NASA’s Webb Telescope Reveals Secrets of Sub-Neptunes

Introduction:

NASA’s James Webb Space Telescope (JWST) is unlocking the secrets of one of the universe’s most common — yet elusive — types of exoplanets: sub-Neptunes. Though not found in our own solar system, these planets dominate the catalog of exoplanets in our galaxy. They are larger than Earth but smaller than Neptune, and until recently, their hazy atmospheres made them nearly impossible to study in detail.

That all changed with a new observation of the exoplanet TOI-421 b. Thanks to the advanced capabilities of the Webb Telescope, astronomers finally got a clear look at a sub-Neptune’s atmosphere — and what they found was surprising.

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What Are Sub-Neptunes — And Why Are They So Mysterious?

Sub-Neptunes are a class of exoplanets that range in size between Earth and Neptune. While they don’t exist in our solar system, they are the most common type of planet discovered by missions like Kepler. That raises a big question: Why are they everywhere in the galaxy — but not here?

Until recently, the lack of data on their atmospheres meant scientists had few answers. The major challenge? Most sub-Neptunes appear to be cloaked in thick layers of haze or clouds that flatten their transmission spectra. In simpler terms, when light from their host star passes through their atmospheres during transit, it doesn’t reveal much — just a flat line, with no telltale chemical “fingerprints.”

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NASA’s Webb Telescope Breaks the Haze Barrier

The James Webb Space Telescope was designed to see what other telescopes couldn’t. With its powerful infrared instruments, Webb can detect molecules in exoplanet atmospheres that were previously out of reach.

Eliza Kempton, principal investigator and astrophysicist at the University of Maryland, had long waited for a tool like Webb. “I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” she said.

The team focused on TOI-421 b — a planet they believed might be free of the hazy veil seen on other sub-Neptunes. Why? Because TOI-421 b is hot — about 1,340°F (727°C) — well above the 1,070°F threshold where scientists think haze-forming chemical reactions, particularly involving methane, become less likely.

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TOI-421 b: The Clear Atmosphere Breakthrough

Their hunch was right.

When TOI-421 b passed in front of its star, Webb captured a clear transmission spectrum, showing distinct features rather than the usual flat lines. For the first time, scientists were able to analyze the planet’s atmospheric composition in detail.

Brian Davenport, a Ph.D. student at the University of Maryland who led the data analysis, said: “We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere.”

Here’s what they found in the atmosphere of TOI-421 b:

• Water vapor
• Possible signs of carbon monoxide and sulfur dioxide
• No signs of methane or carbon dioxide
• A large amount of hydrogen

That last point was unexpected.

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A Hydrogen-Rich Surprise

Until now, sub-Neptunes observed by Webb seemed to have atmospheres filled with heavy molecules. The assumption was that this was the norm. But TOI-421 b bucks that trend. Its atmosphere is light and dominated by hydrogen — much more like the gas giants in our own solar system.

Kempton explained: “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.”

This suggests that TOI-421 b might have formed under different conditions — and maybe even by different processes — than its cooler, hazier cousins.

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Does TOI-421 b Mimic Its Host Star?

One of the most intriguing findings is that TOI-421 b’s atmospheric makeup closely resembles the elemental composition of its host star. This could mean the planet’s atmosphere formed directly from the gas that built the star — a process more like what we see with Jupiter and Saturn in our own solar system.

That’s a big deal because most sub-Neptunes observed so far orbit red dwarf stars — small, cool stars very different from our Sun. TOI-421 b, on the other hand, orbits a Sun-like star. This makes it a particularly valuable case study for understanding how sub-Neptunes might form in systems more like our own.

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One Off or the First of Many?

Now the question becomes: Is TOI-421 b an outlier — or the first in a new category of hot, haze-free sub-Neptunes?

The only way to know is to study more planets like it. By targeting more sub-Neptunes in the same temperature range and around similar stars, astronomers hope to figure out whether this is a broader pattern or a rare exception.

Davenport summed it up: “We’ve unlocked a new way to look at these sub-Neptunes. These high-temperature planets are amenable to characterization.”

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The Future of Sub-Neptune Research

TOI-421 b’s clear atmosphere and hydrogen-rich composition have already changed the way scientists think about sub-Neptunes. But it’s only the beginning. With the James Webb Space Telescope now fully operational, more discoveries like this are likely in the pipeline.

Researchers are particularly excited about the prospect of studying more hot sub-Neptunes. These planets may offer a shortcut to understanding what sub-Neptunes really are — and why we don’t have any in our own solar system.

For Kempton and her team, the work is personal as well as scientific. “By studying their atmospheres, we’re getting a better understanding of how sub-Neptunes formed and evolved,” she said. “And part of that is understanding why they don’t exist in our solar system.”

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Conclusion: A Clearer View of a Foggy Mystery

The discovery of TOI-421 b’s clear, hydrogen-rich atmosphere marks a major milestone in the study of exoplanets. It proves that sub-Neptunes — once considered almost impossible to analyze — can now be studied in detail, at least under the right conditions.

This isn’t just a technical win for space telescopes. It’s a conceptual shift. We now know that sub-Neptunes are not all the same — and that their diversity might hold clues to how planets form across the universe.

The mystery of sub-Neptunes is far from solved. But thanks to Webb, we’ve pulled back the haze — and gotten our first real look at what lies beneath.