James Webb hunts for a haze-less exoplanet to answer longstanding mystery

By Georgina Torbet Published May 6, 2025

Some of the most common exoplanets in our galaxy are also the most mysterious. We don’t have an equivalent to this type of planet, called a sub-Neptune, in our solar system, but now the James Webb Space Telescope is uncovering details about these planets for the first time.

Smaller than ice giants like Neptune or Uranus but with a gas composition more similar to Saturn or Juputer, Sub-Neptunes are thought to be the most common type of exoplanet, but they are hard to study because they are typically obscured by clouds and haze. That means that astronomers haven’t been able to study their atmospheres, or to learn much about how these planets evolve, or why we don’t have one in our solar system.

But recently, astronomers were able to use Webb to study a sub-Neptune called TOI-421 b. “I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said lead researcher 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.”

This particular planet was selected for study because of its extreme heat, with a scorching temperature of around 1,340 degrees Fahrenheit. That is high enough there shouldn’t be methane present in the planet’s atmosphere, which means that it shouldn’t form a haze — and should therefore be easier to observe.

“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.”

Thanks to the lack of haze, the researchers were able to look into the planet’s atmosphere and see what it was composed of. They found water vapor, with a large amount of hydrogen in the atmosphere, as well as suggestions of carbon monoxide and sulfur dioxide — but notably, no indications of methane or carbon dioxide. The large amount of hydrogen was a surprise as it differs from the few other sub-Neptunes that have been observed with Webb — so could this planet be an anomaly? Or perhaps it formed in a different way from these other similar planets?

The researchers hope to observe more sub-Neptunes to find out. “We’ve unlocked a new way to look at these sub-Neptunes,” said researcher Brian 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 research is published in The Astrophysical Journal Letters.