NASA's Webb Reveals Secrets of Milky Way's Most Common Type of Planet

Newsweek

06-05-2025

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Astronomers are pulling back the hazy veil on the Milky Way's most common—and arguably most mysterious—type of planet: so-called sub-Neptunes.
Not seen among the planets of our solar system, these are gassy worlds that, as their names imply, fall in size between that of Earth and Neptune.
First discovered by NASA's Kepler space telescope, the atmospheres of sub-Neptune had proven impossible to probe because of how they are shrouded in haze.
However, the James Webb Space Telescope has revealed that at least one sub-Neptune exoplanet—TOI-421 b—is clear and ready to show off its secrets.
"I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets," said paper author and astronomer professor Eliza Kempton of the University of Maryland, College Park, in a statement.
"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."
Artist's impression of the sub-Neptune exoplanet TOI-421 b.
Artist's impression of the sub-Neptune exoplanet TOI-421 b.
NASA, ESA, CSA, Dani Player STScI
TOI-421 b lies some 245 light-years from Earth in the constellation of Lepus, to the south of Orion. Critically, it also has a temperature of around 1,340 degrees Fahrenheit.
"Why did we observe this planet, TOI-421 b? It's because we thought that maybe it wouldn't have hazes," explained 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."
Below this level, photochemical reactions are expected to occur between sunlight and methane gas, triggering the haze. This threshold is around 1,070 degrees Fahrenheit—well below the temperature of TOI-421 b, which has no methane.
"We saw spectral features that we attribute to various gases—and that allowed us to determine the composition of the atmosphere," added Maryland astronomer Brian Davenport in a statement.
"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 researchers found that TOI-421 b's atmosphere contains water vapour, and also tentative signatures of both carbon monoxide and sulfur dioxide. Alongside a lack of methane, the team also did not detect any carbon dioxide.
The team also believes that the sub-Neptune contains a large amount of lightweight hydrogen in its atmosphere—a finding that came as something of a surprise.
"We had recently wrapped our minds around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmospheres, so that had become our expectation," said Kempton. She added: "And then we found the opposite."
This adds, the researchers note, to the notion that TOI-421 b may have developed in a different way to the cooler sub-Neptunes astronomers have observed before.
A transmission spectrum of exoplanet TOI-421 b, revealing its atmospheric composition.
A transmission spectrum of exoplanet TOI-421 b, revealing its atmospheric composition.
NASA, ESA, CSA, Joseph Olmsted STScI
Another difference between TOI-421 b and the other sub-Neptunes Webb has observed to date is that the newly studied world orbits a sun-like star, not a cooler and smaller red dwarf.
In fact, TOI-421 b's hydrogen-rich atmosphere would seem to mimic that of its 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," said Kempton.
"That process is more in line with the giant planets in our solar system—and it is different from other sub-Neptunes that must have been observed with Webb so far."
As to whether all sub-Neptunes found around sun-like stars are like hot little TOI-421 b, or whether exoplanets are just very diverse in nature, more observations of sub-Neptunes are needed, the researchers said.
"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."
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Reference
Davenport, B., Kempton, E. M.-R., Nixon, M. C., Ih, J., Deming, D., Fu, G., May, E. M., Bean, J. L., Gao, P., Rogers, L., & Malik, M. (2025). TOI-421 b: A Hot Sub-Neptune with a Haze-free, Low Mean Molecular Weight Atmosphere. The Astrophysical Journal Letters, 984(2). https://doi.org/10.3847/2041-8213/adcd76