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NASA’s Webb Investigates Eternal Sunrises, Sunsets on Distant World


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NASA’s Webb Investigates Eternal Sunrises, Sunsets on Distant World

Illustration of a planet, zoomed in on the planet’s dayside/nightside boundary. The planet encompasses takes up the full image. At the bottom left, the image is dark, depicting the nightside covering the planet in a dark shadow. In the right side of the image, the planet has a fuzzy orange-pink atmosphere with hints of latitudinal wispy cloud bands. The right upper corner is bright, where the star (not illustrated) shines.
Artists concept of WASP-39 b (full image below).

Near-infrared spectral analysis of terminator confirms differences in morning and evening atmosphere

Researchers using NASA’s James Webb Space Telescope have finally confirmed what models have previously predicted: An exoplanet has differences between its eternal morning and eternal evening atmosphere. WASP-39 b, a giant planet with a diameter 1.3 times greater than Jupiter, but similar mass to Saturn that orbits a star about 700 light-years away from Earth, is tidally locked to its parent star. This means it has a constant dayside and a constant nightside—one side of the planet is always exposed to its star, while the other is always shrouded in darkness.

Using Webb’s NIRSpec (Near-Infrared Spectrograph), astronomers confirmed a temperature difference between the eternal morning and eternal evening on WASP-39 b, with the evening appearing hotter by roughly 300 Fahrenheit degrees (about 200 Celsius degrees). They also found evidence for different cloud cover, with the forever morning portion of the planet being likely cloudier than the evening.

Image A: Artist Concept WASP-39 b

Illustration of a planet, zoomed in on the planet’s dayside/nightside boundary. The planet encompasses takes up the full image. At the bottom left, the image is dark, depicting the nightside covering the planet in a dark shadow. In the right side of the image, the planet has a fuzzy orange-pink atmosphere with hints of latitudinal wispy cloud bands. The right upper corner is bright, where the star (not illustrated) shines.
This artist’s concept shows what the exoplanet WASP-39 b could look like based on indirect transit observations from NASA’s James Webb Space Telescope as well as other space- and ground-based telescopes. Data collected by Webb’s NIRSpec (Near-Infrared Spectrograph) show variations between the eternal morning and evening atmosphere of the planet.

Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet’s terminator, the boundary that separates the planet’s dayside and nightside. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. When making that comparison, researchers can get information about the temperature, composition, and other properties of the planet’s atmosphere.

“WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author on the study. “It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.”

Previously published Webb spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, represent the entire day/night boundary – there was no detailed attempt to differentiate between one side and the other.

Now, the new analysis builds two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one from the evening, and the other from the morning. Data reveals the evening as significantly hotter, a searing 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius).

Image B: Transmission Spectra

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“It’s really stunning that we are able to parse this small difference out, and it’s only possible due Webb’s sensitivity across near-infrared wavelengths and its extremely stable photometric sensors,” said Espinoza. “Any tiny movement in the instrument or with the observatory while collecting data would have severely limited our ability to make this detection. It must be extraordinarily precise, and Webb is just that.”

Extensive modeling of the data obtained also allows researchers to investigate the structure of WASP-39 b’s atmosphere, the cloud cover, and why the evening is hotter. While future work by the team will study how the cloud cover may affect temperature, and vice versa, astronomers confirmed gas circulation around the planet as the main culprit of the temperature difference on WASP-39 b.

On a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers generally expect the gas to be moving as the planet rotates around its star: Hotter gas from the dayside should move through the evening to the nightside via a powerful equatorial jet stream. Since the temperature difference is so extreme, the air pressure difference would also be significant, which in turn would cause high wind speeds.

Image C: Transit Light Curve

webb-stsci-01j2f16qhy22whj26m4cfbmpc5-2k

Using General Circulation Models, 3-dimensional models similar to the ones used to predict weather patterns on Earth, researchers found that on WASP-39 b the prevailing winds are likely moving from the night side across the morning terminator, around the dayside, across the evening terminator and then around the nightside. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side gets slammed with winds of air that have been cooled on the nightside, while the evening is hit by winds of air heated on the dayside. Research suggests the wind speeds on WASP-39 b can reach thousands of miles an hour!

“This analysis is also particularly interesting because you’re getting 3D information on the planet that you weren’t getting before,” added Espinoza. “Because we can tell that the evening edge is hotter, that means it’s a little puffier. So, theoretically, there is a small swell at the terminator approaching the nightside of the planet.”

The team’s results have been published in Nature.

The researchers will now look to use the same method of analysis to study atmospheric differences of other tidally locked hot Jupiters, as part of  Webb Cycle 2 General Observers Program 3969.

WASP-39 b was among the first targets analyzed by Webb as it began regular science operations in 2022. The data in this study was collected under Early Release Science program 1366, designed to help scientists quickly learn how to use the telescope’s instruments and realize its full science potential.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

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The research results have been published in Nature.

Media Contacts

Rob Gutrorob.gutro@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Hannah Braun hbraun@stsci.edu Christine Pulliamcpulliam@stsci.edu
Space Telescope Science Institute, Baltimore, Md.

ARTICLE: What is an Exoplanet?

VIDEO: How do we learn about a planet’s Atmosphere?

VIDEO: Reading the Rainbow of Light from an Exoplanet’s Atmosphere

VIDEO: Science Snippets – Exoplanets

BLOG: Reconnaissance of Potentially Habitable Worlds with NASA’s Webb

More Webb News – https://science.nasa.gov/mission/webb/latestnews/

More Webb Images – https://science.nasa.gov/mission/webb/multimedia/images/

Webb Mission Page – https://science.nasa.gov/mission/webb/

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      Webb has revealed a new type of galaxy: a distant population of mysteriously compact, bright, red galaxies dubbed Little Red Dots. What makes Little Red Dots so bright and so red? Are they lit up by dense groupings of unusually bright stars or by gas spiraling into a supermassive black hole, or both? And whatever happened to them? Little Red Dots seem to have appeared in the universe around 600 million years after the big bang (13.2 billion years ago), and rapidly declined in number less than a billion years later. Did they evolve into something else? If so, how? Webb is probing Little Red Dots in more detail to answer these questions.
      3. Pulsating stars and a triply lensed supernova are further evidence that the “Hubble Tension” is real.
      How fast is the universe expanding? It’s hard to say because different ways of calculating the current expansion rate yield different results — a dilemma known as the Hubble Tension. Are these differences just a result of measurement errors, or is there something weird going on in the universe? So far, Webb data indicates that the Hubble Tension is not caused by measurement errors. Webb was able to distinguish pulsating stars from nearby stars in a crowded field, ensuring that the measurements weren’t contaminated by extra light. Webb also discovered a distant, gravitationally lensed supernova whose image appears in three different locations and at three different times during its explosion. Calculating the expansion rate based on the brightness of the supernova at these three different times provides an independent check on measurements made using other techniques. Until the matter of the Hubble Tension is settled, Webb will continue measuring different objects and exploring new methods.
      4. Webb has found surprisingly rich and varied atmospheres on gas giants orbiting distant stars.
      While NASA’s Hubble Space Telescope made the first detection of gases in the atmosphere of a gas giant exoplanet (a planet outside our solar system), Webb has taken studies to an entirely new level. Webb has revealed a rich cocktail of chemicals, including hydrogen sulfide, ammonia, carbon dioxide, methane, and sulfur dioxide — none of which had been clearly detected in an atmosphere outside our solar system before. Webb has also been able to examine exotic climates of gas giants as never before, detecting flakes of silica “snow” in the skies of the puffy, searing-hot gas giant WASP-17 b, for example, and measuring differences in temperature and cloud cover between the permanent morning and evening skies of WASP-39 b.
      Image: Spectrum of WASP-107 b
      A transmission spectrum of the “warm Neptune” exoplanet WASP-107 b captured by NASA’s Hubble and Webb space telescopes, shows clear evidence for water, carbon dioxide, carbon monoxide, methane, sulfur dioxide, and ammonia in the planet’s atmosphere. These measurements allowed researchers to estimate the interior temperature and mass of the core of the planet, as well as understand the chemistry and dynamics of the atmosphere. NASA, ESA, CSA, Ralf Crawford (STScI) 5. A rocky planet 40 light-years from Earth may have an atmosphere fed by gas bubbling up from its lava-covered surface.
      Detecting, let alone analyzing, a thin layer of gas surrounding a small rocky planet is no easy feat, but Webb’s extraordinary ability to measure extremely subtle changes in the brightness of infrared light makes it possible. So far, Webb has been able to rule out significant atmosphere on a number of rocky planets, and has found tantalizing signs of carbon monoxide or carbon dioxide on 55 Cancri e, a lava world that orbits a Sun-like star. With findings like these, Webb is laying the groundwork for NASA’s future Habitable Worlds Observatory, which will be the first mission purpose-built to directly image and search for life on Earth-like planets around Sun-like stars.
      6. Webb exposes the skeletal structure of nearby spiral galaxies in mesmerizing detail.
      We already knew that galaxies are collections of stars, planets, dust, gas, dark matter, and black holes: cosmic cities where stars form, live, die, and are recycled into the next generation. But we had never been able to see the structure of a galaxy and the interactions between stars and their environment in such detail. Webb’s infrared vision reveals filaments of dust that trace the spiral arms, old star clusters that make up galactic cores, newly forming stars still encased in dense cocoons of glowing dust and gas, and clusters of hot young stars carving enormous cavities in the dust. It also elucidates how stellar winds and explosions actively reshape their galactic homes.
      Image: PHANGS Phantom Galaxy (M74/NGC 628)
      A near- to mid-infrared image from NASA’s James Webb Space Telescope highlights details in the complex structure of a nearby galaxy that are invisible to other telescopes. The image of NGC 628, also known as the Phantom Galaxy, shows spiral arms with lanes of warm dust (represented in red), knots of glowing gas (orange-yellow), and giant bubbles (black) carved by hot, young stars. The dust-free core of the galaxy is filled with older, cooler stars (blue). NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS team 7. It can be hard to tell the difference between a brown dwarf and a rogue planet.
      Brown dwarfs form like stars, but are not dense or hot enough to fuse hydrogen in their cores like stars do. Rogue planets form like other planets, but have been ejected from their system and no longer orbit a star. Webb has spotted hundreds of brown-dwarf-like objects in the Milky Way, and has even detected some candidates in a neighboring galaxy. But some of these objects are so small — just a few times the mass of Jupiter — that it is hard to figure out how they formed. Are they free-floating gas giant planets instead? What is the least amount of material needed to form a brown dwarf or a star? We’re not sure yet, but thanks to three years of Webb observations, we now know there is a continuum of objects from planets to brown dwarfs to stars.
      8. Some planets might be able to survive the death of their star.
      When a star like our Sun dies, it swells up to form a red giant large enough to engulf nearby planets. It then sheds its outer layers, leaving behind a super-hot core known as a white dwarf. Is there a safe distance that planets can survive this process? Webb might have found some planets orbiting white dwarfs. If these candidates are confirmed, it would mean that it is possible for planets to survive the death of their star, remaining in orbit around the slowly cooling stellar ember.
      9. Saturn’s water supply is fed by a giant fountain of vapor spewing from Enceladus.
      Among the icy “ocean worlds” of our solar system, Saturn’s moon Enceladus might be the most intriguing. NASA’s Cassini mission first detected water plumes coming out of its southern pole. But only Webb could reveal the plume’s true scale as a vast cloud spanning more than 6,000 miles, about 20 times wider than Enceladus itself. This water spreads out into a donut-shaped torus encircling Saturn beyond the rings that are visible in backyard telescopes. While a fraction of the water stays in that ring, the majority of it spreads throughout the Saturnian system, even raining down onto the planet itself. Webb’s unique observations of rings, auroras, clouds, winds, ices, gases, and other materials and phenomena in the solar system are helping us better understand what our cosmic neighborhood is made of and how it has changed over time.
      Video: Water plume and torus from Enceladus
      A combination of images and spectra captured by NASA’s James Webb Space Telescope show a giant plume of water jetting out from the south pole of Saturn’s moon Enceladus, creating a donut-shaped ring of water around the planet.
      Credit: NASA, ESA, CSA, G. Villanueva (NASA’s Goddard Space Flight Center), A. Pagan (STScI), L. Hustak (STScI) 10. Webb can size up asteroids that may be headed for Earth.
      In 2024 astronomers discovered an asteroid that, based on preliminary calculations, had a chance of hitting Earth. Such potentially hazardous asteroids become an immediate focus of attention, and Webb was uniquely able to measure the object, which turned out to be the size of a 15-story building. While this particular asteroid is no longer considered a threat to Earth, the study demonstrated Webb’s ability to assess the hazard.
      Webb also provided support for NASA’s Double Asteroid Redirection Test (DART) mission, which deliberately smashed into the Didymos binary asteroid system, showing that a planned impact could deflect an asteroid on a collision course with Earth. Both Webb and Hubble observed the impact, serving witness to the resulting spray of material that was ejected. Webb’s spectroscopic observations of the system confirmed that the composition of the asteroids is probably typical of those that could threaten Earth.
      —-
      In just three years of operations, Webb has brought the distant universe into focus, revealing unexpectedly bright and numerous galaxies. It has unveiled new stars in their dusty cocoons, remains of exploded stars, and skeletons of entire galaxies. It has studied weather on gas giants, and hunted for atmospheres on rocky planets. And it has provided new insights into the residents of our own solar system.
      But this is only the beginning. Engineers estimate that Webb has enough fuel to continue observing for at least 20 more years, giving us the opportunity to answer additional questions, pursue new mysteries, and put together more pieces of the cosmic puzzle.
      For example: What were the very first stars like? Did stars form differently in the early universe? Do we even know how galaxies form? How do stars, dust, and supermassive black holes affect each other? What can merging galaxy clusters tell us about the nature of dark matter? How do collisions, bursts of stellar radiation, and migration of icy pebbles affect planet-forming disks? Can atmospheres survive on rocky worlds orbiting active red dwarf stars? Is Uranus’s moon Ariel an ocean world?
      As with any scientific endeavor, every answer raises more questions, and Webb has shown that its investigative power is unmatched. Demand for observing time on Webb is at an all-time high, greater than any other telescope in history, on the ground or in space. What new findings await?
      By Dr. Macarena Garcia Marin and Margaret W. Carruthers, Space Telescope Science Institute, Baltimore, Maryland
      Media Contacts
      Laura Betz – laura.e.betz@nasa.gov
      NASA’s Goddard Space Flight Center, Greenbelt, Md.
      Christine Pulliam – cpulliam@stsci.edu
      Space Telescope Science Institute, Baltimore, Md.
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      Last Updated Jul 02, 2025 Editor Marty McCoy Contact Laura Betz laura.e.betz@nasa.gov Related Terms
      James Webb Space Telescope (JWST) Astrophysics Black Holes Brown Dwarfs Exoplanet Science Exoplanets Galaxies Galaxies, Stars, & Black Holes Goddard Space Flight Center Nebulae Science & Research Star-forming Nebulae Stars Studying Exoplanets The Universe View the full article
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