images separated by a wide black gap. The gap obscures the galaxies present between the two square images. Each square image contains thousands of galaxies with many different colors. Some galaxies are shades of yellow, while others are white, blue, orange and red. Most of these galaxies appear as fuzzy ovals, but others appear thin and long. A few galaxies with distinct spiral arms are spread throughout.
This image of the Hubble Ultra Deep Field was taken by the Near-Infrared Camera on NASA’s James Webb Space Telescope. The Webb image observes the field at depths comparable to Hubble – revealing galaxies of similar faintness – in just one-tenth as much observing time. It includes 1.8-micron light shown in blue, 2.1-micron light shown in green, 4.3-micron light shown in yellow, 4.6-micron light shown in orange, and 4.8-micron light shown in red (filters F182M, F210M, F430M, F460M, and F480M). Download the full resolution from the Space Telescope Science Institute. Image Credit: NASA, ESA, CSA, Joseph DePasquale (STScI), Christina Williams (NSF’s NOIRLab).

We spoke with Christina Williams (NSF’s NOIRLab), Sandro Tacchella (University of Cambridge), and Michael Maseda (University of Wisconsin-Madison) to learn more about the first observation of the Hubble Ultra Deep Field through Webb’s eyes.

What is important for people to know about these Webb observations?

Michael Maseda: The fact that we see hot, ionized gas is telling us exactly where stars are being born in these galaxies. Now we can separate those areas from where stars already existed. That piece of information is very important because, billions of years later, we don’t exactly know how galaxies became how they are today. It’s important to note that we still haven’t seen everything there is to see. Our whole program was ~24 hours, which isn’t that much time in the grand scheme of how much time other observatories have looked at it. But, even in this relatively short amount of time, we’re starting to put together a new picture of how galaxies are growing at this really interesting point in the history of the Universe.

What are you interested in learning by exploring the Hubble Ultra Deep Field with Webb?

Christina Williams: We proposed to image the Ultra Deep Field using some of Webb’s NIRCam’s medium-band image filters, which allowed us to take images of spectral features more accurately than we could with broadband filters because medium-band filters span a shorter wavelength range. This gives us more sensitivity in measuring colors, which helps us understand the history of star formation and ionization properties of galaxies during the first billion years of the universe, like in the Reionization Era. Measuring the energy that galaxies produced in that time will help us understand how galaxies reionized the universe, reverting it from being neutral gas to once again being an ionized plasma like it was after the big bang.

Sandro Tacchella: One of the key outstanding questions in extragalactic astrophysics is how the first galaxies form. Since the medium bands cover a range of different wavelengths, we can either directly find the some of the first galaxies in the early universe, or we can age-date the stars in galaxies when the universe was about one billion years old to understand when the galaxy actually formed their stars in the past. This survey helps to pin down the formation of the first galaxies.

Michael: The capabilities that we have with Webb’s medium-band filters are actually quite new. We’re getting a sort of hybrid between imaging and spectroscopy, so we’re getting detailed information for basically all of the galaxies in the field, as opposed to traditional spectroscopy where you could only select a few galaxies in the field of view for study. It’s really a complete picture in the sense that this information complements a lot of existing data, not only from Hubble, but ground-based instrumentation like MUSE (the Multi Unit Spectroscopic Explorer) on the Very Large Telescope, where we have spectroscopy at different wavelengths for a number of these objects. MUSE is very good at finding galaxies that have Lyman-alpha emission, or light from ionized hydrogen in these galaxies, which are the type of galaxies that existed when reionization was ending. This new data is a missing piece that we did not have before in terms of understanding the full population of galaxies in this field.

A comparison between two images, one on the left and one on the right separated by a white line. The image on the left has a caption on the bottom that says, “Hubble UDF (exposure time: 11.3 days)”, while the image on the right has a caption on the bottom that says “Webb (exposure time: 0.83 days)”. The image on the left shows thousands of galaxies with many different colors. Most are orange and yellow while others are white and blue. Many of these galaxies appear as fuzzy ovals, but others look thin and long or have distinct spiral arms. Within the image is a yellow box, angled about 30 degrees off center, which contains a label on its bottom right that says “Webb’s field of view”. The image on the right shows a box with the same angle. Inside this box are many of the same galaxies seen in the box on the left in different colors. The orange galaxies on the left image are white or yellow on the right. There are a few galaxies spread throughout the right image that are clearer than those on the left.
The capabilities of NASA’s James Webb Space Telescope’s Near-Infrared Camera are on full display in this comparison between Hubble’s and Webb’s observation of the Hubble Ultra Deep Field. The left, which demonstrates Hubble’s observation with its Wide Field Camera 3, required an exposure time of 11.3 days, while the right only took 0.83 days. Several areas within the Webb image reveal previously invisible, red galaxies. Download the full resolution from the Space Telescope Science Institute. Image Credit: NASA, ESA, CSA, Joseph DePasquale (STScI).

Was there anything unexpected in these data that surprised you?

 Michael: I don’t know if I was surprised exactly, but the images were even better than I was expecting. In these images, you can actually see by eye that this is ionized gas over a fairly large area. I was expecting everything to be unresolved, but we have a high-enough resolution to actually see it. And I’m pleased to see it because it could have been a lot harder to understand what was happening.

Christina: I think that seeing how beautiful the images are and how high quality they ended up being was definitely a high point. We calculated that we would be able to do things like this, but it was different to see it and have the real data in practice.

Why did you elect to make the data immediately public?

Sandro: Galaxies are very complex systems in which a wide range of different processes work on different spatial and temporal scales, so there are many approaches that can be used to better understand the physics of galaxies. So, making it available to many different groups will facilitate the search for more insight.

Christina: Webb is still very new, and people are still learning the best practices of how to analyze data sets. So, it benefits everyone to have a few data sets that are available immediately to help people understand the best way to make use of Webb data moving forward, and to better plan programs in future cycles that are based on real experience with data.

 About the Authors:

      • Christina Williams is an assistant astronomer at the National Optical Infrared Astronomy Research Lab (NOIRLab) in Tucson, Arizona.
      • Sandro Tacchella is an assistant professor of astrophysics at the University of Cambridge in Cambridge, England.
      • Michael Maseda is an assistant professor of astronomy at University of Wisconsin-Madison in Madison, Wisconsin.

How Webb’s Coronagraphs Reveal Exoplanets in the Infrared

The study of exoplanets is a key part of the James Webb Space Telescope’s science goals. We asked Webb’s Deputy Observatory Project Scientist Christopher Stark of NASA’s Goddard Space Flight Center to tell us about one of the ways Webb studies other worlds.

NASA’s James Webb Space Telescope has many different observing modes to study planets orbiting other stars, known as exoplanets. One way in particular is that Webb can directly detect some of these planets. Directly detecting planets around other stars is no easy feat. Even the nearest 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. At these tiny angular scales, the planet’s faint light is lost in the glare of its host star when trying to observe it.

Fortunately, Webb has the right tools for the job: the Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) coronagraphic modes. Webb’s coronagraphs block the light from a distant star, while allowing the faint planet light through to reach its sensors. This is not unlike how we use our car’s visor during sunset or sunrise to see the cars in front of us, albeit Webb uses a much fancier “visor.”

Webb NIRCam and MIRI coronagraphic images of the exoplanet HIP 65426 b. The white star symbol marks the location of the star blocked out by the coronagraphs. The exoplanet does not display Webb’s hallmark six-spiked diffraction pattern due to the pupil plane coronagraph masks. Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI). Download the full image here.

Along the path light takes through Webb’s optics, there are several important locations called “planes.” The “image plane” is where the distant sky is in focus, including all astrophysical objects. 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 physically mask out unwanted starlight in both the image and pupil planes to optimize performance. Most of Webb’s image plane masks, resembling opaque spots or bars, remove starlight simply by blocking it in the image. The exception to this are MIRI’s “four-quadrant phase masks,” which shift the phase of the light so it cancels out with itself through a process called “destructive interference.”

However, due to the wave nature of light, the image plane masks can’t completely block the star. So Webb uses additional pupil plane masks, also called Lyot stops, to remove much of the remaining starlight. These pupil plane masks look very 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, as shown in the observations above.

Left: NIRCam’s coronagraphic image plane mask hardware, consisting of two wedge-shaped bars and three round spots (from left to right). Right: MIRI’s four coronagraphic image plane mask hardware, consisting of three phase-shifting four quadrant phase masks and one round spot (from left to right).


Illustration of NIRCam’s pupil plane mask/Lyot stop for the round image plane mask (left) and the bar image plane mask (right). Transmission through the mask is limited to the white regions. Webb’s telescope pupil is shown in gray for comparison. Credit: Mao et al. 2011

Webb’s NIRCam instrument has five coronagraphic masks, each of which can each 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. One AU is equivalent to the distance between the Earth and the Sun.

Despite the masks, Webb’s coronagraphs don’t perfectly remove a star’s light. To remove the last remnants of light, Webb’s astronomers will carefully use a variety of “point spread function (PSF) subtraction methods.” Simply put, this means measuring the pattern of the residual starlight, and then subtracting it from the science image. In the end, what remains is a noisy-looking pattern, which ultimately limits the faintest detectable exoplanet. This limit is 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.


Left: Example image of residual starlight after suppression with the MIRI F1065C coronagraph. Right: The same image after PSF subtraction removing most of the remaining stellar residuals. The star is located in the center of the image. The black and yellow pattern in the center of the image set the faintest detectable planet in an observation. Credit: Boccaletti et al. (2022)

Webb’s large primary mirror and infrared capabilities mean that its coronagraphs are uniquely suited to study faint objects in the infrared and will complement other instruments currently observing at other wavelengths, including Hubble’s STIS coronagraph and multiple instruments on ground-based observatories. Exoplanet astronomers will mainly use Webb’s coronagraphs to detect giant extrasolar planets that are still warm from being formed, like those shown above, which are the first images of an exoplanet at wavelengths longer than 5 microns. Webb will also excel at imaging dense circumstellar disks of debris generated by the asteroids and comets in these exoplanetary systems, as well as protoplanetary disks in which planets are still forming. Webb’s coronagraphs can even be used for extragalactic astronomy, to study host galaxies that contain bright active galactic nuclei.

Webb’s coronagraphs won’t be able to reveal all the secrets of a planetary system. To image planets like our own around nearby Sun-like stars, we’ll need to observe even closer to the star and be able to detect planets just one ten billionth the brightness of the star. This will require a future mission fully optimized around next-generation coronagraphs. Fortunately, NASA is already looking into it. The agency’s upcoming Nancy Grace Roman Space Telescope will carry a technology demonstration instrument to test next-generation coronagraph technology. And, 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.  

– Christopher Stark, Webb deputy observatory project scientist, NASA Goddard