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Explore This Section Webb News Latest News Latest Images Webb’s Blog Awards X (offsite – login reqd) Instagram (offsite – login reqd) Facebook (offsite- login reqd) Youtube (offsite) Overview About Who is James Webb? Fact Sheet Impacts+Benefits FAQ Science Overview and Goals Early Universe Galaxies Over Time Star Lifecycle Other Worlds Observatory Overview Launch Deployment Orbit Mirrors Sunshield Instrument: NIRCam Instrument: MIRI Instrument: NIRSpec Instrument: FGS/NIRISS Optical Telescope Element Backplane Spacecraft Bus Instrument Module Multimedia About Webb Images Images Videos What is Webb Observing? 3d Webb in 3d Solar System Podcasts Webb Image Sonifications Team International Team People Of Webb More For the Media For Scientists For Educators For Fun/Learning 6 Min Read NASA’s Webb Lifts Veil on Common but Mysterious Type of Exoplanet
This artist’s concept shows what the hot sub-Neptune exoplanet TOI-421 b could look like. It is based on spectroscopic data gathered by Webb, as well as previous observations from other telescopes on the ground and in space. Credits:
Illustration: NASA, ESA, CSA, Dani Player (STScI) Though they don’t orbit around our Sun, sub-Neptunes are the most common type of exoplanet, or planet outside our solar system, that have been observed in our galaxy. These small, gassy planets are shrouded in mystery…and often, a lot of haze. Now, by observing exoplanet TOI-421 b, NASA’s James Webb Space Telescope is helping scientists understand sub-Neptunes in a way that was not possible prior to the telescope’s launch.
“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said principal investigator Eliza Kempton of the University of Maryland, College Park. “By studying their atmospheres, we’re getting a better understanding of how sub-Neptunes formed and evolved, and part of that is understanding why they don’t exist in our solar system.”
Image A: Artist’s Concept of TOI-421 b
This artist’s concept shows what the hot sub-Neptune exoplanet TOI-421 b could look like. It is based on spectroscopic data gathered by Webb, as well as previous observations from other telescopes on the ground and in space. Illustration: NASA, ESA, CSA, Dani Player (STScI) Small, Cool, Shrouded in Haze
The existence of sub-Neptunes was unexpected before they were discovered by NASA’s retired Kepler space telescope in the last decade. Now, astronomers are trying to understand where these planets came from and why are they so common.
Before Webb, scientists had very little information on them. While sub-Neptunes are a few times larger than Earth, they are still much smaller than gas-giant planets and typically cooler than hot Jupiters, making them much more challenging to observe than their gas-giant counterparts.
A key finding prior to Webb was that most sub-Neptune atmospheres had flat or featureless transmission spectra. This means that when scientists observed the spectrum of the planet as it passed in front of its host star, instead of seeing spectral features – the chemical fingerprints that would reveal the composition of the atmosphere – they saw only a flat-line spectrum. Astronomers concluded from all of those flat-line spectra that at least certain sub-Neptunes were probably very highly obscured by either clouds or hazes.
Image B: Spectrum of TOI-421 b
A transmission spectrum captured by NASA’s James Webb Space Telescope reveals chemicals in the atmosphere of the hot sub-Neptune exoplanet TOI-421 b. Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI) A Different Kind of Sub-Neptune?
“Why did we observe this planet, TOI-421 b? It’s because we thought that maybe it wouldn’t have hazes,” said Kempton. “And the reason is that there were some previous data that implied that maybe planets over a certain temperature range were less enshrouded by haze or clouds than others.”
That temperature threshold is about 1,070 degrees Fahrenheit. Below that, scientists hypothesized that a complex set of photochemical reactions would occur between sunlight and methane gas, and that would trigger the haze. But hotter planets shouldn’t have methane and therefore perhaps shouldn’t have haze.
The temperature of TOI-421 b is about 1,340 degrees Fahrenheit, well above the presumed threshold. Without haze or clouds, researchers expected to see a clear atmosphere – and they did!
A Surprising Finding
“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” said the University of Maryland’s Brian Davenport, a third-year Ph.D. student who conducted the primary data analysis. “Whereas with many of the other sub-Neptunes that had been previously observed, we know their atmospheres are made of something, but they’re being blocked by haze.”
The team found water vapor in the planet’s atmosphere, as well as tentative signatures of carbon monoxide and sulfur dioxide. Then there are molecules they didn’t detect, such as methane and carbon dioxide. From the data, they can also infer that a large amount of hydrogen is in TOI-421 b’s atmosphere.
The lightweight hydrogen atmosphere was the big surprise to the researchers. “We had recently wrapped our mind around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmospheres, so that had become our expectation, and then we found the opposite,” said Kempton. This suggests TOI-421 b may have formed and evolved differently from the cooler sub-Neptunes observed previously.
Is TOI-421 b Unique?
The hydrogen-dominated atmosphere is also interesting because it mimics the composition of TOI-421 b’s host star. “If you just took the same gas that made the host star, plopped it on top of a planet’s atmosphere, and put it at the much cooler temperature of this planet, you would get the same combination of gases. That process is more in line with the giant planets in our solar system, and it is different from other sub-Neptunes that have been observed with Webb so far,” said Kempton.
Aside from being hotter than other sub-Neptunes previously observed with Webb, TOI-421 b orbits a Sun-like star. Most of the other sub-Neptunes that have been observed so far orbit smaller, cooler stars called red dwarfs.
Is TOI-421b emblematic of hot sub-Neptunes orbiting Sun-like stars, or is it just that exoplanets are very diverse? To find out, the researchers would like to observe more hot sub-Neptunes to determine if this is a unique case or a broader trend. They hope to gain insights into the formation and evolution of these common exoplanets.
“We’ve unlocked a new way to look at these sub-Neptunes,” said Davenport. “These high-temperature planets are amenable to characterization. So by looking at sub-Neptunes of this temperature, we’re perhaps more likely to accelerate our ability to learn about these planets.”
The team’s findings appear on May 5 in the Astrophysical Journal Letters.
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).
To learn more about Webb, visit:
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Space Telescope Science Institute, Baltimore, Md.
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Last Updated May 04, 2025 Editor Marty McCoy Contact Laura Betz laura.e.betz@nasa.gov Related Terms
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Help Classify Galaxies Seen by NASA’s James Webb Space Telescope!
The Galaxy Zoo classification interface shows you an image from NASA’s Webb telescope and asks you questions about it. Image credit: Galaxy Zoo, Zooniverse. Inset galaxy: NASA/STScI/CEERS/TACC/S. Finkelstein/M. Bagley/Z. Levay/A. Pagan NASA needs your help identifying the shapes of thousands of galaxies in images taken by our James Webb Space Telescope with the Galaxy Zoo project. These classifications will help scientists answer questions about how the shapes of galaxies have changed over time, what caused these changes, and why. Thanks to the light collecting power of Webb, there are now over 500,000 images of galaxies on website of the Galaxy Zoo citizen science project—more images than scientists can classify by themselves.
“This is a great opportunity to see images from the newest space telescope,” said volunteer Christine Macmillan from Aberdeen, Scotland. “Galaxies at the edge of our universe are being seen for the first time, just as they are starting to form. Just sign up and answer simple questions about the shape of the galaxy that you are seeing. Anyone can do it, ages 10 and up!”
As we look at more distant objects in the universe, we see them as they were billions of years ago because light takes time to travel to us. With Webb, we can spot galaxies at greater distances than ever before. We’re seeing what some of the earliest galaxies ever detected look like, for the first time. The shapes of these galaxies tell us about how they were born, how and when they formed stars, and how they interacted with their neighbors. By looking at how more distant galaxies have different shapes than close galaxies, we can work out which processes were more common at different times in the universe’s history.
At Galaxy Zoo, you’ll first examine an image from the Webb telescope. Then you will be asked several questions, such as ‘Is the galaxy round?’, or ‘Are there signs of spiral arms?’. If you’re quick, you may even be the first person to see the galaxies you’re asked to classify.
“I’m amazed and honored to be one of the first people to actually see these images! What a privilege!” said volunteer Elisabeth Baeten from Leuven, Belgium.
Galaxy Zoo is a citizen science project with a long history of scientific impact. Galaxy Zoo volunteers have been exploring deep space since July 2007, starting with a million galaxies from a telescope in New Mexico called the Sloan Digital Sky Survey and then, moving on to images from space telescopes like NASA’s Hubble Space Telescope and ESA (European Space Agency)’s Euclid telescope. The project has revealed spectacular mergers, taught us about how the black holes at the center of galaxies affect their hosts, and provided insight into how features like spiral arms form and grow.
Now, in addition to adding new data from Webb, the science team has incorporated an AI algorithm called ZooBot, which will sift through the images first and label the ‘easier ones’ where there are many examples that already exist in previous images from the Hubble Space Telescope. When ZooBot is not confident on the classification of a galaxy, perhaps due to complex or faint structures, it will show it to users on Galaxy Zoo to get their human classifications, which will then help ZooBot learn more. Working together, humans and AI can accurately classify limitless numbers of galaxies. The Galaxy Zoo science team acknowledges support from the International Space Sciences Institute (ISSI), who provided funding for the team to get together and work on Galaxy Zoo. Join the project now.
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Last Updated Apr 29, 2025 Related Terms
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A Boeing-built X-66 full-span model underwent testing in the 11-Foot Transonic Unitary Plan Facility at NASA’s Ames Research Center in California’s Silicon Valley between January and March 2025.NASA / Brandon Torres NASA and Boeing are currently evaluating an updated approach to the agency’s Sustainable Flight Demonstrator project that would focus on demonstrating thin-wing technology with broad applications for multiple aircraft configurations.
Boeing’s proposed focus centers on a ground-based testbed to demonstrate the potential for long, thin-wing technology. Work on the X-66 flight demonstrator – which currently incorporates a more complex transonic truss braced wing concept that uses the same thin wing technology as well as aerodynamic, structural braces — would pause for later consideration based on the thin-wing testbed results and further truss-braced configuration studies.
Under this proposal, all aspects of the X-66 flight demonstrator’s design, as well as hardware acquired or modified for it, would be retained while the long, thin-wing technology is being investigated with more focus. NASA and Boeing would also continue to collaborate on research into the transonic truss-braced wing concept.
The proposal is based on knowledge gained through research conducted under the Sustainable Flight Demonstrator project so far.
Since NASA issued the Sustainable Flight Demonstrator award in 2023, the project has made significant progress toward its goal of informing future generations of more sustainable commercial airliners. Boeing and NASA have collaborated on wind tunnel tests, computational fluid dynamics modeling, and structural design and analysis aimed at exploring how best to approach fuel-efficient, sustainable designs.
This research has built confidence in the substantial potential energy-savings benefits that technologies investigated through the Sustainable Flight Demonstrator project and other NASA research can make possible. The Boeing proposal identifies the thin-wing concept as having broad applications for potential incorporation into aircraft with and without truss braces.
NASA and Boeing are discussing potential options for advancing these sustainable flight technologies. NASA’s ultimate goal for this sustainable aircraft research is to achieve substantial improvements for next-generation airliner efficiency, lower costs for travelers, reduced fuel costs and consumption, and increase U.S. aviation’s technological leadership.
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