Sub-Neptunes are high-occurrence exoplanets that have no solar system analog. Much smaller than gas giants and typically cooler than hot-Jupiter exoplanets, these worlds were extremely challenging to observe before the launch of the NASA/ESA/CSA James Webb Space Telescope. Many of sub-Neptunes appear to be very highly obscured by clouds and hazes, which have made it impossible to determine their atmospheric makeup. Now, astronomers using Webb have captured the transmission spectrum of the sub-Neptune TOI-421b and uncovered the chemical fingerprints of its atmosphere.
An artist’s impression of the sub-Neptune exoplanet TOI-421b. Image credit: NASA / ESA / CSA / Dani Player, STScI.
TOI-421 is a solar-type star located about 245 light-years away in the constellation of Lepus.
Also known as BD-14 1137, the star is approximately 10 billion years old, and hosts at least two massive exoplanets.
The inner planet, TOI-421b, is a sub-Neptune with a radius of 2.65 Earth radii and notably a high equilibrium temperature of 647 degrees Celsius (1,197 degrees Fahrenheit).
“Before Webb, scientists had very little information on sub-Neptunes,” said University of Maryland astronomer Brian Davenport and his colleagues.
“While they are a few times larger than Earth, these planets are still much smaller than gas giants 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.”
“They concluded from all of those flat-line spectra that at least certain sub-Neptunes were probably very highly obscured by either clouds or hazes.”
“Why did we observe this planet, TOI-421b? It’s because we thought that maybe it wouldn’t have hazes,” said University of Maryland astronomer Eliza 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 577 degrees Celsius (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-421b is about 727 degrees Celsius (1,340 degrees Fahrenheit), well above the presumed threshold.”
The transmission spectrum of the sub-Neptune TOI-421b reveals the presence of water, and the possible presence of sulfur dioxide and carbon monoxide, but no signs of carbon dioxide or methane. Image credit: NASA / ESA / CSA / Joseph Olmsted, STScI.
Without haze or clouds, the astronomers expected to see a clear atmosphere — and they did.
“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” Davenport said.
“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 water vapor in the atmosphere of TOI-421b, 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 the planet’s atmosphere.
The lightweight hydrogen atmosphere was the big surprise to the scientists.
“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,” Dr. Kempton said.
“This suggests TOI-421b may have formed and evolved differently from the cooler sub-Neptunes observed previously.”
“The hydrogen-dominated atmosphere is also interesting because it mimics the composition of TOI-421b’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.”
The team’s paper was published this week in the Astrophysical Journal Letters.
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Brian Davenport et al. 2025. TOI-421b: A Hot Sub-Neptune with a Haze-free, Low Mean Molecular Weight Atmosphere. ApJL 984, L44; doi: 10.3847/2041-8213/adcd76
