NASA’s James Webb Space Telescope has uncovered groundbreaking insights into the world of sub-Neptunes, a class of exoplanets that have long puzzled astronomers. These small, gassy planets, although not orbiting our Sun, are the most common type of exoplanet observed in our galaxy. In a new study, Webb’s powerful observations of the exoplanet TOI-421 b have provided scientists with the clearest view yet of sub-Neptune atmospheres, which had previously been obscured by haze. As detailed in the article from NASA Science, Webb’s findings are helping to solve the mystery of why sub-Neptunes are so common, yet so different from planets in our solar system.
A Breakthrough for Sub-Neptunes
Sub-Neptunes, smaller than gas giants but larger than Earth, were an unexpected discovery made by NASA’s Kepler Space Telescope. While these planets are abundant, scientists have struggled to fully understand them because of their gaseous, hazy atmospheres. Before Webb, astronomers could only observe flat-line spectra for these planets—meaning the chemical signatures in their atmospheres were hidden behind haze and clouds.
“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said Eliza Kempton, principal investigator at 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.”
TOI-421 b: The Unexpected Breakthrough
The key breakthrough came from Webb’s study of TOI-421 b, a hotter sub-Neptune with a temperature of about 1,340 degrees Fahrenheit. Scientists had hypothesized that planets with temperatures above 1,070 degrees Fahrenheit would lack haze, offering a clearer view of their atmospheres.
“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” said Brian Davenport, a Ph.D. student at the University of Maryland. “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.” These spectral features revealed water vapor, carbon monoxide, and sulfur dioxide in TOI-421 b’s atmosphere, offering vital clues about its composition.
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI)
A New Perspective on Sub-Neptunes
The discovery that TOI-421 b’s atmosphere is dominated by hydrogen was a surprise to the researchers, as previous sub-Neptunes observed by Webb had heavier molecules in their atmospheres. This suggests that TOI-421 b may have evolved differently than other sub-Neptunes observed so far.
“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.”
These insights could lead to a better understanding of how sub-Neptunes form and evolve, and whether TOI-421 b represents a new category of exoplanets or is a unique case. Scientists hope that further observations of hot sub-Neptunes will help answer these questions and continue to unravel the mysteries of planetary systems beyond our own.